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Antimicrobial Agents and Chemotherapy, November 2001, p. 3009-3013, Vol. 45, No. 11
0066-4804/01/$04.00+0 DOI: 10.1128/AAC.45.11.3009-3013.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
In Vitro Antimicrobial Activities of Bakuchiol against Oral
Microorganisms
Harumi
Katsura,
Ryo-Ichi
Tsukiyama,
Akiko
Suzuki, and
Makio
Kobayashi*
Research Laboratory of Higashimaru Shoyu Co.
Ltd., 100-3, Tominaga, Tatsuno, Hyogo 679-4193, Japan
Received 5 March 2001/Returned for modification 5 June
2001/Accepted 9 August 2001
 |
ABSTRACT |
Bakuchiol was isolated from the seeds of Psoralea
corylifolia, a tree native to China with various uses in
traditional medicine, followed by extraction with ether and column
chromatography combined with silica gel and octyldecyl silane. In this
study, the antimicrobial activities of bakuchiol against some oral
microorganisms were evaluated in vitro. The cell growth of
Streptococcus mutans was inhibited in a bakuchiol
concentration-dependent manner, and growth of S. mutans
was completely prevented by 20 µg of bakuchiol per ml. The
bactericidal effect of bakuchiol on S. mutans was
dependent on temperature and stable under the following conditions:
sucrose, 0 to 10% (wt/vol); pH, 3.0 to 7.0; organic acids (3%
[wt/vol] citric and malic acids). Bakuchiol showed
bactericidal effects against all bacteria tested, including
S. mutans, Streptococcus sanguis,
Streptococcus salivarius, Streptococcus
sobrinus, Enterococcus faecalis,
Enterococcus faecium, Lactobacillus
acidophilus, Lactobacillus casei,
Lactobacillus plantarum, Actinomyces
viscosus, and Porphyromonas gingivalis, with
MICs ranging from 1 to 4 µg/ml and the sterilizing concentration for
15 min ranging from 5 to 20 µg/ml. Furthermore, bakuchiol was also
effective against adherent cells of S. mutans in
water-insoluble glucan in the presence of sucrose and inhibited the
reduction of pH in the broth. Thus, bakuchiol would be a useful compound for development of antibacterial agents against oral pathogens
and has great potential for use in food additives and mouthwash for
preventing and treating dental caries.
| |
INTRODUCTION |
Dental plaque, a film of
microorganisms on the tooth surface, plays an important part in the
development of caries and periodontal diseases (20).
Mutans streptococci can colonize the tooth surface and initiate plaque
formation by their ability to synthesize extracellular polysaccharides
from sucrose, mainly water-insoluble glucan, using glucosyltransferase (4, 5, 7). De novo synthesis of
water-insoluble glucan is essential for the adherence of
Streptococcus mutans and other oral microorganisms to the
tooth surface, forming a barrier that prevents the diffusion of acids
produced by the bacteria. The acids accumulate in situ and decalcify
minerals in the enamel. This sucrose-dependent adherence and
accumulation of cariogenic streptococci is critical to the development
of pathogenic plaque. The further accumulation of plaque around the
gingival margin and subgingival region may lead to a shift in its
microbial composition from streptococcus-dominated to a larger number
of Actinomyces spp. and increased numbers of capnophilic and
obligatory anaerobic bacteria, such as Porphyromonas
gingivalis (21). These microorganisms seem to be
involved in root caries and periodontal disease, respectively (26, 27).
To avoid dental caries due to cariogenic bacteria, inhibition of
glucosyltransferase activity by specific enzyme inhibitors (9,
31), inhibition of initial cell adhesion of S. mutans by polyclonal and monoclonal antibodies (24), and
inhibition of cell growth of S. mutans by antibacterial
agents have been investigated. This third line of research in
particular has attracted a great deal of attention, and effective
antimicrobial agents against these oral pathogens could play an
important part in the prevention of dental caries and periodontal
diseases, particularly those that affect plaque formation (1, 12,
13, 14, 29, 30).
