BJID 2006; 10 (August)247
Selective versus Non-selective Culture Medium for Group B Streptococcus Detection in
Pregnancies Complicated by Preterm Labor or Preterm-Premature Rupture of Membranes
Marcelo Luís Nomura1, Renato Passini Júnior1
Ulysses Moraes Oliveira2
1Department of Obstetrics and Gynecology; 2Department of
Clinical Pathology; Medical School, Campinas State University
(UNICAMP), São Paulo State, Brazil
The objective of this study was to identify group B streptococcus (GBS) colonization rates and
compare detection efficiency of selective versus non-selective culture media and anorectal versus
vaginal cultures in women with preterm labor and preterm-premature rupture of membranes
(PROM). A prospective cohort study of 203 women was performed. Two vaginal and two anorectal
samples from each woman were collected using sterile swabs. Two swabs (one anorectal and one
vaginal) were placed separately in Stuart transport media and cultured in blood-agar plates for 48
hours; the other two swabs were inoculated separately in Todd-Hewitt selective media for 24 hours
and then subcultured in blood-agar plates. Final GBS identification was made by the CAMP test. A
hundred thrity-two cultures out of 812 were positive. The maternal colonization rate was 27.6%.
Colonization rates were 30% for preterm PROM and 25.2% for preterm labor. Todd-Hewitt selective
medium detected 87.5% and non-selective medium 60.7% GBS-positive women. Vaginal samples
and anorectal samples had the same detection rate of 80.3%. Anorectal selective cultures detected
75% of carriers; 39% of GBS-positive women were detected only in selective medium. A combined
vaginal-anorectal selective culture is appropriate for GBS screening in this population, minimizing
Key Words: Streptococcus agalactiae, culture media, premature rupture of membranes, preterm labor.
Received on 12 April 2006; revised 26 July 2006.
Address for correspondence: Dr. Marcelo Luís Nomura. Rua
Alexander Fleming, 101, Cidade Universitária Zeferino Vaz – Área
de Obstetrícia - Centro de Atenção Integral a Saúde da Mulher -
Universidade Estadual de Campinas-Campinas, São Paulo, Zip code:
13084-881–Brazil. E-mail: email@example.com.
The Brazilian Journal of Infectious Diseases 2006;10(4):247-250.
© 2006 by The Brazilian Journal of Infectious Diseases and Contexto
Publishing. All rights reserved.
Streptococcus agalactiae, or group B streptococcus
(GBS), is an important infectious agent in perinatology; it is
the predominant bacteria in early-onset neonatal sepsis [1,2].
GBS can be acquired during labor or in utero by transmission
from maternal-vaginal or anorectal-colonized mucosa.
Prematurity is also a risk factor, and mortality is higher in
preterm than in term newborns .
Reported maternal colonization rates are quite variable,
but generally range from 20-30% [4,5]. The differences in
colonization rates depend on the particular population and
especially on the laboratory methods used to identify GBS.
Brazilian data are scarce [6,7], but are in general agreement
with literature reports from other countries.
Since maternal colonization at delivery is the main risk
factor for neonatal disease , microbiological techniques must
be designed in order to maximize detection rates. The use of
selective media containing antibiotics is reported to be both
sensitive and also the most adequate method for detection
. The anatomic site of sampling is also important, and
anorectal and vaginal cultures are recommended for detection
in pregnant women .
We analyzed culture methods for the detection of GBS in
pregnancies complicated by preterm labor (PTL) or preterm
premature rupture of membranes (PPROM), comparing Todd-
Hewitt selective medium with non-selective medium and
anorectal with vaginal samples.
Material and Methods
During the 12-month period from February 2003 to January
2004, all pregnant women (total 203) with preterm labor or
preterm premature rupture of membranes attended at the
Maternity Ward of the Campinas State University, located in
Campinas, in the state of São Paulo, Brazil were enrolled and
included in the study. All participating women signed an
informed consent. Socio-demographic, clinical and obstetrical
data were collected at the time of admission and retrospectively
from patient’s charts.
Samples were collected from the proximal third of the
vaginal introitus and from inside the anus through the anal
sphincter. Two vaginal and two anorectal swabs were obtained
from each woman. Two swabs (one vaginal and one anorectal)
were separately placed in two tubes containing Stuart
transport media and then cultured in non-selective 5% blood-
agar plates for 48 hours. The other two swabs were immediately
inoculated into two tubes containing Todd-Hewitt selective
broth medium (manufactured by PROBAC, Brazil), enriched
with gentamicin and nalidixic acid, incubated for 24 hours,
and then subcultured for 24 hours in non-selective 5% blood-
agar plates. The final identification of GBS in all samples was
made by the CAMP test, which is based on the production of
an arrow-shaped hemolysis zone in conjunction with a
Staphylococcus aureus colony in a blood-agar plate. Four
248 BJID 2006; 10 (August)
types of culture were then identified: selective anorectal, non-
selective anorectal, selective vaginal and non-selective
vaginal. A woman was considered colonized if any of the four
cultures were positive.
Detection rates for each type of culture medium and each
sampling site were calculated. Chi-square or Fischer’s exact
test were used to detect significant differences, and a P value
< 0.05 was considered statistically significant.
One hundred thirty two (16.2%) GBS-positive cultures were
obtained from the 812 specimens collected from the 203
pregnant women. Fifty-six women (27.6%) had at least one of
the cultures positive. Table 1 shows the clinical and obstetric
characteristics of the women enrolled in the study. White race
and low education level were significantly associated with
Colonization rates for women in preterm labor were 25.2%,
and for women with preterm premature rupture of membranes
it was 30%. Sixteen women (7.8%) had all of the four cultures
positive. Selective anorectal cultures were positive in 42
patients (20.7%) and selective vaginal cultures were positive
in 38 (18.7%). Non-selective anorectal cultures were positive
in 24 patients (11.8%) and non-selective vaginal cultures in 28
(13.8%). Table 2 shows the comparison of the selective versus
non-selective cultures. There was discordance in the results
for 29 women. Twenty-two (39)% of the colonized women
were detected only in the selective-media cultures.
