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The Interaction Between
Microorganisms, Metabolites, and
Immune System in the Female
Genital Tract Microenvironment
Huanrong Li
1,2†
, Yuqin Zang
1,2†
, Chen Wang
1,2
, Huiyang Li
1,2
, Aiping Fan
1,2
, Cha Han
1,2
and Fengxia Xue
1,2
*
1
Department of Gynecology and Obstetrics, Tianjin Medical University General Hospital, Tianjin, China,
2
Department of
Gynecology and Obstetrics, Tianjin Key Laboratory of Female Reproductive Health and Eugenic, Tianjin Medical University
General Hospital, Tianjin, China
The female reproductive tract microenvironment includes microorganisms, metabolites, and
immune components, and the balance of the interactions among them plays an important role
in maintaining female reproductive tract homeostasis and health. When any one of the
reproductive tract microorganisms, metabolites, or immunity is out of balance, it will affect the
other two, leading to the occurrence and development of diseases and the appearance of
corresponding symptoms and signs, such as infertility, miscarriage, premature delivery, and
gynecological tumors caused by infectious diseases of the reproductive tract. Nutrients in the
female reproductive tract provide symbiotic and pathogenic microorganisms with a source of
nutrients for their own reproduction and utilization. At the same time, this interaction with the
host forms a variety of metabolites. Changes in metabolites in the host reproductive tract are
related not only to the interaction between the host and microbiota under dysbiosis but also to
changes in host immunity or the environment, all of which will participate in the pathogenesis
of diseases and lead to disease-related phenotypes. Microorganisms and their metabolites
can also interact with host immunity, activate host immunity, and change the host immune
status and are closely related to persistent genital pathogen infections, aggravation of
infectious diseases, severe pregnancy outcomes, and even gynecological cancers.
Therefore, studying the interaction between microorganisms, metabolites, and immunity in
the reproductive tract cannot only reveal the pathogenic mechanisms that lead to
inflammation of the reproductive tract, adverse pregnancy outcomes and tumorigenesis
but also provide a basis for further research on the diagnosis and treatment of targets.
Keywords: microenvironment, female genital tract, immunity, metabolites, microbiota
INTRODUCTION
Different from the high diversity of the gastrointestinal tract, the female genital tract microbiome has
low diversity, and it changes dynamically through the female menstrual cycle (Consortium, 2012;
Chen et al., 2017). Most microbes have a symbiotic relationship with the host. Accounting for 90–95%
of the total bacterial biomass, Lactobacillus spp. represents a healthy female genital tract microbiota
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 6094881
Edited by:
Carla R. Taddei,
University of São Paulo, Brazil
Reviewed by:
Silvia Daher,
Federal University of São Paulo, Brazil
Alison Jane Carey,
Queensland University of Technology,
Australia
*Correspondence:
Fengxia Xue
fengxiaxue1962@gmail.com
†
These authors have contributed
equally to this work
Specialty section:
This article was submitted to
Microbiome in Health and Disease,
a section of the journal
Frontiers in Cellular
and Infection Microbiology
Received: 23 September 2020
Accepted: 18 November 2020
Published: 23 December 2020
Citation:
Li H, Zang Y, Wang C, Li H, Fan A,
Han C and Xue F (2020) The
Interaction Between Microorganisms,
Metabolites, and Immune System in
the Female Genital Tract
Microenvironment.
Front. Cell. Infect. Microbiol. 10:609488.
doi: 10.3389/fcimb.2020.609488
REVIEW
published: 23 December 2020
doi: 10.3389/fcimb.2020.609488
that produces lactic acid to maintain an acidic microenvironment.
It can also inhibit pathogens through competition, adhesion
prevention, and the secretion of antibacterial and
immunomodulatory substances (Anahtar et al., 2018;Van der
Veer et al., 2019). Vaginitis, cervicitis, and pelvic inflammatory
disease (PID) will occur if pathogenic bacteria surpass lactobacilli
in the female genital tract and can cause uncomfortable
symptoms such as increased vulvovaginal discharge, itching,
odor, and lower abdominal pain (Workowski and Bolan, 2015).
However, there are differences in the microbes between
subjects and in the ability of the host to resist dysbiosis that
may be related to race, diet, age, living habits, immunity, disease
susceptibility, and genetic polymorphism (Consortium,
2012;Anahtar et al., 2018;Chu et al., 2018;Serrano et al.,
2019). Furthermore, the dominance of different microflora is not
necessarily related to symptoms because partial non-Lactobacillus-
dominant women do not experience uncomfortable symptoms of
vulvovaginitis; hence, we cannot define disease by the number of
bacteria alone, and we cannot define dysbiosis without the internal
milieu of the host and disease environment (Anahtar et al., 2018;
Scott et al., 2019).
Metabolites in the reproductive tract play an important role
in female genital tract inflammation, pregnancy and tumors and
can be considered biomarkers of disease severity, diagnosis, and
prognosis (Ghartey et al., 2015;McMillan et al., 2015;Ilhan et al.,
2019;Song et al., 2019). Metabolites in the female reproductive tract
are the substrates, intermediates and byproducts of biochemical
reactions caused by the interaction of human nutrients and bacteria,
reflecting downstream events of gene expression (Altmäe et al.,
2014;McMillan et al., 2015;Turkoglu et al., 2016;Watson and Reid,
2018)(Figure 1). These metabolites are better than genome,
transcriptome, and proteome substances at predicting the disease
phenotype (Altmäe et al., 2014). Genital infections, adverse
pregnancy outcomes, and cancers possess different metabolic
signatures that are often accompanied by dysbiosis of the female
genital tract (Ghartey et al., 2015;Ceccarani et al., 2019;Ilhan et al.,
2019). The metabolic pathways affected by these metabolites mainly
include amino acids, carbohydrates, and lipid metabolism, which
are closely related to life activities (Srinivasan et al., 2015). These
activities further affect host cell function, immunity, and disease
susceptibility and help maintain the balance of the host’s
reproductive tract microenvironment.
The host’s innate and adaptive immune systems perform
complex interactions with microorganisms and metabolites
(Agostinis et al., 2019;Delgado-Diaz et al., 2019). Microbial
ligands bind to host receptors to produce inflammatory factors,
chemokines and antimicrobial products to regulate the immune
response of the reproductive tract (Hooper et al., 2012).Vaginal
dysbiosis cannot only directly cause vaginal epithelial injury
through pathogens (Tao et al., 2019), but also indirectly cause
vaginal epithelial injury through immune components, which in
turn release metabolites into the microenvironment (Olive and
Sassetti, 2016;Serrano et al., 2019). This metabolite may be
ingested by the vaginal microbiota, leading to increased
microbial metabolism, which is beneficial to the growth and
reproduction of the microbiota (Serrano et al., 2019). The local
competition between the host, pathogen and different immune
cells for metabolic precursors will also affect the ability of immune
cells to respond effectively to infection, affecting the growth and
immunogenicity of the pathogen and further affecting the host
response (Hooper et al., 2012;Olive and Sassetti, 2016;Postler and
Ghosh, 2017). Therefore, the interaction between microorganisms,
metabolites, and immunity in the host reproductive tract
microenvironment plays an important role in maintaining the
balance of the reproductive tract (Pruski et al., 2018). An
imbalance in any part will result in host phenotype changes,
disease, and even serious complications. Therefore, this article
intended to review the relationship and importance of
FIGURE 1 | Microenvironmental disorders of the female reproductive tract are closely related to inflammations, adverse pregnancy outcomes, and tumors. Modified
from PawełŁaniewski et al. (2020).
Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 6094882
reproductive tract microorganisms, metabolites, and immunity to
obtain a deeper understanding of the reproductive tract
microenvironment, reproductive tract diseases and adverse
reproductive tract outcomes.
NORMAL VAGINAL MICROENVIRONMENT
The vaginal microbiota community state types (CSTs) of women
of childbearing age are divided into five categories (Ravel et al.,
2011). CST I is dominated by Lactobacillus crispatus; CST II by L.
gasseri; CST V by L. jensenii; and CST III by L. iners. CST IV
belongs to the Lactobacillus-deficient type, which is dominated
by anaerobic bacteria (classified by bacterial vaginosis, BV),
partial aerobic bacteria (classified by aerobic vaginitis, AV) or a
modest proportion of Lactobacillus spp. (Gajer et al., 2012). The
vaginal microbiota is dynamic and occasionally transitions to an
intermediate state or a disease state in most normal non-
pregnant women. A high Nugent score does not indicate the
disease status or microecological disorders (Gajer et al., 2012).
The internal milieu of the host will make the microbiota return to
a stable state. Factors that cause vaginal microbiota changes are
primarily related to menstruation. Others include sexual
intercourse, hormonal contraception, antimicrobial agents, use
of lubricants, and vaginal douching, but they have less impact
than menstrual periods and cause changes in the vaginal
microbiota for a shorter duration (Gajer et al., 2012;Mitchell
et al., 2012). In addition, in a few women, the vaginal CST does
not change with the menstrual cycle and hormonal
contraception, but further study by future researchers is
needed to determine whether it is related to the metabolic
function of bacteria (Gajer et al., 2012;Song et al., 2020).
Lactobacillus abundance in the female genital tract is strongly
positively correlated with lactate and 4-hydroxyphenylacetate
and correlated to a lesser extent with isoleucine, leucine,
tryptophan, phenylalanine, aspartate, dimethylamine, sarcosine
and pi-methylhistidine, all of which are typically associated with
vaginal health (Srinivasan et al., 2015;Ceccarani et al., 2019)
(Table 1). L. crispatus and L. jensenii have similar metabolic
patterns (Srinivasan et al., 2015), while, L. crispatus and L. iners
have different metabolic characteristics (Pruski et al., 2018). For
example, studies have found that the genome of L. crispatus is
almost twice that of L. iners (France et al., 2016). However, the
carbon metabolism of L. iners is fermented by fewer compounds
than L. crispatus (Pruski et al., 2018). When the dominant
bacteria are L. crispatus and/or L. jensenii,mostofthe
metabolites in the vagina are amino acids and dipeptide, such
as higher levels of ornithine, lysine, glycylproline, phenylalanine
(Srinivasan et al., 2015). Similar to BV-related flora, L. iners are
correlated with amino acid catabolites, such as higher levels of
TABLE 1 | Current existing articles analyzing the correlation between the genital tract flora and metabolites in normal non-pregnant women of reproductive age.