A few recent studies have demonstrated antimicrobial activity against
selected oral pathogens from natural sources. From the native American
plant Ceanothus americanus, ceanothic acid and ceanothetric
acid demonstrated growth-inhibitory effects against S. mutans, Actinomyces viscosus, and P. gingivalis (19). Ethanolic extracts of propolis, a
resinous hive product, showed antimicrobial activity against these
three oral microorganisms (10). Natural products have been
used for thousands of years in folk medicine for several purposes. For
example, bakuchiol (Fig. 1)
isolated from the seeds and leaves of Psoralea corylifolia
Linn, a tree native to China with various uses in traditional Oriental
medicine, is a phenolic isoprenoid and exhibits antimutagenic,
antimicrobial, and insect juvenile hormone activities (16, 22,
25). However, these activities were assayed using crude extracts
from the seeds of P. corylifolia, and the antimicrobial
activity of an oily hexane extract from the seeds against only
Staphylococcus aureus was reported (8). Thus,
the antimicrobial activity of bakuchiol has not been thoroughly
investigated, and little is known about its antimicrobial activity
against oral microorganisms or its effects on dental plaque formation
in vitro.
In this study, we purified bakuchiol from the seeds
of P. corylifolia and evaluated the in vitro
antimicrobial activity of bakuchiol against some oral
microorganisms, especially the effects on adherent mutans streptococci.
| |
MATERIALS AND METHODS |
Microorganisms.
The following 18 oral microorganisms were
used in this study: Streptococcus mutans JCM 5175, Streptococcus mutans IFO 13955, Enterococcus
faecalis IFO 3989, Enterococcus faecium IFO 3826, Lactobacillus acidophilus AKU 1122, Lactobacillus
acidophilus AKU 1124, Lactobacillus plantarum AKU 1130, and Porphyromonas gingivalis ATCC 33277 were obtained from
stock culture collections. Streptococcus sanguis 179-3, Streptococcus sanguis 254-4, Streptococcus salivarius 70-2, Streptococcus salivarius 160-2, Actinomyces viscosus 19246, Lactobacillus
plantarum 8016, and Lactobacillus casei 4646 were
obtained from the Department of Conservative Dentistry, School of
Dentistry, Tokushima University, Tokushima, Japan. Streptococcus mutans GS5, Streptococcus mutans JC2, and
Streptococcus sobrinus 6715 were obtained from the
Department of Oral Bacteriology, School of Dentistry, Hokkaido
University, Sapporo, Japan.
Extraction and isolation of bakuchiol.
Seeds of
P. corylifolia Linn were purchased from Tochimoto
Tenkai-do, Osaka, Japan. Bakuchiol was extracted and isolated from the
seeds as described by Mehta et al. (22). Percolation of whole seeds (1 kg) with diethyl ether (5 liters) at room temperature followed by removal of solvent gave a dark brown gummy residue (30 g).
This extract was taken up in diethyl ether (200 ml), and the strongly
acidic compounds were removed by washing with 1% (wt/vol) KOH solution
(50 ml, three times). The organic phase was washed with water followed
by drying on Na2SO4 and
evaporated to furnish a bakuchiol-rich residue (20 g), which
was chromatographed on a silica gel column (BW-200, 100 cm by 4.2 cm;
Fuji Silysia Chemical Ltd., Kasugai, Japan), gradually eluting with 10 to 30% (vol/vol) ethyl acetate (EtOAc) in n-hexane
(a total of 9 liters). The aliquots of each fraction were subjected to
thin-layer chromatography (TLC) (silica gel, high-performance TLC
[HPTLC] plate, 1 mm; Merck) in a solvent system consisting of 20%
(vol/vol) EtOAc in n-hexane, with monitoring of absorption
at 254 nm. Crude bakuchiol (12 g) was recovered from the active
fractions at 254 nm in the silica gel column and then purified with a
reversed-phase octyldecyl silane (ODS) column (DM1020T, 100-200 mesh,
50 cm by 2.2 cm; Fuji Silysia Chemical Ltd.) with chromatography,
eluting with 90% (vol/vol) methanol (MeOH). Each fraction was
