R E S E A R C H A R T I C L E Open Access
Antimicrobial effects of mustard oil-
containing plants against oral pathogens:
an in vitro study
, Anne Schüller
, Klaus Biehler
, Ali Al-Ahmad
and Uwe Frank
Background: The present study examines the antimicrobial activity of nasturtium herb (Tropaeoli maji herba) and
horseradish root (Armoraciae rusticanae radix) against clinically important oral bacterial pathogens involved in
periodontitis, gingivitis, pulpitis, implantitis and other infectious diseases.
Methods: A total of 15 oral pathogens, including members of the genera Campylobacter, Fusobacterium, Prevotella,
Parvimonas, Porphyromonas, Tanerella, Veillonella, and HACEK organisms, were exposed to  a combination of
herbal nasturtium and horseradish using a standardized gas test and  a mixture of synthetic Isothiocyantes (ITCs)
using an agardilution test. Headspace gas chromatography mass spectrometry was employed to quantify the
amount of allyl-, benzyl-, and 2- phenyl- ethyl-ITC.
Results: With exception of Veillonella parvula, all tested species were highly susceptible to herbal nasturtium and
horseradish in the gas test with minimal inhibitory concentrations (MICs) between 50/20 mg and 200/80 mg and to
synthetic ITCs in the agardilution with MICs between 0.0025 and 0.08 mg ITC/mL, respectively. Minimal bactericidal
concentrations extended from 0.005 mg ITC/mL to 0.34 mg ITC/mL.
Conclusions: ITCs may be considered an interesting alternative to antibiotics for prevention and treatment of
oropharyngeal infections, periodontitis and related diseases. Furthermore, the suitability of ITCs for endocarditis
prophylaxis in dental procedures might be worth further investigation.
Keywords: Periodontitis, Isothiocyanates, Mustard oils, Endocarditis prophylaxis, Horseradish, Nasturtium
With increasing spread of antibiotic-resistant pathogens,
and better understanding of the effects of antibiotics on
the microbiota, alternatives to antibiotics must be con-
sidered for therapy and prevention.
The discovery in 1928 of penicillin by Alexander Flem-
ing heralded the golden era of antibiotics, which, lasting
until the late 1960s, saw the development of different
novel antibiotic classes [1,2]. Many bacterial pathogens,
however, developed resistance to most of these antibi-
otics, and the paucity in the development of new antibi-
otics from the 1970s to the present day threatens a
return to the preantibiotic era [1–4]. Additionally, resist-
ance to disinfectants such as chlorhexidine digluconate
may correlate with antibiotic resistance [5–7]. Due to
the correlation observed between resistance against dis-
infectants and antibiotics, widespread use of disinfec-
tants should be reassessed . Considering the
aforementioned points, there is substantial need for al-
ternative treatment methods to control oral infections.
Nasturtium (Tropaeolum majus L., TR) herb and
horseradish (Armoracia rusticana P.Gaertn., B.Mey. &
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* Correspondence: firstname.lastname@example.org
Center for Infectious Diseases, Heidelberg University Hospital, Im
Neuenheimer Feld 324, 69120 Heidelberg, Germany
Full list of author information is available at the end of the article
Medicine and Therapie
Eichel et al. BMC Complementary Medicine and Therapies (2020) 20:156
Scherb., AR) root release high amounts of isothiocya-
nates (ITCs) when glucosinolates - the inactive prodrugs
of ITCs found naturally in Brassica plants - are hydro-
lysed by myrosinases, which are also present in Brassica
plants [8,9]. Several in-vitro studies have demonstrated
that herbal ITCs display antimicrobial effects against a
variety of bacteria including multidrug-resistant (MDR)
bacteria, such as methicillin-resistant Staphylococcus
aureus, vancomycin-resistant Enterococci,MDR Escheri-
chia coli, penicillin-resistant Streptococcus pneumoniae,
biofilm-producing Pseudomonas aeruginosa and also
against viruses [10–14]. Clinical studies have demon-
strated the non-inferiority of TR/AR compared to stand-
ard antibiotics in upper respiratory tract infections such
as acute sinusitis and bronchitis treatment [15,16] and
their efficacy in prophylaxis of both respiratory and urin-
ary tract infections [17,18].