Year Author Population Sample CST Metabolites
2019 (Ceccarani
et al.,
2019)
Healthy women (n = 21), women
with BV (n = 20) and women with
Chlamydia trachomatis infection
(n = 20)
Vaginal
swabs
Contains all
Lactobacillus spp.
without
distinguishing CST
High levels of lactate, 4-hydroxyphenylacetate, isoleucine, leucine, tryptophan,
phenylalanine, aspartate, dimethylamine, sarcosine and pi-methylhistidine
2018 (Parolin
et al.,
2018)
Healthy women (n = 22), women
with Chlamydia trachomatis
infection (n = 20), and women with
BV (n = 19)
Vaginal
swabs
Contains all
Lactobacillus spp.
without
distinguishing CST
Higher levels of lactate, 4-hydroxyphenylacetate, diverse amino acids
(phenylalanine, glutamate, leucine, threonine, tryptophan, aspartate) and
amino acid derivatives(sarcosine)
2015 (Srinivasan
et al.,
2015)
Women with BV (n = 40) and
women without BV (n = 20)
Vaginal
fluid
CST-I 1) Higher levels of sugars (maltose, maltotriose, and maltohexose), lipid
metabolism biochemicals (such as arachidonate and carnitine), amino acids
(ornithine, lysine), dipeptide (glycylproline, phenylalanine) as well as lactate,
and urea
2) Lower levels of N-acetylneuraminate, succinate, the carnitine precursor
deoxycarnitine, the eicosanoid 12-hydroxyeicosatetraenoic acid, the fatty acid
13-hydroxyoctadecadienoic acid, the nucleobase uracil, and glutathione
CST-III 1) Higher levels of proline, threonine, aspartate, serine, and valinylglutamate
2) Lower levels of glutamate and glycylleucine
CST-V Same as CST-I:
1) Higher levels of sugars (maltose, maltotriose, and maltohexose), lipid
metabolism biochemicals (such as arachidonate and carnitine), amino acids
(ornithine, lysine), dipeptide (glycylproline, phenylalanine) as well as lactate,
and urea
2) Lower levels of N-acetylneuraminate, succinate, the carnitine precursor
deoxycarnitine, the eicosanoid 12-hydroxyeicosatetraenoic acid, the fatty acid
13-hydroxyoctadecadienoic acid, the nucleobase uracil, and glutathione
2015 (McMillan
et al.,
2015)
Pregnant women (n = 67) and non-
pregnant women (n = 64)
Vaginal
fluid
CST-I Higher levels of succinate
2012 (Gajer
et al.,
2012)
Reproductive age women (n = 32) Vaginal
swabs
CST-III 1) Higher levels of lactate
2) Lower levels of succinate and acetate
CST, Community state types; BV, bacterial vaginosis.
Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 6094883
proline, threonine, aspartate, serine, and valinylglutamate
(Srinivasan et al., 2015). In addition, L. iners often has a
symbiotic relationship with G. vaginalis, and both produce
similar levels of cholesterol-dependent cytolysin (Macklaim
et al., 2013). Therefore, the metabolic characteristics of L.
crispatus and/or L. jensenii dominance can be defined as a
healthy vaginal microenvironment. However, whether the
metabolic characteristics of the non-Lactobacillus abundance of
some asymptomatic women are similar to those of L. crispatus
and/or L. jensenii dominance needs to be further explored.
The mucous layer on the surface of female genital tract
epithelial cells plays an important role as the first line of
defense against microbial invasion (Mirmonsef et al., 2011;
Aldunate et al., 2015). When microorganisms break through
the line of defense, epithelial cells use pattern recognition
receptors (PPRs) to identify microorganisms to produce
inflammatory factors and recruit inflammatory cells to resist
microbial invasion and colonization. The dominance of
Lactobacillus in the genital tract is essential, as it inhibits
pathogens and maintains immune equilibrium (Smith and
Ravel, 2017). Studies have found that the concentration of
inflammatory factors in the vagina is very low when L.
crispatus and L. jensenii are dominant (Kyongo et al., 2012).
Lactic acid, as a metabolite derived primarily from Lactobacillus
spp., is also related to reproductive tract immunity (Delgado-
Diaz et al., 2019). L-lactic acid produced by Lactobacillus spp. can
cause an anti-inflammatory response and inhibit the production
of proinflammatory cytokines and chemokines induced by toll-
like receptor (TLR) in cervical and vaginal epithelial cells at low
pH (Delgado-Diaz et al., 2019). In addition, lactic acid can
induce the secretion of the anti-inflammatory cytokine
interleukin (IL)-10, reduce the production of the
proinflammatory cytokine IL-12 in dendritic cells (DCs), and
reduce the cytotoxicity of natural killer cells (Ilhan et al., 2019).
The anti-inflammatory activity of lactic acid also requires the
presence of organic acids produced by microorganisms to
maintain vaginal health, mainly by increasing the production
of the anti-inflammatory cytokine IL-1RA, inhibiting the
proinflammatory signal of the IL-1 cytokine, and slightly
reducing the production of the proinflammatory cytokines IL-6
and macrophage inflammatory protein 3 alpha (MIP-3a)
(Delgado-Diaz et al., 2019). Therefore, the interaction between
the flora, metabolites, and immunity in the healthy reproductive
tract is very important for maintaining the health of the
reproductive tract. When any one party is imbalanced, it will
affect the balance of the reproductive tract.
COMMON REPRODUCTIVE TRACT
INFECTIONS
Female genital tract infections mainly include vaginitis, cervicitis,
and PID (Sherrard et al., 2018). The main cause is exogenous
pathogen interference or endogenous dysbiosis (Workowski and
Bolan, 2015;Song et al., 2020). At present, the relevant research
on infectious diseases caused by the interaction between
reproductive tract microorganisms, metabolites and the host is
mainly focused on BV, Chlamydia trachomatis (C. trachomatis),
and AV (Ceccarani et al., 2019). In addition, inflammation of the
reproductive tract caused by bacterial flora disorders involving
BV, AV, and C. trachomatis infection is closely related to adverse
pregnancy outcomes and tumors (Sherrard et al., 2018). Other
diseases, such as trichomoniasis (Trichomonas vaginalis),
vulvovaginal candidiasis, and gonorrhea, have received less
relevant research in this area and may become future research
directions. Therefore, this section mainly discusses the
interaction between the microorganisms, metabolites, and host
immunity of three common RTIs: BV, C. trachomatis infection,
and AV.
BV
BV is the most common vaginal microbial disorder of women of
childbearing age, and can lead to adverse obstetrics and
gynecological outcomes such as infertility, miscarriage,
premature rupture of membranes, and premature delivery
(Workowski and Bolan, 2015;Baqui et al., 2019;Peebles et al.,
2019). It also increases the risk of sexually transmitted infections
(Shipitsyna et al., 2020). BV is characterized by an increase in the
diversity of vaginal microbiota, a decrease in Lactobacillus spp. in
the vagina, and an increase in BV-related anaerobic and
microaerobes (Srinivasan et al., 2015). BV-related bacteria
mainly include Gardnerella,Atopobium,Mycoplasma,
Megasphaera,Mobiluncus,Roseburia,Dialister.,Sneathia and
Prevotella spp. (McMillan et al., 2015;Ceccarani et al., 2019).
However, flora analysis alone cannot distinguish between a
normal vaginal environment and BV because Atopobium spp.,
Prevotella spp. and Mycoplasma hominis can also be detected in
healthy people, therefore, the vaginal microenvironment needs to
be analyzed in combination with metabolomics (Vitali
et al., 2015).
BV is closely related to metabolites in the genital tract
(Spiegel et al., 1980;Wolrath et al., 2002)(Table 2). The
metabolites of amines, organic acids, short chain fatty acids
(SCFAs), amino acids, nitrogenous bases and monosaccharides
of BV patients are significantly different from those of
healthy individuals (Vitali et al., 2015). Current studies suggest
that metabolites better reflect the disease phenotype than
microorganisms. Before disease symptoms appear, the
appearance or disappearance of certain metabolites in the
vagina has a positive or negative correlation with the metabolic
function of certain microorganisms (Yeoman et al., 2013).
Changes in maltose, kynurenine, nicotinate, malonate, acetate
and nicotinamide adenine dinucleotide (NAD
+
) represent the
occurrence of BV and can be used as metabolic biomarkers to
distinguish BV from a healthy vagina (Vitali et al., 2015). When
BV is cured, the metabolites associated with BV decrease
significantly (Stanek et al., 1992;Srinivasan et al., 2015). In
addition, genital tract metabolic analysis plays a prominent
role in the diagnosis of BV (Watson and Reid, 2018). In 2015,
McMillan et al. (2015) foundthatanincreasein2-
hydroxyisovalerate and g-hydroxybutyrate and a decrease in
lactic acid and tyrosine in the vagina are the most sensitive
Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 6094884
TABLE 2 | Current existing articles analyzing the correlation between the genital tract flora and metabolites in women with vaginitis.