subjected to TLC (RP18, HPTLC plate, 1 mm; Merck) in a solvent system
of 90% (vol/vol) MeOH, with monitoring of absorption at 254 nm,
followed by recovery, evaporation and drying in a desiccator to furnish
pure bakuchiol as a colorless liquid (8 g). The purity of
bakuchiol was examined by high-pressure liquid chromatography
(HPLC) and mass spectra as described (15, 16, 22). The
purified bakuchiol was dissolved in the mobile-phase solvent
and applied to an ODS column (YMC-Pack ODS-A, S-5 µm, 120 A, A-312, 6 mm by 150 mm; YMC Co. Ltd., Kyoto, Japan). Bakuchiol was eluted by 85%
(vol/vol) MeOH at 1.0 ml/min and analyzed with a UV detector (SPD-6AU;
Shimadzu, Kyoto, Japan) at 263 nm. Mass spectrometry (MS) of the
purified bakuchiol was carried out on a JMS-AM II (Jeol Ltd.,
Tokyo, Japan) recorded at 70 eV with a source temperature of 250°C:
MS: m/z 256 (M+, 8%), 213 (13%), 173 (100%), 158 (20%), 145 (36%), 107 (28%), and 83 (17%). Pure
bakuchiol (>98%) was dissolved in dimethyl sulfoxide (DMSO)
at 5% (wt/vol), and the bakuchiol solution was used as an
antimicrobial agent in the experiments described below.
Antimicrobial activity against S. mutans
JCM
5175 was inoculated into brain heart infusion broth (Difco
Laboratories, Detroit, Mich.) in test tubes and grown in stationary culture for 24 h at 37°C. Aliquots of 10 ml of culture of
S. mutans JCM 5175 were inoculated into 100 ml of fresh
brain heart infusion broth containing bakuchiol at 0, 1.0, 5.0, 10, and 20 µg/ml in 200-ml Erlenmeyer flasks. The 5%
(wt/vol) bakuchiol solution in DMSO was diluted in the medium
and added to each flask. Solvent controls were included, though no
adverse effect had been noted at the highest concentration employed.
Culture was continued at 37°C for 24 h, and then the
turbidity at various time points was measured at 610 nm.
To determine viable-cell counts in the broth containing
bakuchiol at each incubation time point, the culture broth was
diluted
with sterile water containing 0.1% (wt/vol) Tween 80 for
inactivation
of bakuchiol, and aliquots were inoculated into 10 ml of fresh
brain heart infusion agar containing 0.1% (wt/vol) Tween
80 for
inactivation of bakuchiol in plates. The plates were
incubated
for 48 h at 37°C, and CFU were
counted.
Determination of MIC and sterilizing concentration of
bakuchiol against oral microorganisms.
All oral
microorganisms used in this study were inoculated into brain heart
infusion broth in test tubes and grown to stationary phase for 24 h at 37°C up to 2.0 × 108 to 1.0 × 109 CFU/ml. The MIC was estimated as described by
Li et al. (19). The culture of each oral microorganism was
diluted 10-fold with sterile water. Aliquots of 100 µl of the diluted
cultures of each oral microorganism were inoculated at
105 to 106 CFU/ml into 10 ml of fresh brain heart infusion broth containing bakuchiol at
0, 0.4, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 3.0, and 4.0 µg/ml in test
tubes, serially diluted bakuchiol, and growth medium. The MIC
of bakuchiol was determined by the turbidity at 610 nm after
48 h of incubation at 37°C. The MIC for each oral microorganism
was defined as the minimum concentration of bakuchiol limiting
turbidity to <0.05 absorbance unit at 610 nm.
The sterilizing concentration for 15 min against each oral
microorganism was estimated as described below. Aliquots of 100
µl of
culture of each oral microorganism were inoculated at
10
6 to 10
7 CFU/ml into 10 ml of fresh brain heart infusion broth containing
bakuchiol at
0, 2.5, 5.0, 10, 20, and 40 µg/ml in test tubes,
serially diluted
bakuchiol, and growth medium. After cultures
of each oral
microorganism were incubated with bakuchiol at 37°C
for 15 min, aliquots of 100 µl of the mixtures were inoculated
into 10 ml of
fresh brain heart infusion broth containing 0.1%
(wt/vol) Tween 80 for
inactivation of bakuchiol in test tubes.