Table 3 shows culture results by site of sampling.
Discordance between results was observed in 22 women.
However, the number of colonized women detected by each
of the cultures was the same (45 each, or 80.3% of all colonized
women). However, these 22 women were not the same in each
Selective media detected 87.5% of the 56 colonized women,
compared to 60.7% detected by non-selective cultures. There
was no difference in the detection rate between anorectal and
vaginal samples (80.3%).
Screening of group B streptococcus colonization is
recommended for all pregnant women between 35 and 37
weeks, and in situations of risk of preterm delivery, which are
preterm labor and preterm premature rupture of membranes
. Antibiotic prophylaxis of colonized women, during labor,
greatly reduces the risk of neonatal disease .
It has been reported that GBS isolation is 20% to 40%
greater when combined vaginal and anorectal cultures are
collected [8,10-12]. A significant proportion of women were
found to have only one of these sites colonized; this proportion
was 18% to 24% higher in anorectal compared to vaginal
samples [11,12]. In an analysis of 651 specimens, the
combination of anorectal and vaginal cultures reduced the
number of false-negative results, allowing detection of 97.8%
of GBS carriers . Since there are no published data
comparing the risk of neonatal disease by colonized anatomic
site, it is recommended that both vagina and rectum be cultured
In our study, 11 patients were detected only in vaginal
and another 11 only in anorectal cultures; the number of
carriers detected would have been the same if only one of
these sites was sampled. However, anorectal-selective medium
detected 75% of GBS carriers.
Todd-Hewitt selective medium enriched with gentamicin
and nalidixic acid inhibits growth of Gram-negative bacteria,
and it has greater sensitivity when compared to other non-
selective media, such as blood or Granada agar . Twenty-
two cases, or 39% of all colonized women in this study, were
detected only in the samples incubated in Todd-Hewitt
medium. The proportion of colonized women detected by the
selective medium was 87.5%, and 15 affected patients would
not have been detected if only non-selective medium had
been used; the detection rate would have fallen 30%. This
finding supports the current view that selective medium is
fundamental to maximize detection rates and should be
employed by laboratories involved in screening of pregnant
In the clinical scenario, which is where this screening was
done, a critical issue is the time needed to obtain reliable results
from the cultures, which is 48 hours when using the methods
recommended by CDC. Preterm labor and premature rupture
of membranes are high-risk situations for early-onset sepsis,
for which adequate intrapartum antibiotic prophylaxis is
necessary. Rapid identification tests would be more suitable,
since reliable and immediate results would be available,
avoiding unnecessary antibiotics prescription. The only FDA-
approved rapid test is a real-time PCR assay (IDI-Strep B®),
which has high sensitivity and specificity .
A study evaluating maternal colonization rates in 34 reports
from developing countries found a GBS-prevalence rate of
12.7%. However, when considering only those studies in which
adequate laboratory methods were used (use of selective
culture medium and collection of vaginal specimens), this rate
increases to 17.8%. Inappropriate microbiological methods
were used on almost half of the patients included in this
The economic costs of universal maternal screening must
be considered. We believe that given the enormous amount
of money spent in intensive care units to care for infected
newborns, and later to treat long-term disabilities caused by
neonatal disease, the cost-benefit ratio favors culture
screening. The Centers for Disease Control and Prevention
estimated that $300 million dollars were spent in a year to treat
almost 7,500 cases of early-onset GBS disease . There is
no published evaluation of this issue in developing countries.
In women with preterm labor, which had a prevalence of 25%
Culture Medium for Group B Streptococcus Detection in Pregnancies
BJID 2006; 10 (August) 249 Culture Medium for Group B Streptococcus Detection in Pregnancies
Table 2. Group B Streptococcus detection results by type of
Table 3. Group B Streptococcus culture results by anatomic
GBS-positive mothers, initiating penicillin treatment before
culture results would result in overtreatment of three out of
four patients. This combined strategy of culturing women at
risk while prescribing antibiotics has not been evaluated on a
cost/benefit basis, which involves not only culture and
antibiotic costs, but also pediatric care after birth. In our
country, penicillin is a low-cost antibiotic.
We found that selective culture medium yielded the
highest detection rates, with no difference between
anorectal and vaginal samples. The proposed combined
anorectal and vaginal swab directly inoculated into a tube
containing Todd-Hewitt medium would cost about two U.S.
dollars in Brazilian currency per patient. We believe that
this low cost would be feasible even for universal screening
at our maternity. This strategy was previously compared
through delayed inoculation after sampling in transport
media and gave better results, with enhanced detection
The use of Todd-Hewitt selective medium yielded greater
detection rates than non-selective medium. There was no
difference in detection rates between vaginal and anorectal
samples, however selective anorectal cultures detected more
colonized women than the other three types of cultures.
From a practical standpoint, a single combined vaginal-
anorectal sample incubated in selective medium would be
appropriate. This strategy would minimize laboratory costs
without compromising maximum detection capacity, which is
crucial for the prevention of early-onset neonatal disease.
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Table 1. Clinical and obstetric characteristics and maternal colonization by group B Streptococcus
Low level of education*
≥ 32 weeks
Mean birth weight (g)
1.38 (0.87–2.18) 0.178
1.44 (0.88–2.36) 0.134
RR = Risk Ratio CI = Confidence Interval; PPROM = Preterm premature rupture of membranes; PTL = Preterm labor;
SD = Standard deviation; *less than elementary school completed.
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