Year Author Population Sample CST Metabolites
BV
2019 (Ceccarani
et al., 2019)
Healthy women (n = 21), women with BV (n = 20) and women
with Chlamydia trachomatis infection (n = 20)
Vaginal
swabs
CST-IV 1) Higher levels of organic acids (i.e.: formate, pyruvate,
propionate, acetate, 2-hydroxyisovalerate), amines (i.e.:
trimethylamine, putrescine), amino acids (i.e.: proline
and alanine) and 5-aminopentanoate
2) Lower levels of lactate, 4-hydroxyphenylacetate,
phenylalanine, pi-methylhistidine, glycine, isoleucine,
leucine, tryptophan, aspartate, dimethylamine, and
sarcosine
2018 (Parolin et al.,
2018)
Healthy women (n = 22), women with Chlamydia trachomatis
infection (n = 20), and women with BV (n = 19)
Vaginal
swabs
CST-IV Higher levels of biogenic amines (methylamine,
putrescine, trimethylamine, tyramine, desaminotyrosine),
organic acids (succinate, malonate, 2-
hydroxyisovalerate, and short-chain fatty acids) and
alanine
2015 (McMillan et al.,
2015)
Pregnant women (n = 67) and non-pregnant women (n = 64) Vaginal
fluid
CST-IV 1) Organic acid: higher levels of 2-hydroxyisovalerate, g
-hydroxybutyrate, 2-hydroxyglutarate and 2-
hydroxyisocaproate; lower levels of lactate
2) Amines: higher levels of tyramine, putrescine, and
cadaverine
3) Amino acids: lower levels of tyrosine
2015 (Srinivasan
et al., 2015)
Women with BV (n = 40) and women with non-BV (n = 20) Vaginal
fluid
CST-IV 1) Amino acid: higher levels of cadaverine, pipecolate,
tyramine, 4-hydroxyphenylacetate, 3- (4-hydroxyphenyl)
propionate, tryptamine, citrulline and putrescine; lower
concentrations of arginine, ornithine, spermine and
dipeptides
2) Carbohydrates: higher levels of N-acetylneuraminate,
galactose, threitol and succinate; lower levels of
glucosamine, maltotriose, maltotetraose,
maltopentaose, maltohexaose, lactate, fructose, and
mannitol
3) NAD: lower levels of nicotinamide; higher levels of
nicotinate
4) Lipids: higher levels of 12-hydroxyeicosatetraenoic
acid, deoxycarnitine, 4-hydroxybutyrate and
13-hydroxyoctadecadienoic acid; lower levels of
arachidonate, carnitine, ascorbic acid, acetylcarnitine,
propionylcarnitine, butyrylcarnitine, glycerol and
glycerol-3-phosphate
2015 (Vitali et al.,
2015)
BV-affected patients (n = 43) and healthy controls (n = 37) Vaginal
fluid
CST-IV 1) Amines: higher levels of tyramine, ethanolamine,
trimethylamine, methylamine, cadaverine
2) Organic acids: higher levels of formate, malonate,
succinate, pyruvate, acetate
3) Short-chain fatty acids: higher levels of propionate,
butyrate, 2-hydroxyisovalerate
4) Amino acids: higher levels of proline; lower levels of
tryptophan, phenylalanine, tyrosine, glutamate,
isoleucine, leucine
5) Nitrogenous bases: higher levels of nicotinate, uracil;
lower levels of NAD+, inosine
6) Sugars: higher levels of glucose; lower levels of
maltose
7) Others: higher levels of urocanate, 2-aminoadipate,
3-methyl-2-oxovalerate; lower levels of kynurenine, sn-
glycero-3-phosphocholine, sarcosine
2013 (Yeoman et al.,
2013)
Pre-menopausal women of reproductive age (n = 36) Vaginal
lavage
fluid
CST-IV 1) Higher levels of putrescine, cadaverine, 2-methyl-2-
hydroxybutanoic acid, hydroxylamine, glycolic acid,
tetradecanoic acid, and butyrolactone
2) Lower levels of 2,3-hydroxypropyl-2-aminoethyl
phosphate, cis-11-octadecanoic acid, and ribose-5-
phosphate
2012 (Gajer et al.,
2012)
Reproductive age women (n = 32) Vaginal
swabs
CST-IV 1) Higher levels of succinate and acetate
2) Lower concentrations of lactate
(Continued)
Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 6094885
and specific indicators for the diagnosis of BV. Therefore, not
only the microbiota but also metabolites can be used as effective
reference indicators for clinical diagnosis.
The vaginal microbiota and metabolites of BV patients are also
closely related to the clinical symptoms and signs of the host. The
odor of vaginal secretions in patients with BV is related to the
increase in tyramine, trimethylamine, cadaverine, and putrescine
and the decrease in the aromatic substances 2 (5H)-furanone and
2-ethyl-4-methyl-1,3-dioxolane (Yeoman et al., 2013;Srinivasan
et al., 2015;Vitali et al., 2015). Odor is also closely related to
Dialister spp. (Yeoman et al., 2013;McMillan et al., 2015;
Srinivasan et al., 2015). Thin and homogeneous secretions are
positively related to cadaverine, and cadaverine is related to
Streptococcus spp. and Mycoplasma spp. (Yeoman et al., 2013;
Srinivasan et al., 2015). Clue cells are positively correlated
with deoxycarnitine and pipecolate, while deoxycarnitine is
positively correlated with BV-associated bacterium 1 (BVAB1),
Megasphaera sp. type 2, and several Prevotella species (Srinivasan
et al., 2015). Vaginal discharge is related to 2-methyl-2-
hydroxybutanoic acid and Mobiluncus spp. (Yeoman et al.,
2013). In addition, the metabolic pathways of amino acids,
carbohydrates, NAD, and lipids in the vaginal flora of BV
patients are active and are closely related to cellular life activities
(Srinivasan et al., 2015;Ceccarani et al., 2019).Therefore,
understanding the interaction between the flora and metabolites
of BV patients provides a basis for understanding the molecular
mechanisms of microbe-microbe and microbe-host interactions
(Ilhan et al., 2019).
BV-related bacteria can activate the host’sgenitaltract
immune response, but they do not cause obvious inflammatory
symptoms such as redness, swelling, heat and pain (Smith and
Ravel, 2017). The reason may be related to the influence of BV-
related microorganisms and their metabolites on immunity. In
2019, Delgado-Diaz et al. (2019) found that the sustained action
of organic acids, metabolites of the vaginal microbiota associated
with BV, led to dysregulation of the immune response of cervical
and vaginal epithelial cells in vitro. SCFAs can recruit and
activate female reproductive tract innate immune cells, such as
neutrophils and monocytes (Vitali et al., 2015). However, SCFAs
can also inhibit the production of proinflammatory cytokines
and affect the migration and phagocytic response of immune
cells to regulate the immune response (Al-Mushrif et al., 2000).
In addition, succinic acid produced by Prevotella spp. and
Mobiluncus spp. in the genital tract can also inhibit leukocyte
chemotaxis and regulate the immune response (Al-Mushrif et al.,
2000;McMillan et al., 2015;Vitali et al., 2015). In 1985, Rotstein
et al. (1985) demonstrated that succinic acid has the strongest
chemotaxis inhibitory effect at pH 5.5 and at concentrations of
20–30 mM. Therefore, the current research has proven that the
interaction between BV flora, metabolites and immunity is of
great significance for understanding clinical symptoms and signs.
However, more research on the interaction mechanism between
immunity and metabolites in BV patients is needed to confirm
the influence of metabolites on flora and immunity.
Chlamydia trachomatis
In 2016, the World Health Organization announced the newest
global C. trachomatis prevalence rate of 1.5–7% for women aged
15–49 years and an estimated 127 million new cases women
worldwide that year (Organization, 2020). Most women infected
with C. trachomatis are asymptomatic (Ceccarani et al., 2019).
Approximately 10% of C. trachomatis infections will progress to
PID without timely treatment, which will cause severe ectopic
pregnancy, reproductive dysfunction and cancer (Workowski
and Bolan, 2015;Idahl et al., 2020). Lactic acid is an important
inhibitor of C. trachomatis infection (Gong et al., 2014).
However, L. iners produces less lactic acid, so the microbiota
dominated by L. iners increases the risk of C. trachomatis
infection (Van Houdt et al., 2018). Similarly, BV also increases
the risk of C. trachomatis infection due to a reduction in the
lactate concentration (Shipitsyna et al., 2020). Therefore, C.
trachomatis infection is greatly affected by lactic acid in the
reproductive tract microenvironment.
The interactions among microorganisms, C. trachomatis and
metabolites in the reproductive tract are closely related (Table 2).
In 2016, Ceccarani et al. (2019) performed a combined
metagenomic and metabolomics analysis on the vaginal
secretions of non-pregnant Caucasians of childbearing age with
risk factors for C. trachomatis infection. The study showed that C.
trachomatis infection was dominated by Lactobacillus in most
TABLE 2 | Continued
Year Author Population Sample CST Metabolites
2002 (Wolrath et al.,
2002)
Women of childbearing age with various lower genital tract
disorders (n = 61)
Vaginal
fluid
CST-IV Higher levels of trimethylamine
1980 (Spiegel et al.,
1980)
Women with non-specific vaginitis (n = 53) Vaginal
fluid
CST-IV 1) Higher levels of succinate, acetate, butyrate, and
propionate
2) Lower levels of lactate
Chlamydia trachomatis
2018 (Parolin et al.,
2018)
Women with Chlamydia trachomatis infection (n = 20), healthy
women (n = 22), and women with BV (n = 19)
Vaginal
swabs
CST-III Lower levels of tyramine, dimethylamine, cadaverine,
succinate, valine, isoleucine, glycine, sarcosine,
creatinine, 4-aminobutyrate
2019 (Ceccarani
et al., 2019)
Healthy women (n = 21), women with BV (n = 20) and women
with Chlamydia trachomatis infection (n = 20)
Vaginal
swabs
CST-IV Lower levels of lactate, certain amino acids and
biogenic amines
AV
2012 (Gajer et al.,
2012)
Reproductive age women (n = 32) Vaginal
swabs
CST-IV Higher levels of acetate and lactate
CST, Community state types; BV, bacterial vaginosis; AV, aerobic vaginitis.
Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 6094886
people, and L. iners was increased, and some patients had
anaerobic bacteria as the dominant bacteria. Compared with
healthy controls, women infected with C. trachomatis showed
only slight changes in vaginal metabolites that were mainly
manifested as a reduction in certain amino acids and biogenic
amines (Ceccarani et al., 2019). In 2018, Parolin et al. (2018)
studied the characteristics of vaginal microbes and metabolites in
thecaseofC. trachomatis infection and found that vaginal valine,
isoleucine, tyramine, cadaverine, and succinate in patients with C.
trachomatis infection were significantly decreased compared with
those in healthy controls, indicating that C. trachomatis may use
nitrogen as the first nutrient source or that C. trachomatis may
affect the nitrogen metabolism of infected host cells. There is a
correlation between the vaginal microbiome, metabolites, and
genital symptoms of C. trachomatis infection (Parolin et al.,
2018). More than half of C. trachomatis-infected patients are
completely asymptomatic, while symptomatic patients mainly
manifest with abnormal vaginal discharge, dyspareunia, dysuria,
and abnormal bleeding. The concentration of 4-aminobutyrate is
significantly different between asymptomatic and symptomatic
women with C. trachomatis infection. However, all asymptomatic
women have L. crispatus as the dominant bacteria in the vagina,
and only half of symptomatic women have L. crispatus as the
dominant bacteria (Parolin et al., 2018). Therefore, C. trachomatis
infection is related to the genital tract flora, metabolites and
clinical symptoms. However, the effect of 4-aminobutyrate on
host immunity against C. trachomatis infection needs
further study.
There are complicated interactions between C. trachomatis,
microorganisms, genital tract metabolites and immunity (Ziklo
et al., 2016b). Epithelial cells and immune cells infected by C.
trachomatis can secrete several proinflammatory cytokines and
chemokines to eliminate pathogen infection (Rasmussen et al.,
1997;Johnson, 2004;Brunham and Rey-Ladino, 2005).