The sterilizing
concentration of bakuchiol for 15 min was determined
by the
turbidity at 610 nm after 48 h of incubation at 37°C. The
sterilizing concentration for 15 min against each oral microorganism
was defined as the minimum concentration of bakuchiol limiting
turbidity to <0.05 absorbance unit at 610
nm.
Antimicrobial activity against adherent S.
mutans in water-insoluble glucan.
For formation of
water-insoluble glucan by S. mutans JCM 5175, aliquots
of 100 µl of a culture of S. mutans JCM 5175 were inoculated into 10 ml of fresh brain heart infusion broth containing 2% (wt/vol) sucrose in test tubes and incubated at 37°C for 24 h at an angle of 30°. The fluid was gently removed, and then the cells in water-insoluble glucan in test tubes were gently washed in 10 ml of sterile water. To the cells of S. mutans JCM 5175 in water-insoluble glucan in test tubes was then added 10 ml of citrate
buffer (10 mM, pH 6.0) containing 100 µg of bakuchiol per ml,
followed by incubation at 37°C for 5 min. The mixture was gently
removed and rinsed twice with 10 ml of sterile water containing 0.1%
(wt/vol) Tween 80 to inactivate bakuchiol. To the
bakuchiol-treated cells of S. mutans JCM 5175 in water-insoluble glucan in test tubes was added 10 ml of fresh brain
heart infusion broth containing both 2% (wt/vol) sucrose and 0.1%
(wt/vol) Tween 80 for inactivation of bakuchiol, followed by
incubation at 37°C for 24 h. After incubation for 7.5, 16, or
24 h, the acidogenicity of the cultures was measured with a pH
meter. The fluid containing free cells of S. mutans JCM
5175 was gently removed, and 10 ml of sterile water was added to the
test tube. The water-insoluble glucan formed in the test tube was
homogenized by five 30-s bursts of ultrasonication (UT-204, Silent
Sonic; Sharp, Osaka, Japan), and then the turbidity was measured at 610 nm.
| |
RESULTS |
Antimicrobial activity against S. mutans.
Figure 2A shows the inhibitory effect of
bakuchiol on growth of S. mutans JCM 5175. Growth was inhibited in a bakuchiol concentration-dependent manner. At 5 µg/ml, bakuchiol inhibited growth of
S. mutans JCM 5175 for 7 h before cell growth
started. At 10 µg/ml, cell growth was inhibited even after 24 h
of incubation. To examine viable cells in the cultures, CFU of
S. mutans JCM 5175 were counted in plates containing
0.1% (wt/vol) Tween 80 for inactivation of bakuchiol (Fig.
2B). After incubation with bakuchiol at 5 µg/ml, CFU did not
increase in plates, and thus growth of S. mutans JCM 5175 was bacteriostatically inhibited by bakuchiol at 5 µg/ml. After incubation with bakuchiol at 10 µg/ml, CFU
decreased to the order of 103 from
107, and thus the growth was bactericidally
partially blocked by bakuchiol at 10 µg/ml. After incubation
with bakuchiol at 20 µg/ml, growth was completely blocked,
and thus the cell was sterilized by bakuchiol at 20 µg/ml.
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FIG. 2.
Cell growth (A) and viable-cell counts (B) of
S. mutans JCM 5175 in the presence and absence of
bakuchiol. Bakuchiol concentration:  , 1 µg/ml;  , 5 µg/ml;
 , 10 µg/ml;  , 20 µg/ml;  , none.
|
|
Figure
3 shows the relationship between
bakuchiol treatment period and bakuchiol concentration
for the bactericidal effect
against
S. mutans JCM 5175. After incubation with bakuchiol for
each treatment time, CFU of
S. mutans JCM 5175 were counted in
plates containing
0.1% (wt/vol) Tween 80 for inactivation of bakuchiol.
The
bactericidal effect of bakuchiol was treatment time dependent.