The objective of this in-vitro study was to assess the
antimicrobial effects of TR/AR on clinically important
oral pathogens. A total of 15 bacterial strains, (Campylo-
bacter concisus, Campylobacter rectus, Fusobacterium
naviforme, Fusobacterium nucleatum, Parvimonas micra,
Prevotella baroniae, Prevotella intermedia, Porphyromo-
nas gingivalis, Tannerella forsythia, Veillonella parvula),
including 5 HACEK organisms (Haemophilus aphrophi-
lus, Aggregatibacter actinomycetemcomitans, Cardiobac-
terium hominis,Eikenella corrodens,Kingella kingae),
involved in periodontitis and other diseases and one ref-
erence strain (Clostridium perfringens) were assessed.
Table 1gives an overview of the diseases these strains
Headspace gas chromatography mass spectrometry
To identify the amount of allyl–ITC, benzyl–ITC, and 2-
phenyl-ethyl–ITC in TR and AR, headspace gas chroma-
tography mass spectrometry (GC-MS) was employed.
For calibration 0.1, 0.5, 1, 2.5, or 5 μL of a ITC stock so-
lution (ITC:Methanol = 1:100) was added to 500 μLH
in a 10 mL glass vial. After 30 min shaking at 60 °C
500 μL gas were analyzed using a GC-MS-QP2010S (Shi-
madzu) equipped with a 30 m × 0.32 mm HP-VOC capil-
lary column. The flow rate of the carrier gas helium was
1 mL/min. The column temperature was programmed
from 60 °C to 220 °C at a rate of 10 °C/min. The temper-
atures of the injector and detector were set to 200 °C
and 280 °C, respectively. The experiments were carried
out with electron impact ionization (EI) mode at elec-
tron energy of 70 eV. The degradation products were
identified by matching the recorded mass spectra with
the NIST 107 mass spectrum library of the GC-MS data
system. Mass-to-charge ratios for benzyl-ITC were 149
m/z and 91 m/z, for allyl–ITC 99 m/z and 41 m/z, and
for 2-phenyl-ethyl–ITC 163 m/z and 91 m/z,
respectively. For testing TR and AR, 1 mg of the dried
plant was activated with 500 μLH
0 and ITCs amounts
were measured in the same way as the calibration stock.
Cultivation of bacterial strains
The bacterial strains were chosen from the collection of
the microbiological institute of the university hospital of
Freiburg, Germany. They were cultivated under different
conditions corresponding to their special needs. Yeast-
cysteine-blood (YCB) agar plates containing 5% sheep
blood were prepared. The anaerobes Aggregatibacter
actinomycetemcomitans, Campylobacter concisus, Cam-
pylobacter rectus, Clostridium perfringens, Parvimonas
micra, Prevotella baroniae, Prevotella intermedia, Por-
phyromonas gingivalis, Tannerella forsythia, and Veillo-
nella parvula were cultured on the agar plates in
Anaerocult pots (Becton Dickinson and Merck) in which
the anaerobic conditions were monitored by Dry Anaer-
obic Indicator Strips (Becton Dickinson). The micro-
aerophilic strains, Fusobacterium naviforme and
nucleatum, Haemophilus aphrophilus, Kingella kingae,
Eikenella corrodens and Cardiobacterium hominis grew
at 5% CO
The plates were incubated at 36.5 °C, and
32.5 °C for Prevotella intermedia and Prevotella baronie.
Preparation of bacterial stock solution
To achieve quantitative results, a defined amount of bac-
teria should be used for sensitivity tests. The bacterial
stock solutions were prepared by harvesting bacterial
colonies from the plates with sterile cotton swabs and
suspending them in 1 mL PBS (Dulbecco Biochrom).
The turbidity of the suspensions was adjusted to McFar-
land standard 0.5, which corresponds to a concentration
of approximately 10
cells/mL. The solution was diluted
with PBS to a final concentration of 10
Table 1 Diseases caused by the bacterial strains tested
(adapted from (Lamont and Jenkinson 2010))
Disease Involved species
Gingivitis Fusobacterium spp., Prevotella spp., Campylobacter spp.
Periodontitis Aggregatibacter actinomycetemcomitans, Campylobacter
spp., Eikenella corrodens, Fusobacterium spp.,
Porphyromonas gingivalis, Prevotella spp., Tannerella
forsythia, Veillonella parvula
Implantitis Porphyromonas gingivalis, Prevotella spp.