Interferon (IFN)-gis an important factor that inhibits the
reproduction of C. trachomatis (Shemer and Sarov, 1985). The
ability to synthesize tryptophan in the IFNg-rich infection
microenvironment is an important virulence factor of the
genital C. trachomatis serovars (Aiyar et al., 2014). IFN-g
mediates the activation of host indoleamine 2,3-dioxgenase
(IDO), leading to the consumption of tryptophan necessary for
the growth of C. trachomatis and inhibiting the growth of C.
trachomatis (Beatty et al., 1994;Aiyar et al., 2014;Olive and
Sassetti, 2016;Molenaar et al., 2018). The tryptophan needed for
the growth of C. trachomatis is reduced, and C. trachomatis
forms a static state (Byrne et al., 1989). It is known that BV
infection increases the risk of C. trachomatis infection. BV-
related bacteria, such as partial Prevotella species, can produce
indole (Romanik et al., 2007;Sasaki-Imamura et al., 2011), which
is increased in the vaginal discharge of BV patients (Lewis et al.,
2014). C. trachomatis in the genital tract can use indole produced
by microorganisms as a substrate to activate alternative
tryptophan synthesis pathways- the trpA,trpB and trpR genes,
synthesize tryptophan, and make C. trachomatis evade the
clearance of IFN-gin the genital tract (Fehlner-Gardiner et al.,
2002;Wood et al., 2003;Ziklo et al., 2016b). However, in patients
who are coinfection with BV and C. trachomatis, the inhibitory
response to IFN-gis not exactly the same, which may be related
to the level of indole in the vaginal microenvironment (Lewis
et al., 2014). In addition, the low oxygen environment formed
under BV may result in insufficient energy supply for the IFN-g
signaling pathway, which further reduces its function (Roth et al.,
2010). IFN-ginduces the production of nitric oxide and further
inhibits the growth of C. trachomatis (Agrawal et al., 2011). The
most recent in vitro experiments have confirmed that C.
trachomatis induces the expression of ornithine decarboxylase
(ODC), deprives the inducible nitric oxide synthase (iNOS)
substrate arginine, and actively promotes polyamine synthesis
while downregulating iNOS expression and inhibiting the
activity of iNOS to reduce nitric oxide production in the host
and further escape the host’s innate immunity (Abu-Lubad et al.,
2014;Olive and Sassetti, 2016). Studies have shown that the
amino acids and sugars in the environment are critical to the
ability of C. trachomatis to infect (Harper et al., 2000). However,
the detailed metabolic and immune interaction mechanisms still
need further study. After C. trachomatis escapes the host’s
immunity, the host’s immune surveillance is reduced, and the
environment is conducive to the growth of C. trachomatis, which
is reactivated (Belland et al., 2003). After C. trachomatis
reinfection or chronic infection, T helper (Th)1-, Th2- and
Th17-typecellsaretriggeredtomediatetissuedestruction,
fibrosis, and scarring, further leading to the progression of PID
and its sequelae (Ziklo et al., 2016a;Molenaar et al., 2018).
AV
The incidence of AV in women of childbearing age is approximately
10% (Donders et al., 2017). Significantly different from BV patients
and those with a normal flora, AV patients have increased aerobic
bacteria or enterococci, such as Escherichia coli,Streptococcus
agalactiae,Staphylococcus aureus,Staphylococcus epidermidis,
Streptococcus anginosus,andEnterococcus faecalis, in the vagina
(Donders et al., 2017;Tao et al., 2019;Wang et al., 2020). Studies
have also shown that BV-related bacteria often appear in the vaginal
flora of AV patients, which may be related to the symbiotic
relationship between BV- and AV-related bacteria in the state of
flora disorder (Wang et al., 2020). Different from the clinical
symptoms and signs of BV, AV mainly manifests as foul and
yellow purulent discharge, but similar to BV, it easily causes adverse
obstetrics and gynecology complications that may be related to
bacterial ascending infection (Donders et al., 2017).
It is known that AV-related bacteria and their metabolites are
involved in the host’sinflammatory state and immune response,
but their correlation with host disease phenotypes and diagnostic
applications have not yet been studied. Previous studies have
found that when Streptococcus sp. increase in the vaginal
secretions of non-pregnant women, acetate also increases
(Gajer et al., 2012)(Table 2). Acetate is a SCFAs that directly
activates the host’simmune-inflammatory pathway and
promotes the expansion of T-regulatory (Treg) cells (Olive and
Sassetti, 2016;Postler and Ghosh, 2017). However, the detailed
mechanism by which acetic acid is produced by Streptococcus sp.
and genital tract immunity still needs to be studied.
Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 6094887
AV patients mainly present with a local immune imbalance in
the reproductive tract caused by pathogens (Benner et al., 2018).
In 2020, Budilovskaya et al. (2020) found that the expression of
IL1b, IL-6, IL-8, IL10, tumor necrosis factor-a(TNFa) and
CD68 messenger ribose nucleic acids (mRNAs) in AV patients
was significantly increased, and this change was related to itching
or burning as well as increases in leukocytes and parabasal
epithelial cells under the microscope (Smith and Ravel, 2017).
Purulent vaginal discharge and vaginal redness may be related to
the toxic effect of the virulence gene sag of Streptococcus
anginosus on epithelial cells, leading to epithelial cell lysis (Tao
et al., 2019).
The molecular mechanism of inflammatory genital tract
symptoms in AV patients may be related to the interaction of
metabolism and immunity. Nitric oxide plays an important role
in host resistance to pathogens. Nitric oxide is synthesized by
iNOS in inflammatory cells (Richardson et al., 2006;Jones et al.,
2010). After the cells secrete nitric oxide, they can kill pathogens
directly beside the inflammatory cells. However, Staphylococcus
aureus can evade the host nitric oxide response by changing
metabolism (Olive and Sassetti, 2016). Staphylococcus aureus
induces the expression of flavohaemoglobin (Hmp) through the
SrrAB system, quickly and enzymatically hydrolyzes nitric oxide,
and resists the host’s inhibitory effect on pathogens (Richardson
et al., 2006); at the same time, Staphylococcus aureus upregulates
L-lactate dehydrogenase 1 (Ldh1), enabling it to survive lactic
acid fermentation under aerobic and anaerobic conditions
(Richardson et al., 2008). The virulence of Staphylococcus
aureus requires hexose produced by glycolysis, and an increase
in the glucose concentration will enhance the resistance of
pathogens to nitric oxide and subsequently the host immune
response (Vitko et al., 2015;Olive and Sassetti, 2016).
Polyamines are toxic to Staphylococcus aureus.Staphylococcus
aureus strains with arginine catabolic mobile element (ACME)
encode the acetyltransferase SpeG, which makes the strains
resistant to polyamines and facilitates colonization in host cells
(Diep et al., 2008;Olive and Sassetti, 2016). Pathogens evade
the killing effect of the host’s immune system, facilitating the
colonization of pathogens in the host’s reproductive tract. The
colonization of toxic shock syndrome toxin-1 (TSST-1)
Staphylooccus aureus strains will increase the production of
proinflammatory cytokines and chemokines in human vaginal
epithelial cells, further destroying the mucosal barrier and
increasing the penetrating effect of TSST-1, leading to severe
symptoms and signs of vulvovaginitis (Pereira et al., 2013). This
also explains why vaginal inflammation in AV is more serious
than that in BV. However, previous studies have mainly focused
on of BV-related bacteria, and there are few studies on AV-
related bacteria, metabolites, and immunity. Future research may
reveal the significance if it is used as a future research direction.
NORMAL PREGNANCY
Unlike non-pregnant women, healthy pregnant women are
affected by estrogen-progesterone, and the vaginal microflora
tends to be stable from the first trimester to the third trimester,
that is, the low richness and low diversity dominated by
Lactobacillus spp. inhibits the growth of CST IV pathogenic
bacteria such as Gardnerella vaginalis, Atopobium vaginae,
Sneathia amnii,Prevotella Bivia,andPrevotella cluster 2
(MacIntyre et al., 2015;Brown et al., 2018;Serrano et al.,
2019). The vaginal flora during pregnancy is less transformed,
mostly between Lactobacillus species (Romero et al., 2014;
TABLE 3 | Current existing articles analyzing the correlation between the genital tract flora and metabolites in pregnant women.
Year Author Population Sample CST/
microorganisms
Metabolites
Normal pregnant women
2016 (Prince
et al.,
2016)
Women who delivered at term (n = 27), women
who delivered preterm (n = 44)
Placental
membranes
swabs
Bradyrhizobium
spp., streptococcus
thermophilus
Term cohorts: lower levels of the amino sugar and nucleotide
sugar metabolism, butanoate metabolism, riboflavin
metabolism, and amino-benzoate degradation
2015 (McMillan
et al.,
2015)
Pregnant women (n = 67) and non-pregnant
women (n = 64)
Vaginal fluid CST-I Similar with non-pregnant women :
Higher levels of succinate
CST-IV Similar with non-pregnant women :
1) Organic acid: higher levels of 2-hydroxyisovalerate,
g-hydroxybutyrate, 2-hydroxyglutarate and 2-
hydroxyisocaproate; lower levels of lactate
2) Higher levels of amines: tyramine, putrescine, and
cadaverine
3) Lower levels of amine precursors: tyrosine, lysine, ornithine
Preterm birth
2016 (Prince
et al.,
2016)
Women who delivered at term (n = 27), women
with spontaneous preterm birth (n = 44)
Placental
membranes
swabs
Lactobacillus
crispatus,
Acinetobacter
johnsonii
Preterm cohorts: higher levels of pentose phosphate pathway,
glycerophopholipid metabolism, and biosynthesis of the
siderophore group non-ribosomal peptides
2014 (Aagaard
et al.,
2014)
Women who delivered with preterm birth (n =
16), women with remote antenatal infection (n =
16), and controls (n = 16)
Placenta
tissue
Burkholderia spp. 1)Higher levels of methane metabolism, isoquinoline alkaloid
biosynthesis, and glycine/serine/threonine metabolism
2) Lower levels of biotin metabolism and
glycosylphosphatidylinositol anchor pathways
CST, Community state types.
Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 6094888
Serrano et al., 2019). Pregnant women with CST I as the
dominant bacteria have the most stable vaginal flora
throughout pregnancy, followed by those with CST V, CST II,
and CST IV (MacIntyre et al., 2015). The vaginal microbiota
during the third trimester is similar to that of non-pregnant
women. One week after delivery, estrogen decreases, and
glycogen-supported Lactobacillus spp. also decreases sharply.