The required time for the bactericidal effect against
S. mutans JCM 5175 was shorter with bakuchiol at 20 µg/ml
than at 5 µg/ml.
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FIG. 3.
Relationship between bakuchiol treatment time
and bakuchiol concentration for the bactericidal effect against
S. mutans JCM 5175. Bakuchiol concentration: , 5 µg/ml; , 20 µg/ml.
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|
Figure
4 shows the antimicrobial
activity of bakuchiol against
S. mutans
JCM 5175 under several treatment conditions, such
as different
temperatures, pHs, sugars, and organic acids. The
bactericidal effect
of bakuchiol on
S. mutans JCM 5175 was
dependent
on temperature of incubation and stable under the following
conditions:
sucrose, 0 to 10% (wt/vol); pH, 3.0 to 7.0; organic acids,
3%
(wt/vol) citric and malic acids).
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FIG. 4.
Antimicrobial activity of bakuchiol against
S. mutans JCM 5175 under several treatment
conditions, including variations in temperature, pH, sucrose, and
organic acids (citric and malic acids). Standard treatment conditions:
temperature, 37°C; pH, 7.0; sucrose, 0% (wt/vol); organic acid, 0%
(wt/vol). Bars (bakuchiol concentration): open, 2.5 µg/ml;
stippled, 5.0 µg/ml; solid, 20 µg/ml.
|
|
MIC and sterilizing concentration of bakuchiol against oral
microorganisms.
To evaluate the antimicrobial activity of
bakuchiol against oral microorganisms, the MICs and the
sterilizing concentrations for 15 min were determined (Table
1). The bacteriostatic effects of
bakuchiol required concentrations of 1 to 4 µg/ml, and the bactericidal effects required sterilizing concentrations for 15 min of
5 to 20 µg/ml for all microorganisms tested. The antimicrobial activity of bakuchiol was effective for both gram-negative
anaerobic periodontal pathogens, such as P. gingivalis,
and gram-positive cariogenic bacteria, such as S. mutans and A. viscosus.
Antimicrobial activity against adherent S.
mutans in water-insoluble glucan.
The adherence of cells
to a glass surface was evident in test tubes when S. mutans JCM 5175 was grown in broth containing 2% (wt/vol) sucrose
for 24 h. After the adherent cells of S. mutans JCM 5175 were treated with bakuchiol at 100 µg/ml for 5 min,
the bakuchiol-treated cells were then incubated in broth
containing both 2% (wt/vol) sucrose and 0.1% (wt/vol) Tween 80 for
inactivation of bakuchiol for 7.5, 16, and 24 h. Figure
5 shows the antimicrobial activity of
bakuchiol against the adherent cells of S. mutans JCM 5175 and pH in broth containing sucrose. The adherent
cells of S. mutans JCM 5175 without bakuchiol
treatment grew well, accompanied by synthesis of water-insoluble glucan
in the presence of sucrose. The pH of the broth also decreased rapidly
to 4.0 after 7.5 h of incubation. On the other hand, the adherent
cells treated with bakuchiol at 100 µg/ml grew slightly,
and water-insoluble glucan synthesis was almost completely inhibited in
the presence of sucrose. The decrease in pH after 16 h of
incubation in the presence of sucrose was negligible, and the pH of the
broth finally decreased to 5.0 after 24 h. When the adherent cells
were treated with bakuchiol at 20 µg/ml, these effects were
weak (data not shown).
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FIG. 5.
Inhibitory effects of bakuchiol on the formation
of water-insoluble glucan (A) and the decrease in pH in broth (B) in
adherent cells of S. mutans JCM 5175 in the
presence of sucrose. Bakuchiol concentration: , 100 µg/ml; ,
none.