Pulpitis Fusobacterium spp., Parvimonas micra, Porphyromonas
Halitosis Fusobacterium spp., Porphyromonas gingivalis, Prevotella
Pharyngitis Haemophilus aphrophilus
Tonsillitis Haemophilus aphrophilus
Meningitis Veillonella parvula
Endocarditis Haemophilus aphrophilus, Aggregatibacter actinomycetemcomitans,
Cardiobacterium hominis, Eikenella corrodens, Kingella kingae
Eichel et al. BMC Complementary Medicine and Therapies (2020) 20:156 Page 2 of 7
Phytotherapeutic drug susceptibility testing with gas test
The antimicrobial effects of the herbal drugs were
assessed using a modified gas test (Fig. 1). Cover plates
were filled with the native substances TR herb powder
(18,834, supplier Martin Bauer) and AR root powder
(17,604, supplier Peter) at a ratio of 2.5:1 (REPHA
GmbH, Langenhagen, Germany). Amounts of TR/AR
ranged from 12.5/5 mg, which corresponds to 1/16 of
the commercially available tablet “Angocin®”and to 400/
160 mg, which corresponds to 2 tablets. A negative con-
trol with an empty cover plate was added to each series.
The bacterial stock solutions were spread on YCB agar
plates and the drugs were activated by stirring in 2 mL
PBS. The plates were assembled, closed with parafilm,
and incubated at 32.5 °C and checked for bacterial
growth after 48 and 72 h, respectively, by counting col-
ony forming units (CFU). Minimal inhibitory concentra-
tions (MIC) were defined as the lowest concentration of
TR/AR to prevent visible bacterial growth. Every strain
was tested at least twice with this assay. MIC values are
ITC susceptibility testing with agardilution
To verify the effects observed in the gas test, synthetic
ITCs were used to perform agar dilution tests. To reflect
the proportions of active agents in Angocin®, a mixture
of 38% allyl-ITC (Merck), 50% benzyl-ITC (Sigma-Al-
drich), and 12% 2-phenyl-ethyl-ITC (Fluka) was pre-
pared. 1% (v/v) polysorbate 80 (Merck Schuchardt)
served as a solvent to dilute the lipophilic mixture. The
ITC/polysorbate mixture was diluted nine times 1:1 to
prepare the desired concentrations. Series of YCB agar
plates with ITC concentrations from 0.0025 mg/mL to
0.34 mg/mL were poured by adding 2 mL ITC/polysor-
bate mixture to 18 mL liquid YCB agar. The bacterial
stock solution with a concentration of 10
inoculated using a multipoint-inoculator (Mast). As a
negative control, 2 mL polysorbate without ITC was
added to the YCB agar. The inoculated agar plates were
incubated at 32.5 °C up to 5 days, depending on the par-
ticular bacterial growth rate. Anaerobes were cultivated
in Anaerocult pots, microaerophilic bacteria at 5% CO
The plates were checked for bacterial growth after 48
and 72 h, respectively. Each strain was tested at least
twice with this assay. The minimal bactericidal concen-
tration (MBC) is the lowest ITC concentration required
to kill the bacterium. For determination of the MBC
values, inoculated areas which showed no visible bacter-
ial growth after 48 h incubation were transferred with a
sterile swab to an ITC-free YBC agar plate, and checked
for bacterial growth after 48 and 72 h, respectively. Each
strain was tested at least twice with this assay. MIC and
MBC values are mean values.
Horseradish and Nasturtium powder release high
amounts of Isothiocyanates
With headspace GC-MS we measured considerable
amounts of ITCs in the gases, when Tropaeolum majus
and Armoraciae rusticanae powder was activated with
water (Fig. 2). While TR mainly contained benzyl–ITC
(0.046 ± 0.001 μL/mg), AR released huge amounts of
allyl–ITC (0.033 ± 0.001 μL/mg), and 2-phenyl-ethyl–
ITC (0.0023 ± 0.0005 μL/mg).