The stability and compliance of the vaginal flora decreased
significantly, and the diversity increases, especially that of CST
IV, which leads to disorders of the postpartum vaginal flora and
even postpartum endometritis and puerperal morbidity
(DiGiulio et al., 2015;MacIntyre et al., 2015). There are also
microflora in the placenta and amniotic fluid of women who
experience normal-term delivery (Collado et al., 2016;Nuriel-
Ohayon et al., 2016). Some studies suggest that placental bacteria
may be derived from oral flora, mainly non-pathogenic
symbiotic flora such as Firmicutes,Tenericutes,Proteobacteria,
Bacteroidetes, and Fusobacteria phyla (Aagaard et al., 2014). The
origin of the placental microbiota is also related to the migration
of the intestinal flora to the fetus-placenta interface, which
promotes colonization of the fetus after birth (Collado et al.,
2016). However, some studies indicate that there are no
microorganisms in the normal placenta and amniotic fluid,
which may be caused by the contamination of laboratory
reagents or equipment or by different methods used to
obtaining specimens (Leiby et al., 2018;Lim et al., 2018;De
Goffau et al., 2019).
Analysis of the metabolic characteristics of the vaginal
microbiota revealed that the microbial metabolic activity in the
first trimester is the highest to adapt to changes in pregnancy
(Serrano et al., 2019)(Table 3). As pregnancy progresses, the
vaginal microbiota tends to become stable, and its metabolic
capacity tends to be simplified, mainly reflected in the low
activity of carbohydrate metabolism, cell wall/membrane
biochemical pathways, protein synthesis pathways, and nucleic
acid metabolism pathways (Serrano et al., 2019). Another study
also found that the carbohydrate metabolism and lipid
metabolism of cervicovaginal secretions in full-term women
were downregulated during the second and third trimesters
(Ghartey et al., 2015). Carbohydrate metabolism was
significantly downregulated, especially in the third trimester of
pregnancy, and was related to the large amount of glycogen
deposition and metabolizationintolacticacidinahighly
estrogen state (Ghartey et al., 2015). This change is conducive
to the colonization of lactobacilli in the host reproductive tract
and maintains the necessary acidic pH in the “healthy”
reproductive tract. It also helps maintain the integrity of the
cervix and is related to a reduction in adverse pregnancy
outcomes. In women who give birth at term, the lipid
metabolism of cervicovaginal secretions is significantly reduced
in the third trimester, which may be related to the acidic
environment inhibiting the growth of pathogenic bacteria, and
the antimicrobial component of cervicovaginal secretions,
methyl-4-hydroxybenzoate, increases by approximately 8.8
times from the second to the third trimester, helping maintain
a stable vaginal microenvironment (Ghartey et al., 2015). The
metabolic pathways of amniotic fluid and placental flora are
mainly involved in membrane transport, carbohydrate
metabolism, amino acid metabolism and energy metabolism,
which are closely related to the life activities of the fetal placenta
(Aagaard et al., 2014;Collado et al., 2016). Therefore, the
metabolic function of the genital tract flora during pregnancy
is of great significance for maintaining pregnancy stability.
In normal pregnancy, the mother has increased immune
tolerance to fetal-expressed paternal antigens through
“extended-self”antigens to maintain the growth of the fetus in
the body (Bromfield et al., 2017;Deshmukh and Way, 2019).
Maternal forkhead box P3 (FOXP3) Treg cells expand locally at
the maternal-fetal interface and expand systemically during
pregnancy to maintain allogeneic fetal tolerance (Agostinis et al.,
2019;Deshmukh and Way, 2019;Ghaemi et al., 2019). Metabolites
are closely related to host immunity during pregnancy. In humans,
the metabolism of L-arginine is related to the temporary
suppression of the maternal immune response during pregnancy
(Kropf et al., 2007). The activity of arginase expressed in the full-
term placenta of pregnant women increases significantly, and the
high enzyme activity leads to a decrease in its substrate L-arginine,
which in turn induces the downregulation of T-cell receptor
(TCR) associated z-chain (CD3z) and the hyporesponsiveness of
functional T cells (Ismail, 2018). IDO also uses a similar approach
to silence T cells to induce and maintain immune tolerance (Kropf
et al., 2007). The normal reproductive tract flora plays an
important role in the establishment and consolidation of
mother-placental-fetal immunity to resist the interference of
external pathogenic bacteria (Mei et al., 2019). Studies have
demonstrated a correlation between Bacteroides species and
TCRgd+ T cells (participating in mucosal immunity) (Ghaemi
et al., 2019). The microbiota can induce the accumulation of Treg
cells, which are essential for maintaining immune tolerance, timely
endometrial receptivity, and correct placental implantation
(Benner et al., 2018). However, dysbiosis in the reproductive
genital tract can lead to immune disorders and participate in the
occurrence of adverse pregnancy outcomes (Smith and Ravel,
2017). Therefore, a comprehensive interpretation of the
reproductive tract microbes, metabolism, and immunity during
normal pregnancy provides a reference for discovering the causes
and mechanisms of adverse pregnancy outcomes (Wang
et al., 2016).
PREGNANCY-RELATED ADVERSE
OUTCOMES
Spontaneous Abortion and Infertility
Statistics from the Centers for Disease Control in the United
States showed that among married women aged 15–44 years, 6%
hadinfertilityand12%hadimpairedfecundity,andthe
incidence increased yearly (Prevention, 2019). RTI is a risk
factor leading to reproductive dysfunction (such as infertility,
miscarriage, and repeated fertility failures), which in turn leads to
a clinical pregnancy rate of only 29.7–43.3% with embryo
Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 6094889
transfer technology (Baker et al., 2010;Franasiak et al., 2016;
Koedooder et al., 2019). For example, the prevalence of BV in
infertile women is 19–28%, while the clinical pregnancy success
rate is only 8% (Haahr et al., 2016;Bracewell-Milnes et al., 2018).
This observation may be related to abnormal vaginal microbiota
and pelvic pathogens (such as C. trachomatis) ascending to the
upper genital tract through the cervix, leading to PID and
reduced fertility (Witkin et al., 1995;Franasiak et al., 2016;
Haahr et al., 2016).
The normal reproductive tract flora provides a favorable
environment for embryo implantation, which can increase the
success rate and live birth rate of in vitro fertilization-embryo
transfer (IVFET) (Hyman et al., 2012;Sirota et al., 2014;
Koedooder et al., 2019). The genital tract microbiota in females
with fertility disorders is mainly manifested as a decrease in
Lactobacillus spp., an increase in non-Lactobacillus spp., a high
concentration of Candida spp., and an increase in the prevalence
of asymptomatic BV (Franasiak et al., 2016;Babu et al., 2017;
Campisciano et al., 2017;Wee et al., 2018;Koedooder et al.,
2019). The live birth rate will be reduced if harmful bacteria, such
as Gardnerella vaginalis,Atopobium vaginae,Acidovorax spp.,
Enterococcus spp., and Streptococcus spp., are found in the lower
reproductive tract (Hyman et al., 2012;Haahr et al., 2016;Wee
et al., 2018;Koedooder et al., 2019). Studies have shown that
before spontaneous abortion, endometrial aspiration fluid has
higher bacterial diversity and lower Lactobacillus abundance
than before a healthy pregnancy (Moreno et al., 2020). The
presence of CST IV microbiota in the endometrium is related to
asignificant reduction in the incidence of implantation,
pregnancy, and continuous pregnancy (Moreno et al., 2016). In
2016, Verstraelen et al. (2016) performed endometrial biopsy on
19 women with fertility disorders (infertility, repeated
implantation failures, and repeated miscarriages) and found
that 90% of women with fertility disorders mainly had
Bacteroides phylum as the dominant bacteria in their
endometrium. In addition, when Gardnerella and Streptococcus
genra are detected in the endometrium, they have a particularly
adverse effect on reproductive outcomes (Moreno et al., 2016).
The endometrium microbiota may be carried by sperm and affect
the microbial composition of the female reproductive tract
(Koedooder et al., 2019). For example, when the detection rate
of Mycoplasma hominis, Neisseria genus, Klebsiella genus and
Pseudomonas genus in semen increases, it is not only related to a
low sperm concentration, abnormal sperm morphology, high
semen viscosity, and oligospermia but also indirectly leads to a
decline in female fertility (Ahmadi et al., 2017;Monteiro et al.,
2018). Microorganisms also colonize in the follicular fluid, and
the low success rate of embryo transfer is related to the
colonization of Propionibacterium spp. and Streptococcus spp.
in the follicular fluid (Pelzer et al., 2013). Therefore, the normal
genital tract flora is of great significance to the maintenance
of fertility.
Endometrial receptivity and follicle quality in people with
reproductive disorders are closely related to metabolites in the
reproductive tract. Lipid homeostasis is essential for maintaining
health (Braga et al., 2019;Hernandez-Vargas et al., 2020). In
2019, Braga et al. (2019) analyzed the lipid metabolism of the
endometrial secretions taken from patients with IVFET cycles
before transplantation and found that phosphoethanolamine,
phosphatidic acid, diacylglycerol, triacylglycerol, glycosyl
diacylglycerol, phosphatidylcholine, neutral sphingolipid,
and lysophosphatidylglycerol are possible biomarkers of
endometrial receptivity and are associated with implantation
failure (Altmäe et al., 2014). Follicular fluid is the
microenvironment for the growth of oocytes, and the
metabolism of follicular fluid indirectly affects the growth
and development of oocytes (Bracewell-Milnes et al., 2017). In
2019, Song et al. (2019) conducted a targeted metabolomics
analysis of the follicular fluid of patients with recurrent
spontaneous abortion after IVFET treatment and found that
eight metabolites, namely, dehydroepiandrosterone,
lysophosphatidylcholine (lysoPC) (16:0), lysoPC(18:2), lysoPC
(18:1), lysoPC(18:0), lysoPC(20:5), lysoPC(20:4), and lysoPC
(20:3) were upregulated in the recurrent abortion group, and
10 metabolites, namely, phenylalanine, linoleate, oleic acid,
docosahexaenoic acid, lithocholic acid, 25-hydroxyvitamin D3,
hydroxycholesterol, 13-hydroxy-alpha-tocopherol, leucine, and
tryptophan were downregulated. The above indicators can also
predict the success rate of transplantation. Therefore, it is very
meaningful to analyze metabolites in people with reproductive
disorders. However the metabonomic analysis of cervicovaginal
secretions in women with fertility disorders still needs more
research to fully prove the role of metabolism in fertility disorders
and the interaction between immunity and the flora.
