Isolation and characterization of a novel equol-producing bacterium from human feces

Article abstract of DOI:10.1271/bbb.80329

An equol-producing bacterium was newly isolated from the feces of healthy humans and its morphological and biochemical properties were characterized. The cells were obligate anaerobes. They were non-sporulating, non-motile, gram-positive bacilliform bacteria with a pleomorphic morphology. The strain was catalase-positive, and oxidase-, urease-, and indole-negative. The only other sugar utilized by the strain was glycerin. The strain also degraded gelatin, but not esculin. It was most closely related to Eggerthella hongkongensis HKU10, with 93.3% 16S rDNA nucleotide sequence homology. Based on these features, the isolate was identified as a novel species of the genus Eggerthella. It was named Eggerthella sp. YY7918. Strain YY7918 converted substrates daidzein and dihydrodaidzein into S-equol, but did not convert daidzin, glysitein, genistein, or formononetin into it. An antimicrobial susceptibility assay indicated that strain YY7918 was susceptible to aminoglycoside-, tetracycline-, and new quinolone-antibiotics.

Full text of DOI:10.1271/bbb.80329

Biosci. Biotechnol. Biochem., 72 (10), 2660–2666, 2008
Isolation and Characterization of a Novel Equol-Producing Bacterium
from Human Feces
y
1;
2
Shin-ichiro YOKOYAMA and Tohru SUZUKI
1Department of Applied Microbiology, Gifu Prefectural Research Institute for Bioengineering,
3481-2 Kamihachiya, Hachiya, Minokamo, Gifu 505-0004, Japan
2The United Graduate School of Agricultural Science, Gifu University,
1-1 Yanagido, Gifu 501-1193, Japan
Received May 12, 2008; Accepted June 27, 2008; Online Publication, October 7, 2008
[doi:10.1271/bbb.80329]
An equol-producing bacterium was newly isolated
from the feces of healthy humans and its morphological
and biochemical properties were characterized. The
cells were obligate anaerobes. They were non-sporulat-
ing, non-motile, gram-positive bacilliform bacteria with
a pleomorphic morphology. The strain was catalase-
positive, and oxidase-, urease-, and indole-negative. The
only other sugar utilized by the strain was glycerin. The
strain also degraded gelatin, but not esculin. It was most
closely related to Eggerthella hongkongensis HKU10,
with 93.3% 16S rDNA nucleotide sequence homology.
Based on these features, the isolate was identified as a
novel species of the genus Eggerthella. It was named
Eggerthella sp. YY7918. Strain YY7918 converted sub-
strates daidzein and dihydrodaidzein into S-equol, but
did not convert daidzin, glysitein, genistein, or formo-
nonetin into it. An antimicrobial susceptibility assay
indicated that strain YY7918 was susceptible to amino-
glycoside-, tetracycline-, and new quinolone-antibiotics.
workers have reported that a bacterial consortium
comprising Bacteroides ovatus, Ruminococcus produc-
tus and Streptococcus intermedius,9) or Lactobacillus
mucosae, Enterococcus faecium, Finegoldia magna
and Veillonella sp.15) produces it from daidzein. An
anaerobic gram-positive strain HGH-6,16) an anaerobic
bacterium Niu-O16,17) the Clostridium-like bacterium
TM-40,18) which converts daidzein to dihydrodaidzein,
and the strain SNU-Julong 732 which converts dihy-
drodaidzein to equol,19) have also been isolated individ-
ually from humans. Recently, bacteria that convert
daidzein to equol were isolated from rat cecal contents20)
and human feces21) respectively. Little is known about
equol-producing bacteria. In this study, we isolated a
novel species of equol-producing bacterium from the
feces of healthy humans and characterized the morpho-
logical and biochemical properties of the species.
Materials and Methods
Key words: Eggerthella sp. YY7918; isolate from hu-
man feces; S-equol; substrate specificity;
antimicrobial susceptibility
Materials. The culture broths used in this study were
as follows: Brain heart infusion (BHI), heart infusion
(HI), Bactoꢀ tryptic soy, and Difcoꢀ Lactobacilli MRS
from Becton-Dickinson (Sparks, MD, USA); and GAM
from Nissui Pharmaceuticals (Tokyo). The following
reagents were used as standard substances: Equol was
purchased from LC Laboratories (Woburn, MA, USA),
daidzein from Tokyo Chemical (Tokyo), daidzin from
Wako Pure Chemical (Osaka), formononetin from EMD
Biosciences (La Jolla, CA, USA), genistein from Sigma
(St. Louis, MO, USA), and dihydrodaidzein from
Toronto Research Chemicals (North York, Canada). A
silica gel 60 F254 thin layer chromatography (TLC) plate
was obtained from Merck KGaA (Darmstadt, Germany).
Equol, a daidzein metabolite produced by intestinal
microflora, is known to have higher estrogenic1,2) and
antioxidative3) activities than other major soy isoflavo-
noids, such as daidzein and genistein. Epidemiological
studies suggest that it might be beneficial in the
prevention of breast cancer,4,5) prostate cancer,6,7)
osteoporosis,8) and menopausal syndromes.9) Conse-
quently, there is growing interest in the physiological
effects of equol on human health. However, not every-
body is able to produce equol. It is reported that only
20–60% of healthy adults can produce it.9–14) It is
thought that certain bacteria in the intestinal microflora
are greatly involved in equol bioconversion. Some
Culture conditions. The culture broth was supple-
mented with daidzein or other isoflavonoids (final
y
To whom correspondence should be addressed. Tel: +81-574-25-3803; Fax: +81-574-25-3804; E-mail: yokoyama@bio.rd.pref.gifu.jp
Abbreviations: GAM, Gifu anaerobic medium; CD, circular dichroism; BHI, brain heart infusion; HI, heart infusion; TLC, thin layer
chromatography; HPLC, high performance liquid chromatography; PDA, photodiode array; MIC, minimal inhibitory concentration