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|
| |
DISCUSSION |
It is now clear that S. mutans plays an essential
role in the pathogenesis of dental caries and consequently provides the three prime targets for prevention of caries: antimicrobial
agents against S. mutans, inhibition of adhesion of
S. mutans on the tooth surface, and inhibition of
glucosyltransferase of S. mutans from forming
water-insoluble glucan. Extensive screening for biologically active
compounds from natural sources with these effects has been performed. For example, the ethanolic extract of propolis inhibited both the growth of S. mutans and the activity of
glucosyltransferase (10). Tea polyphenol also
inhibited the growth of S. mutans as well as the
production of insoluble glucans by glucosyltransferases (12, 13,
14). In contrast, mutastein and ribocitrin, which were isolated
from culture supernatants of Aspergillus terreus (2) and a Streptomyces sp. (23),
respectively, inhibited the glucosyltransferase of S. mutans but did not show antibacterial activity. The methanol
extract of the native American plant Ceanothus americanus
demonstrated antimicrobial activity against selected oral pathogens
(19). Thus, ceanothic acid and ceanothetric acid had
growth-inhibitory effects against S. mutans, A. viscosus, and P. gingivalis, with MICs ranging
from 42 to 625 µg/ml.
In this study, we evaluated in vitro the antimicrobial activities
of bakuchiol against some oral microorganisms and showed that bakuchiol had bactericidal effects against all bacteria
tested, including S. mutans, S. sanguis, S. salivarius, S. sobrinus, E. faecalis, E. faecium, L. acidophilus, L. casei, L. plantarum, A. viscosus, and P. gingivalis, with MICs
ranging from 1 to 4 µg/ml and sterilizing concentrations for 15 min
ranging from 5 to 20 µg/ml (Table 1). The antimicrobial activity
of bakuchiol was also effective against adherent cells of
S. mutans in water-insoluble glucan in the presence of
sucrose (Fig. 5). Bakuchiol was found to have cytotoxic activity
against L929 cells in cell culture, and this cytotoxic activity was
considered to be due to injury of the cell membrane, based on
electron microscopic observation and hemolytic activity
(15). Recently, various biological activities of
bakuchiol have been reported, e.g., anti-inflammatory effects (3), inhibition of mitochondrial lipid peroxidation
(6), stimulation of the immune system (18),
inhibition of DNA polymerase (28), inhibition of papilloma
formation (17), and prevention of diabetes
(11). Therefore, bakuchiol might be a promising lead compound for development of antimicrobial agents against oral
pathogens in humans. The mechanism of the antimicrobial activity of
bakuchiol is also under investigation in our laboratory.
Bakuchiol was isolated from the seeds and leaves of P. corylifolia Linn, a tree native to China with various uses in
traditional Oriental medicine (16, 22, 25). However, its
safety was not confirmed completely, and the safety of
bakuchiol needs to be tested in detail. Since the bactericidal
effect of bakuchiol was stable under various environmental
conditions simulating those that occur in the mouth, such as different
temperatures, sugars, pHs, and organic acids (Fig. 4),
bakuchiol is potentially useful for development of
antibacterial agents applicable to food additives for candy and chewing
gum. Furthermore, bakuchiol will also be applicable for use in
mouthwash preparations because of the short treatment time (30 s)
required for the bactericidal effect against S. mutans
(Fig. 3). Research is in progress in our laboratory to evaluate the
antimicrobial effects of bakuchiol against oral microorganisms
using in vivo models.
| |
ACKNOWLEDGMENTS |
We are very grateful to Hiromichi Yumoto and Takashi Matsuo
(Department of Conservative Dentistry, School of Dentistry,
Tokushima University) and Ken-ichiro Shibata (Department of Oral
Bacteriology, School of Dentistry, Hokkaido University) for providing
the oral microorganisms used and for helpful suggestions during this work.
| |
FOOTNOTES |
*
Corresponding author. Mailing address: Research
Laboratory of Higashimaru Shoyu Co. Ltd., 100-3, Tominaga,
Tatsuno, Hyogo 679-4193, Japan. Phone: 81 791 634567. Fax: 81 791 634852. E-mail: mkobayashi{at}higashimaru.co.jp.
| |
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Antimicrobial Agents and Chemotherapy, November 2001, p. 3009-3013, Vol. 45, No. 11
0066-4804/01/$04.00+0 DOI: 10.1128/AAC.45.11.3009-3013.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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Abstract
Introduction