Fertility dysfunction may be related to the destruction of
immune tolerance caused by a decline in the number and
function of Treg cells (Deshmukh and Way, 2019).Through
endometrial biopsies of women with infertility in the mid-
secretory phase of the menstrual cycle, it was found that the
expression of Foxp3 mRNA was reduced, suggesting that the
differentiation of uterine T cells into a Treg cell phenotype is
impaired, which may lead to reduced endometrial receptivity
(Jasper et al., 2006). Moreover, immunoglobulin-like transcript 4
+ (ILT4+) DCs may be involved in the process of recurrent
miscarriage and recurrent implantation failure induced by Foxp3
+ Treg cells (Liu et al., 2018). A reduction in maternal Treg cell
inhibitory ability caused by microbial infection can also cause
placental inflammation, leading to the release and activation of
fetal-specific maternal CD8+ T cells, which infiltrate the
decidua and lead to abortion (Deshmukh and Way, 2019). The
microbiota is important for basic CCL2 (monocyte chemotactic
protein-1, MCP-1) secretion to control the homeostasis of
plasmacytoid DCs, macrophage recruitment and polarization,
and local T cell balance (Sierra-Filardi et al., 2014;Swiecki et al.,
2017). DCs are a key regulator of immune tolerance during
pregnancy. Patients with elevated dehydroepiandrosterone and
dehydroepiandrosterone sulfate (DHEAS) in the follicular fluid
have DC damage, which can cause infertility or spontaneous
abortion by causing the abnormal immunity of oocytes or
embryos (Song et al., 2019). When combined with a bacterial
flora disorder, it may aggravate the dysfunction of DCs, and
reproductive dysfunction is likely. It is known that plasma
Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 60948810
tryptophan metabolism is closely related to abortion (Fei et al.,
2016). While BV-related bacteria are involved in the metabolism
of tryptophan and a variety of amino acids (Srinivasan et al.,
2015), it is necessary to further study whether genital tract
bacteria and their metabolites cause an imbalance of immune
tolerance, affect plasma metabolite levels and participate in the
occurrence of reproductive dysfunction. Future research should
focus on the local immune effects of genital tract flora and
metabolites on people with reproductive dysfunction as the
main research direction to explore the impact of the three
interactions on reproductive disorders.
Preterm Birth
Every year, 15 million babies are born premature worldwide,
accounting for approximately 11% of the live birth population
(Blencowe et al., 2012). Preterm birth caused by ascending
genitourinary tract infection accounts for 40–50% of all
preterm births (Goldenberg et al., 2008). CST-IV vaginal
microflora is closely related to premature delivery (DiGiulio
et al., 2015;Dunlop et al., 2015;Workowski and Bolan, 2015;
Brown et al., 2018;Han et al., 2019). Additionally, studies have
confirmed that pregnant women with BV have an increased risk
of premature birth (Callahan et al., 2017;Anahtar et al., 2018;
Chu et al., 2018;Fettweis et al., 2019;Serrano et al., 2019).
Preterm birth is the second leading cause of neonatal death (Liu
et al., 2016). Premature babies are prone to diabetes, chronic
inflammation and cardiovascular disease in the long term
(Snyers et al., 2020). Therefore, the prevention of premature
birth is the top priority of medical work.
The Human Microbiome Project Multi-Omic Microbiome
Study showed that L. crispatus decreased and that BVAB1,
Sneathia amnii, TM7-H1 (BVAB-TM7), and partial Prevotella
species increased in the first and second trimesters of women
who deliver prematurely, thus, these factors can be used as
markers for predicting preterm birth (Fettweis et al., 2019).
Studies have also shown that the colonization of vaginal
Streptococcus agalactiae and Klebsiella pneumonia in the
second trimester is significantly associated with late
miscarriage and very premature delivery (before 28 weeks)
(Son et al., 2018;Koedooder et al., 2019). Changes in the
cervicovaginal flora of women who deliver prematurely greatly
alter the metabolome and are involved in premature cervical
remodeling (Table 3). Ghartey et al. (2015;2017) found that
women with symptoms of preterm birth and eventually
spontaneous preterm birth (sPTB) have significant changes in
cervicovaginal metabolites. Lipid metabolism and carbohydrate
metabolism in the cervicovaginal secretions of women who
deliver prematurely are significantly upregulated, and peptide
levels are significantly reduced (Ghartey et al., 2015;Ghartey
et al., 2017). Upregulation of lipid and carbohydrate pathways is
associated with positive energy utilization and may be related to
early cervical remodeling and sPTB microbiota utilization
(Ghartey et al., 2015;Ghartey et al., 2017). A decrease in
dipeptides may reflect the decreased level of proteolysis
in women who deliver prematurely and changes in the
activities of proteases and are associated with asymptomatic
sPTB (Ghartey et al., 2015). In addition, the level of N-
acetylneuraminate in the cervix of women who deliver
prematurely increased significantly (by 4.9 times), which may
be related to the increased affinity of cells for infection and
participate in host immunity (Ghartey et al., 2015).
Embryo development and growth depend to a large extent on
placental function, and the placental microbiome may affect fetal
and pregnancy outcomes. Chorioamnionitis and intrauterine
infection are closely related to premature delivery (Chu et al.,
2018). However, the specific source of infection may be the
ascending infection of BV bacteria (Fettweis et al., 2019),
the ascending carrying of sperm (Svenstrup et al., 2003), the
colonization of endometrial bacteria (Cowling et al., 1992;Chu
et al., 2018), the retrograde infection of salpingitis, and the blood-
borne infection of oral bacteria (Chu et al., 2018). Studies have
shown that the bacteria in the uterus of women who deliver
prematurely are mainly derived from vaginal bacteria, such as
Burkholderia taxa, which is significantly enriched in the placenta
(Goldenberg et al., 2000;Aagaard et al., 2014). The metabolic
enrichment of the lipopolysaccharide biosynthetic pathway of
the microbiota in the placenta may be related to the expansion
and reproduction of the microbiota (Aagaard et al., 2014). The
premature birth rate of women with bacteria detected in
amniotic fluid is higher, and the metabolomics of the amniotic
fluid of women who deliver prematurely are significantly altered,
contributing to the initiation of preterm birth (Menon et al.,
2014;Collado et al., 2016). Studies have shown that there are
flora on the fetal membranes and that the composition of the
fetal membranes is closely related to the degree of inflammation
of chorioamnionitis (Prince et al., 2016). Analysis of the
metabolic function of the fetal membrane microbiome revealed
that a reduction in the pentose phosphate pathway and
glycerophospholipid metabolism is related to chorioamnionitis,
and a reduction in glycerophosopholipid metabolism will lead to
an increase in the production of arachidonic acid, which is
related to inflammation and prostanoid synthesis and is
involved in premature birth (Prince et al., 2016).
Inflammation and antimicrobial peptide reactions involved in
certain vaginal microorganisms play a role in destroying and
invading cervical mucus plugs or amniotic membranes and
ultimately trigger proinflammatory reactions, leading to
premature delivery (Goldenberg et al., 2000;Yarbrough et al.,
2015;Smith and Ravel, 2017;Strauss et al., 2018). It is known
that Gardnerella vaginalis ascends to infect the amniotic membrane
and irritate the cervix, leading to premature delivery. Gardnerella
vaginalis may activate the NACHT, LRR and PYD domains-
containing protein 3 (NLRP3) inflammasomes through monocyte
NLRs; then, NLRP3 binds to and cleaves caspase-1, induces IL-1b,
IL-18, and TNF-asecretion, and ultimately leads to premature
delivery (Vick et al., 2014). Metabolites are involved in the
occurrence and development of preterm labor. When L. iners and
BV-related microorganisms are increased in the vagina, the ratio of
D-type/L-type lactic acid decreases, and matrix metalloproteinase
(MMP-8) increases (Witkin et al., 2013;MacIntyre et al., 2015). This
process eventually leads to premature cervix maturation and
ascending infection of the amniotic membrane and thus,
Li et al. The Female Genital Tract Microenvironment
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premature delivery (Yoon et al., 2001). Four proinflammatory
cytokines, eotaxin, IL-1b,IL-6andMIP-1b, are increased
significantly in the vagina of women who deliver prematurely
(Fettweis et al., 2019). BVAB1, Sneathia amnii,TM7-H1,
Prevotella timonensis,andPrevotella buccalis are closely related to
the levels of cytokines in vaginal secretions (Fettweis et al., 2019).
Both BVAB1 and TM7-H1 can produce pyruvate, acetate, L-lactate
and propionate. These SCFAs reduce antimicrobial activity and
promote the production of host proinflammatory cytokines and are
also involved in the occurrence of preterm labor (Fettweis et al.,
2019). Therefore, there is a close correlation between the microbiota,
metabolites and host immune status in reproductive tracts of
women who deliver prematurely, and future researchers need to
study the relevant pathogenesis. However, the relationship of
microbial metabolites in the reproductive tract, preterm delivery
immunity and the onset of preterm delivery still needs
further exploration.
GYNECOLOGICAL ONCOLOGY
Dysbiosisisrelatedtotumorcarcinogenicity(Ilhan et al., 2019;Scott
et al., 2019). An imbalance in specific microorganisms can lead to
host epithelial barrier dysfunction, genome integration,
genotoxicity, inflammatory activation, immune abnormalities and
metabolic abnormalities, creating a microenvironment that allows
tumor growth and further leading to the occurrence, development
and/or transfer of gynecological malignancies (Scott et al., 2019;
Laniewski et al., 2020). Among them, inflammation is the central
feature of carcinogenesis and the main carcinogenic mechanism
related to cancer (Scott et al., 2019). Microbial virulence factors can
induce chronic inflammation in host tissues, stimulate cell
proliferation, cause cell proliferation disorders, and combine with
the failure of cell apoptosis to ultimately lead to a malignant
phenotype (Scott et al., 2019;Laniewski et al., 2020). Metabolic
changes in cancer are the core of tumorigenesis and phenotypic
changes (Buckendahl et al., 2011;Zhang et al., 2012;Turkoglu et al.,
2016;Hopkins and Meier, 2017;Icard et al., 2018). Human
microorganisms are also involved in the formation of
carcinogenic metabolites and even exert genotoxicity to cause host
deoxyribonucleic acid (DNA) damage and participate in tumor
carcinogenicity (Kassie et al., 2001;Scott et al., 2019). Flora disorders
can also destroy the host-based anticancer immune monitoring to
promote tumor development and progression (Di Pietro et al., 2018;
Klein et al., 2019;Scott et al., 2019;Laniewski et al., 2020). Therefore,
the balance of the microenvironment of the reproductive tract has a
positive effect on maintaining the stability of the microbiota and
antitumor effects.