Phytotherapeutic drugs inhibit bacterial growth
All species were highly susceptible to herbal TR/AR in
the gas test and to synthetic ITC in the agar dilution
test, except for Veillonella parvula. MIC values were de-
termined between 50/20 mg and 200/80 mg TR/AR, and
0.0025 and 0.08 mg ITC/mL, as shown in Fig. 3. The
highest susceptibility was shown for Tannerella forsythia
in both, the gas test and the agardilution. However, in
the gas test growth of Porphyromonas gingivalis, Fuso-
bacterium nucleatum, and Prevotella baroniae was also
inhibited at 50/20 mg TR/AR. The agardilution experi-
ments revealed that after Tannerella Porphyromonas
ginigvalis, Cardiobacterium hominis, and Kingella klin-
gae were next susceptible to ITCs. MBCs extended from
0.005 mg ITC/mL for Tannerella forsythia to 0.34 mg
ITC/mL for Fusobacterium naviforme, Fusobacterium
nucleatum, and Eikenella corrodens. Growth of Veillo-
nella parvula was not influenced by TR/AR or synthetic
ITCs at the concentrations tested. Negative control
plates without antibiotic agents showed normal bacterial
Fig. 1 Opened apparatus for gas test experiments
Eichel et al. BMC Complementary Medicine and Therapies (2020) 20:156 Page 3 of 7
High antimicrobial effects
The antimicrobial effects of Tropaeolum majus L. (TR)
and Armoracia rusticana P.Gaertn., B.Mey. & Scherb
(AR) against oral pathogenic bacteria have not been
studied sufficiently, so far. Hence, the aim of our study
was to determine the susceptibilities of clinically import-
ant oral pathogens and to show that TR and AR are
feasible for the usage in antimicrobial therapy in
In addition to the frequently used standard MIC-
evaluate the antimicrobial effects of the dried plant
Prior to testing the antimicrobial activity, the active
substances of TR and AR were analyzed in detail
using headspace gas chromatography mass spectrom-
etry (GC-MS). We found chemically different ITCs in
the two plants, which favors the use of a combination
of ITC-containing plants. For our susceptibility tests,
we therefore used a mixture of TR and AR at a pro-
portion of 2.5:1 and a combination of synthetically
produced ITCs with matches the proportions of ITCs
in the plants.
With the exception of Veillonella parvula, all tested
species were highly susceptible to herbal TR/AR in the
Fig. 2 GC-MS defined ITC amounts in Nasturtium and Horseradish;
AITC: allyl–ITC; BITC: benzyl–ITC, 2PEITC: 2- phenyl- ethyl–ITC; n=3
Fig. 3 blue: MIC values of TR/AR in gas tests (right scale); orange, red: MIC and MBC values of synthetic isothiocyanates in agardilution tests (left
scale); >: maximum test concentration reached; n=2
Eichel et al. BMC Complementary Medicine and Therapies (2020) 20:156 Page 4 of 7
gas test and synthetic ITCs in the agar dilution, with
MICs ranging between 50/20 mg and 200/80 mg TR/AT,
and 0.01 and 0.08 mg ITC/mL, respectively. Tannerella
forsythia, Porphyromonas gingivalis, Fusobacterium
nucleatum and Prevotella baroniae were found to be the
most sensitive of the tested species, as 50/20 mg TR/AR,
which equates to only 1/4 tablet of the commercially
available drug (Angocin®), was enough to stop growth.
MBC values were about 4 to 10 times higher than MIC
values. Although the order of herbal MIC values does
not exactly correlate with the sequence of synthetic ITC
MIC and MBC values, all determined concentrations
could easily be reached in the oral cavity by topical ap-
plication as mouthwash, gel, chip, or, in combination
with fluoride, as toothpaste.
Our GC-MS experiments demonstrated, that a mixture
of herbal TR/AR powder can release a spectrum of ITCs
in high amounts. Previous studies tested TR alone for its
bactericidal effect on certain oral pathogens. However, the
ITC solution was sucked in a paper disk and placed on
the agar. The MBC values of the ITCs extracted from TR
and the synthetic allyl- ITC were higher than in our ex-
periments . This could mean that the combination of
TR and AR is more effective than TR alone.
Furthermore, Salvadora persica, sticks of which have
been used as natural toothbrushes for centuries, were
found to contain high amounts of benzyl-ITC and show
considerable antimicrobial effects on Aggregatibacter
actinomycetemcomitans and Porphyromonas gingivalis
. This is consistent with the results presented in this
study for ITCs.
Why phytotherapeutic drugs?
Although TR and AR are cultivated and have been
used for hundreds of years, relevant bacterial resis-
tances to ITCs have not as yet been reported. The
safety of systemic TR/AR administration up to 1200/
480 mg daily was demonstrated in a clinical trial .
Adverse side effects were significantly lower in a TR/
ARgroupthananantibioticgroup. Negative ef-
fects on the gut microbiota were not observed. More-
over, the treatment costs with TR/AR are
substantially lower than with antibiotic prescriptions.
Additionally, chlorhexidine, which has been consid-
ered the gold standard in dental plaque control ,
is also cytotoxic, as reported for human gingival fi-
broblasts, osteosarcoma cells and osteoblasts [24,25].
Moreover, human saliva can to some extent inactivate
the antibacterial effects of chlorhexidine against some
oral bacteria, inducing selective processes in the bac-
terial populations of human saliva . Furthermore,
a correlation of resistance towards chlorhexidine and
different medically relevant antibiotics cannot be ex-
cluded due to the similar mechanisms of resistance
which include multidrug efflux pumps and cell mem-
brane changes as reported in an own review of the
literature . Another frequently used oral health
product is Listerine® . Although there is accumu-
lating evidence that Listerine® is effective in improving
oral health, the absence of systematic toxicological
studies means that an accurate safety assessment can-
notbemade. Hence, new natural antibacterial
ising components for dental oral care. However, the
direct comparison of ITCs effects on oral pathogens
with standard antibiotics or chlorhexidine is still
pending, which must be acknowledged as a limitation
of our study approach.