Cervical Intraepithelial Lesions and
Cervical Cancer
Cervical cancer (CC) is the most common human papillomavirus
(HPV)-related malignant tumor and the fourth most common
malignant tumor in women worldwide. In 2018, there were an
estimated 570,000 new cases and 311,000 deaths from this disease
(Bray et al., 2018). Approximately 85-90% of high-risk HPV
infections can be cleared spontaneously, and only 10-15% that
persist lead to cervical intraepithelial neoplasia (CIN) and invasive
cervical cancer (ICC). HPV-16 and HPV-18 are the main pathogens
of CC (Chase et al., 2015). The surface of the cervical mucosa is
susceptible to environmental influences. When dysbiosis occurs, the
local cervicovaginal microenvironment may promote the
progression of malignant tumors together with HPV (Ma et al.,
2014;Laniewski et al., 2018;Chorna et al., 2020).
A decrease in Lactobacillus spp. and an increase in vaginal pH
are closely related to HPV infection, cervical lesions and CC
(Laniewski et al., 2018;Ilhan et al., 2019;Laniewski et al., 2020).
Cervical squamous intraepithelial lesions or CC also increase the
TABLE 4 | Current existing articles analyzing the correlation between the genital tract flora and metabolites in women with cervical intraepithelial lesions and cervical
cancer.
Year Author Population Sample CST Metabolites
2020 (Borgogna
et al.,
2020)
HPV-negative participants (n = 13) and HPV-positive
participants (n = 26)
Vaginal swabs CST-I Higher concentrations of histamine, 3-n-acetyl-LL-cysteine-
S-yl acetaminophen, and gammaaminobutyrate
CST-III High levels of 3-n-acetyl-L-cysteine-S-yl acetaminophen
CST-IV Low levels of heme, glycerophosphorylcholine, and
oxidized glutathione
2019 (Ilhan
et al.,
2019)
78 premenopausal, non-pregnant women and grouped as
follows: healthy HPV-negative (n = 18) and HPV-positive
participants (n = 11), low-grade squamous intraepithelial
lesions (n = 12), high-grade squamous intraepithelial lesions
(n = 27) and invasive cervical carcinoma (n = 10)
Cervicovaginal
lavages and
vaginal swabs
CST-IV 1) Higher levels of cadaverine, putrescine, tyramine,
tryptamine, agmatine, and glutathione synthesis
intermediate, 2-hydroxybutyrate, branched chain amino
acid metabolism product, alpha-hydroxy-isovalerate, and
L-isoleucine metabolism product, 2-hydroxy-3-methyl-
valerate
2) Lower levels of nucleotides adenosine and cytosine and
xenobiotics such as 2-keto-3-deoxy-gluconate and 1,2,3-
benzenetriol
2019 (Kwon
et al.,
2019)
Normal women (n = 18), cervical intraepithelial neoplasia two
or three patients (n = 17), and cervical cancer patients
(n = 12)
Cervical swabs CST-IV 1) Cervical cancer patients: enriched in peptidoglycan
biosynthesis (ko00550) pathway
2) Cervical intraeptithelia neoplasia 2/3 patients: enriched in
ko00300 (lysine biosynthesis), ko00680 (methane
metabolism), and ko05211 (renal cell carcinoma)
CST, Community state types; HPV, human papillomavirus.
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diversity of the vaginal flora, limited not only to BV-related
microorganisms but also to non-BV bacteria, such as
Streptococcus agalactiae,Clostridium spp., Pseudomonadales order,
and Staphylococcus spp. (Klein et al., 2019;Laniewski et al., 2020).
An increase in the CIN stage is also related to an increase in the
diversity of the vaginal microbiota, suggesting that microorganisms
play a role in the regulation of persistent viral infection and disease
progression (Van Ostade et al., 2018). Compared with patients with
low-grade squamous intraepithelial lesions (LSILs), Sneathia
sanguinegens,Anaerococcus tetradius and Peptostreptococcus
anaerobius in patients with high-grade squamous intraepithelial
lesions (HSILs) are more enriched in the vagina, and
Mycoplasmatales order, Pseudomonadales order, and
Staphylococcus spp.aremoreenrichedinthecervix(Mitra et al.,
2015;Klein et al., 2019;Laniewski et al., 2020). Another study found
that Sneathia spp. and Fusobacterium spp. exist only in women with
cervical lesions or cancer but not in women without lesions
(Audirac-Chalifour et al., 2016). Therefore, the presence of
Sneathia in the vaginal microbiome may be a characteristic
microorganism of cervical lesions and CC (Laniewski et al., 2018;
Laniewski et al., 2020). Analysis of the metabolic pathways of the
bacterial flora in patients with CC showed that the peptidoglycan
biosynthesis (ko00550) pathway is significantly enriched (Kwon
et al., 2019). Research has also found that the bacterial cell wall
peptidoglycan is not only essential for maintaining the overall
antiosmotic pressure of the bacteria to ensure cell survival but
also participates in the occurrence of inflammation, affecting the
function of host neutrophils and the innate immune response.
Therefore, cervical microbes may promote the development of CC
and precancerous lesions by acting as a modulators of host
inflammatory pathways.
The vaginal metabolism characteristics of HPV-infected and
uninfected patients are different, and the vaginal CST status drives
the metabolic characteristics of HPV-infected patients (Borgogna
et al., 2020)(Table 4). In vaginal CST III, HPV-infected women
have higher levels of biogenic amines than HPV uninfected women
(Borgogna et al., 2020). In CST IV, HPV-infected women have
lower concentrations of glutathione (GSH), oxidized glutathione
(GSSG), glycogen, and phospholipid-related metabolites than
uninfected women. There are also differences in the metabolic
characteristics of HPV infection, cervical lesions, and CC. Several
researchers conducted a study on the metabolome of cervicovaginal
secretions in HPV-mediated cervical tumors and found that
compared with HPV-negative group, HPV-positive group, and
cervical lesion group, the number and diversity of cervical vaginal
metabolites in CC patients were increased (Ilhan et al., 2019).
ComparedwiththeHPV-negativegroup,theHPV-positive,LSIL
and HSIL groups had fewer amino acids, and their metabolites in
cervical and vaginal secretions and the subpathways and depletion
levels under the amino acid superpathway were different (Ilhan
et al., 2019;Laniewski et al., 2020). In addition, in the vulvovaginal
secretions of patients with CC, volatile organic compounds, such as
alkanes, and methylated alkanes are different from those of healthy
women, which may be related to the oxidation of cell membrane
lipids and proteins during the carcinogenic process and the
production of volatile organic compounds (Rodriguez-Esquivel
et al., 2018;Ilhan et al., 2019). These metabolites can be used as
potential biomarkers for CC. In addition, the unique metabolic
characteristics in the cervicovaginal microenvironment can assist in
the diagnosis and differentiation of health, HPV infection
(Borgogna et al., 2020), HSILs, LSILs, and CC (Ilhan et al., 2019).
For example, long chain fatty acids, ketone bodies, steroids,
ceramides, and plasmalogens can distinguish individuals with ICC
from those with HPV (−).
The interaction between the host and reproductive tract
microorganisms forms a metabolic network that participates in
the formation of the local tumor environment during the process
of continuous HPV infection and cancer progression (Ilhan et al.,
2019). In HSILs and CC, the vaginal microbial community
disrupts amino acid and nucleotide metabolism in a manner
similar to that in BV (Ilhan et al., 2019). Compared with healthy
individuals, the abundance of lipid metabolites in the vaginas of
ICC patients is higher (Ilhan et al., 2019;Szewczyk et al., 2019).
This phenomenon may be related to the interaction between
microbes and the host, which activates the carcinogenic
pathways in the tumor microenvironment and increases cell
proliferation and cell membrane synthesis, thus enhancing
the carcinogenic activity of the microflora. Changes in the
cervicovaginal microbial community cannot only change the
cervicovaginal metabolome but also further affect immunity
and participate in cancer progression (Ilhan et al., 2019). For
example, glycochenodeoxycholate (GCDC) is a key metabolite of
host-Lactobacillus cometabolism and can inhibit vaginal flora
disorders (Ilhan et al., 2019). A decrease in GCDC and
Lactobacillus species in CC patients leads to a decrease in the
ability to induce inflammation and toxic reactions and further
leads to a weakened antitumor effect.
The interaction between immunity and metabolites forms a
special tumor microenvironment. In the CIN group, the
concentrations of IL-8, IL-10, and nitric oxide in cervicovaginal
secretions were higher than those in the control group, indicating
that these mediators play a role in the tumor immune
microenvironment (Tavares-Murta et al., 2008). Since IL-8 is a
Th1-type cytokine and has a proinflammatory effect and IL-10 is a
Th2-type cytokine and has an anti-inflammatory effect (Fernandes
et al., 2015), the interaction mechanism between nitric oxide and the
two needs to be further studied. ICC patients with high genital
inflammation (high IL-1a,IL-1b,IL-8,MIP-1b,CCL20,regulation
on activation normal T-cell expressed and secreted (RANTES), and
TNFaexpression) had the strongest correlation with lipids. An
increase in plasmalogens and long chain polyunsaturated fatty acids
in ICC not only indicates abnormal cell metabolism but also has a
proinflammatory cytokine precursors effect, inducing abnormal
gene expression and disordered cytokine production (Ilhan et al.,
2019). Metabolites are also closely related to CC progression and
tumor cell growth. CC is characterized by an immunosuppressive
microenvironment and Th2-type cytokines (Bedoya et al., 2014). In
females with CC, Th2-type cytokines (IL-10 and IL-13) induce the
expression of arginase (ASE), which converts L-arginine into L-
ornithine and polyamines, and a reduction in L-arginine is related
to the downregulation of the immune response, further promoting
tumor progression (Bedoya et al., 2014). Therefore, an increase in
Li et al. The Female Genital Tract Microenvironment
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polyamines in the vagina flora of CST IV HPV-positive patients is a
metabolic feature that HPV uses to escape host immunity and
promote tumor progression (Borgogna et al., 2020). More research
is needed to support the impact of HPV infection, cervical lesions,
and the direct mechanism of action between the bacterial flora,
metabolites and immunity in the cervicovaginal secretions of
patients with CC on tumor progression and tumor metastasis.
Endometrial Cancer
In 2018, there were an estimated 382,069 new cases and 89,929
deaths related to corpus uteri cancer (Bray et al., 2018).
Endometrial cancer (EC) is a perimenopausal and
postmenopausal tumor, divided into two categories: type I and
type II (Troisi et al., 2018). Type I EC is the most common (Troisi
et al., 2018). Environmental factors, including obesity,
inflammation, postmenopausal estrogen metabolism imbalance
and estrogen therapy, are themainriskfactorsforthe
development of type I EC (Laniewski et al., 2020). Type II EC is
rare and is mainly related to endometrial atrophy (Troisi et al.,
2018). Environmental factors are related to changes in the
intestinal and vaginal microbiomes. The close relationship
between the flora, estrogen metabolism and obesity indicates the
potential role of the microbiome in the etiology of EC (Laniewski
et al., 2020). EC is also closely related to PID, and an imbalance in
the vaginal flora can cause PID through ascending infection, so an
imbalance in the vaginal flora may be indirectly related to EC
(Ness et al., 2005;Yang et al., 2015;Champer et al., 2018).
The reproductive tract microbiota is involved in the
pathogenesis of EC (Laniewski et al., 2020). In 2016, Walther-
Antonio et al. (2016) analyzed the genital tract flora of 17
patients with EC, four with endometrial hyperplasia and 10
with benign uterine diseases and found that in the EC cohort,
Porphyromonas sp. was common in the vagina and cervix,
Bacteroides and Faecalibacterium sp.werecommoninthe
endometrium, and Bacteroides sp.was common in the ovary.
The endometrial microbiota of the EC and hyperplasia cohorts
was similar but differed to some degree from that of the benign
cohort. The endometrial hyperplasia and benign cohorts had
different microbiota structures, indicating that the microbiota
plays a role in the early stages of cell transformation (Walther-
Antonio et al., 2016). EC patients usually have a high vaginal pH.
The detection of vaginal Atopobium vaginae and Porphyromonas
sp. combined with a high vaginal pH is statistically correlated
with EC (Walther-Antonio et al., 2016). Studies have found that
Atopobium vaginae can induce proinflammatory cytokines and
antimicrobial peptides, cause chronic inflammation and local
immune disorders, promote Porphyromonas sp. infection in
cells, destroy normal cell regulatory functions, and ultimately
lead to carcinogenic processes (Walther-Antonio et al., 2016;
Laniewski et al., 2020). The link between Atopobium vaginae and
Porphyromonas sp. supports the link between BV-related
bacteria, immunity and EC (Laniewski et al., 2020).
EC has a unique endometrial metabolic signature. In 2017,
Altadill et al. (2017) studied the metabolomics of endometrial
tissue samples from 39 EC patients and 17 healthy women and
found lipids, kynurenine, endocannabinoids and RNA editing
pathway disorders in EC patients. Through further research on
RNA editing pathways, we found that adenosine deaminases
acting on RNA2 (ADAR2) are overexpressed in EC and are
positively correlated with tumor aggressiveness. ADAR2 may
contribute to the carcinogenicity of EC and can be used as a
potential marker for EC treatment. However, whether the genital
tract flora metabolites involved in EC carcinogenesis remains to
be studied. It is known that the concentration of hydroxybutyric
acid in the reproductive tract of BV patients is elevated. In the
intestine, SCFAs (such as hydroxybutyrate) induce Treg cells
through histone deacetylases (HDACs) and exert an
immunosuppressive effect in innate immune cells (McMillan
et al., 2015;Chen and Stappenbeck, 2019). Whether
hydroxybutyrate in the reproductive tract induces immune
suppression through HDACs and promotes the growth of
endometrial tumors requires further in vitro experiments.
Ovarian Cancer
Ovarian cancer (OC) is one of the deadliest malignant tumors in
women and the main cause of death from gynecological
malignancies (Turkoglu et al., 2016;Laniewski et al., 2020). In
2018, an estimated 295,414 new cases of OC were diagnosed
worldwide, and 184,799 women died from the disease, ranking
fifth among cancer-related deaths (Bray et al., 2018). More than 80%
of patients have advanced disease, and the five-year overall survival
rate is between 15 and 45% (Turkoglu et al., 2016). Similar to EC,
chronic infection of sexually transmitted pathogens and ascending
infection of genital tract inflammation are related to the occurrence
of OC (Shanmughapriya et al., 2012;Idahl et al., 2020).
In 2019, Nene et al. (2019) first published a study on the
presence of abnormal uterine flora in women with OC or at risk
of OC and found a strong correlation of OC or the breast cancer
susceptibility gene 1 (BRCA1) mutation status with participants
aged <50 years and those with a non-Lactobacillus dominant
microbiota. Women who have used oral contraceptive pills or
combined hormones for more than 5 years are more likely to
have Lactobacillus dominance and a lower risk of OC than
women who are using oral contraceptive pills or women who
have used combined hormones for less than 5 years. Compared
with the healthy surrounding ovarian tissue of the same
individual, OC tissue has unique microbial characteristics
(Banerjee et al., 2017). Potentially pathogenic intracellular
microorganisms, such as Brucella spp., Chlamydia spp. and
Mycoplasma spp., are present in 60–76% of ovarian tumors
(Banerjee et al., 2017). In addition, an increase in
Proteobacteria and Firmicutes phyla in ovarian tumors,
especially an increase in Actinobacteria phyla, may cause
double-stranded breaks by releasing bacterial toxins (such as
colibactin and cytolethal distending toxin) and directly damage
cellular DNA (Banerjee et al., 2017;Nene et al., 2019). In
addition, several pathogenic viruses, intracellular bacteria,
fungi and parasites also exist in ovarian tissue (Banerjee et al.,
2017). These microorganisms may induce cancer through direct
or indirect mechanisms (Laniewski et al., 2020). It is known that
the integration of the HPV genome into the human genome is an
important reason for the development of CC. In 2017,
Li et al. The Female Genital Tract Microenvironment
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Banerjee et al. (2017) found HPV signals in the tumor tissues of
OC patients, and there was also an integration phenomenon. For
example, HPV16 has the largest number of viral integration sites
in human chromosomes. It can be integrated into various
intronic regions and genetic regions within 56 kb upstream of
many cancer-related human genes. In addition, the coding
sequence of the E1 gene of HPV18 is integrated in the intronic
region of the non-coding RNA gene of the host chromosomes.
All of the above factors may lead to the dysregulation of gene
expression and participate in the occurrence and development of
cancer (Banerjee et al., 2017). This research provides new ideas
for exploring the molecular mechanism of OC.
It is known that the metabolic characteristics of OC cells, OC
tissues, and ascites are significantly changed and are closely
related to tumor tissue energy utilization, reproductive tract
inflammation, the invasion and migration of OC cells, and the
chemotherapy resistance of OC (Fong et al., 2011;Poisson et al.,
2015). However, there are still few studies on the correlation
between the characteristics of microbial metabolism in the
reproductive tract and OC. The ovary is the end organ of the
upper genital tract and is affected by the ascending bacteria of
the genital tract, the bacteria of the ovary and the flora in the
abdominal cavity. Whether the metabolic characteristics of the
above bacteria are related to the occurrence and development of
ovarian tumors and the carcinogenicity of tumors, leading to a
unique tumor microenvironment, needs further research.
Metabolites are closely related to tumor immunity and tumor
development (Turkoglu et al., 2016;Clifford et al., 2018;
Szewczyk et al., 2019). Whether the metabolites of the genital
tract flora of OC patients participate in tumor immunity and
host antitumor immunity, which affects the occurrence,
development and metastasis of OC, still needs further research.
CONCLUSION
The host reproductive tract microenvironment includes
microorganisms, metabolites and immunity, and the balance of
the interactions among them is essential to maintain reproductive
health. Existing research on the relationship between female
reproductive tract microbes or immunity and reproductive tract
inflammation, pregnancy, and tumors is becoming increasingly
detailed. In contrast to research on intestinal metabolites, research
on female reproductive tract metabolites is still in the preliminary
stage. Moreover, the reproductive tract metabolic characteristics of
AV, reproductive dysfunction, EC, and OC still need more research
at present, as relevant data are lacking.Additionally,therelationship
between microbial metabolites and host immunity in inflammation,
pregnancy, and tumors of the female reproductive tract is relatively
unclear, and it may become a new direction for future research.
According to the current research, BV-related/CST IV bacteria and
the microenvironment formed by the reproductive tract have the
most comprehensive research on the adverse pregnancy outcomes
and tumor pathogenesis. The reproductive tract microenvironment
produced by BV/CST IV not only participates in the ascending
infection causing PID, which leads to an increased risk of
infertility, miscarriage, and premature delivery, but also
participates in the increased risks of precancerous lesions and
malignancy of the reproductive tract. The pathogenic mechanism
of adverse pregnancy outcomes and tumor diseases in the AV
microenvironment of the reproductive tract needs to be further
explored. Molecular detection technology combined with immune
and metabolomics can be used to better describe the function and
metabolic status of the flora, infer the possible pathogenic pathways
and immune response status, and analyze the complicated
relationship of the local microenvironment with inflammation,
pregnancy, and tumor diseases. This method is also the best way
to study the pathogenic mechanism and disease characteristics of
female reproductive tract inflammation, pregnancy, and
tumor diseases in the reproductive tract microenvironment. The
study of microorganisms, metabolites, and immunity in the
microenvironment of the reproductive tract under different
diseases (and then the development of targeted therapies for the
above three) was the main purpose of this article. At present, there
are many studies on the treatment of microorganisms with
antibiotics and probiotics. Studies have proven that probiotic
supplementation is very helpful in reducing the risk of
inflammation, adverse pregnancy outcomes, and cancers (Nene
et al., 2019;Laniewski et al., 2020) and improving the ability to
respond to cancer treatments and quality of life (Postler and Ghosh,
2017;Champer et al., 2018;Laniewski et al., 2020). However,
microbial therapy is still prone to relapse and associated with a
high risk of recurrence. Whether targeted therapy for metabolites
(Sevin et al., 2015) and immunity (Ventriglia et al., 2017;Deshmukh
and Way, 2019) can be used to treat diseases or improve the effect of
microbial therapy still needs further research. Therefore, studying
theroleandmechanismofreproductivetractflora, metabolites, and
immunity in disease pathogenesis will aid in disease diagnosis and
treatment and improve female reproductive health.
AUTHOR CONTRIBUTIONS
HL, CH, and FX conceived the study question, and all authors
were involved in the study design. HL created the first draft of the
manuscript.YZ,CW,HYL,andAFmadesubstantial
contributions to drafting the article and revising it
critically. All authors contributed to the article and approved
the submitted version.
FUNDING
This work was supported by Tianjin Municipal Science and
Technology Commission Special Foundation for Science and
Technology Major Projects in Control and Prevention of Major
Diseases (Grant No. 18ZXDBSY00200), General Project of the
National Natural Science Foundation of China (Grant No.
82071674) and Tianjin Health Science and Technology Project
(Grant No. KJ20003).
Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 60948815
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Conflict of Interest: The authors declare that the research was conducted in the
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potential conflict of interest.
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Li et al. The Female Genital Tract Microenvironment
Frontiers in Cellular and Infection Microbiology | www.frontiersin.org December 2020 | Volume 10 | Article 60948820
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