Out of the tested species, Aggregatibacter actinomyce-
temcomitans, Campylobacter rectus, Eikenella corrodens,
Fusobacterium nucleatum, Porphyromonas gingivalis,
Prevotella intermedia, Tanerella forsythia, and Veillo-
nella parvula are highly associated with periodontitis
[30,31]. With the exception of Veillonella parvula, all
these pathogens were found to be highly susceptible to
ITCs. The topical use of herbal TR/AR, e.g. as antiseptic
mouthwash, gel or chip, should be considered, but also
systemic administration, since the compliance to phy-
totherapy is usually good, and spread of antibiotic resist-
ance could be avoided. Activity exhibited by ITCs against
biofilms was demonstrated by the example of Pseudo-
monas aeruginosa . The effects against the diverse
array of oral bacteria tested in the present study suggest
an anti-biofilm effect of ITCs. Such potential should be
examined in future studies to clarify inhibition of forma-
tion or degradation of already formed oral biofilm.
Endocarditis prophylaxis for dental procedures should
predominantly cover Staphylococci,Streptococci, Entero-
cocci, and Candida spp.,but also incidental pathogens
such as HACEK organisms . Our in vitro-study dem-
onstrated that HACEK organisms are highly susceptible
to TR/AR. These results support and expand our previ-
ous findings of the antibacterial effect of mustard oil-
containing plants against the predominant endocarditis
relevant oral bacteria .
This study showed that different components of mustard
oil-containing plants have a high antimicrobial activity
against various oral bacteria. The presented results sug-
gest a high potential activity against oral biofilm forma-
tion which should be tested in vivo in future clinical
studies to evaluate their beneficial protective effects to
prevent oral diseases such as caries, periodontitis and
Eichel et al. BMC Complementary Medicine and Therapies (2020) 20:156 Page 5 of 7
Supplementary information accompanies this paper at https://doi.org/10.
Additional file 1: Table S1. Exposed species and its corresponding MIC
values of nasturtium herb and horseradish root in gas tests, as well as
MIC and MBC values of synthetic isothiocyanates in agardilution tests; n.d:
AR: Armoracia rusticana P.Gaertn., B.Mey. & Scherb; CFU: Colony forming
units; ITC: Isothiocyanate; MBC: Minimal bactericidal concentration;
MDR: Multidrug-resistant; MIC: Minimal inhibitory concentration;
TR: Tropaeolum majus L.; YCB: Yeast-cysteine-blood
We thank Ms. Lawrie-Blum for proofreading.
VE contributed to conception, conducted the gas chromatic experiments,
interpreted and analyzed the data, and has drafted the manuscript. AS
contributed to conception, performed the phytotherapeutic drug testing,
interpreted and analyzed the data. KB contributed to conception, supervised
the phytotherapeutic drug testing, interpreted and analyzed the data, and
contributed to the manuscript. AAA contributed to interpretation of the data
and substantively revised the manuscript. UF initiated the project and
received the financial support, contributed to conception and design,
supervised all experiments and critically revised the manuscript. All authors
read and approved the final manuscript.
This work was supported by Repha GmbH, Langenhagen, Germany [Grant
number D: 100 60 723, C: 50 30 25]. Repha GmbH was neither involved in
conducting the experiments, interpreting the results nor in the writing of
We acknowledge financial support by Deutsche Forschungsgemeinschaft
within the funding programme Open Access Publishing, by the Baden-
Württemberg Ministry of Science, Research and the Arts and by Ruprecht-
Availability of data and materials
The datasets used and/or analysed during the current study are available
from the corresponding author on reasonable request.
Ethics approval and consent to participate
Consent for publication
The authors declare that they have no competing interests.
Center for Infectious Diseases, Heidelberg University Hospital, Im
Neuenheimer Feld 324, 69120 Heidelberg, Germany.
Institute for Infection
Prevention and Hospital Epidemiology, University of Freiburg, Breisacher
Straße 115 B, 79106 Freiburg, Germany.
Department of Operative Dentistry
and Periodontology, Freiburg University Hospital, Hugstetterstrasse 55, 79106
Received: 14 February 2020 Accepted: 14 May 2020
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