A Novel Equol-Producing Bacterium from Human Feces
2661
concentration, 50 mM) and used as a growth medium.
Fecal samples from healthy volunteers were obtained
from the authors’ relatives. Two grams of freshly voided
fecal sample was homogenized and suspended in 5 ml of
growth medium. Fifty microliters of the fecal homoge-
nate or 10 ml of bacterial culture (OD600 ¼ 0:02) was
inoculated into a 24-well Multiwellꢀ plate (Becton-
Dickinson) containing 1 ml of growth medium. The
plates were incubated at 37 ꢀC for 72 h using the
AnaeroPackꢁ/Kenki culture system (disposable O2
absorbing and CO2 generating agent, Mitsubishi Gas
Chemical, Tokyo). After the addition of 20% (v/v)
glycerol, culture samples were stored at ꢁ80 ꢀC.
Biotechnology Information using the BLAST algo-
rithm23) and the BLASTN program for comparison of
a nucleotide query sequence against a nucleotide
sequence database. A phylogenetic tree was constructed
according to the neighbor-joining method with the
CLUSTAL W program24) and visualized using the
FigTree program.
Chiral column high performance liquid chromatog-
raphy (HPLC) analysis. After preparative TLC, the
silica gel at the position that corresponded to the Rf
value of equol was scratched up and eluted with
methanol. Then the eluate was analyzed with a HPLC-
photodiode array (PDA) detection system. The assay
method of Wang et al.19) was adopted for analysis
of enantioisomers. One milligram of the lyophilized
sample was dissolved in 500 ml of H2O/acetonitrile
(60:40), and then diluted 10-fold with the mobile buffer.
Then aliquots (last dosage, 2 mg/sample) were applied
to a Sumichiral OA-7000 column (4.6 mm ID ꢂ 250
mm, Sumika Chemicals, Osaka) and eluted with 40%
acetonitrile in 20 mM potassium phosphate buffer (pH
3.0) at 40 ꢀC. The flow rate was 1 ml/min, and the UV
spectra of the peaks were recorded from 200 to 400 nm.
The lyophilized sample was also dissolved in ethanol
(0.5 mg/ml), and the optical circular dichroism (CD)
spectrum was recorded on a J-820 CD spectrometer
(Jasco, Tokyo).
Screening method. Serial dilutions in sterile saline
were spread on GAM agar plates and incubated at 37 ꢀC
for 96 h. Separated colonies were transferred to a 96-
well Multiwellꢀ plate (Becton-Dickinson) containing
200 ml of growth medium incubated under anaerobic
conditions. After 72 h of incubation at 37 ꢀC, samples
were taken for TLC analysis.
TLC analysis for isoflavonoids, without ꢀ-glucuroni-
dase treatment, has been described by Yokoyama
et al.22) Some modifications were made for the screening
assay. Briefly, culture supernatant was transferred onto
MultiScreenꢀ-BV (pore size, 1.2 mm, Millipore, Bed-
ford, MA, USA), and filled with 80 ml of Wakogelꢁ
50C18 (38–63 mm in diameter). The columns were
washed with 200 ml of water and 200 ml of 30% meth-
anol together, and then eluted with 200 ml of methanol.
The eluate was vacuum dried. The samples were
dissolved in 10 ml of methanol and analyzed by normal
phase TLC with a solvent system of toluene:acetone
(2:1). The isoflavonoids developed on TLC were visu-
alized with an UV transilluminator (312 nm) and/or
iodine vapor.
Antimicrobial susceptibility test. The antimicrobial
susceptibility of the isolated bacterium was estimated
using an Optopanel MP (OP-1) Kit from Kyokuto
Pharmaceuticals (Tokyo). Briefly, 0.05 ml of bacterial
preculture (OD600 ¼ 0:161{0:164) was mixed with 12
ml of GAM broth, and a 0.1-ml aliquot of the broth was
applied to the OP-1 Kit. The minimal inhibitory concen-
tration (MIC) was determined after 5 d of incubation
under anaerobic conditions. The test was confirmed
in duplicate with freshly prepared media on separate
occasions.
Morphological and biochemical analysis. The iso-
lated bacterium was grown on GAM agar plates, and its
colony form was observed. It was stained using Favor G
Nissui (Nissui Pharmaceutical) and observed by light
microscopy (Olympus model BX50F4, Tokyo). The
biochemical features of the isolate were estimated using
Results
´
an API system 20A Kit (bioMerieux, Lyon, France).
Isolation of equol-producing bacteria
Initially we examined five culture media to select a
suitable one for isolation of equol-producing bacteria.
BHI, HI, Tryptic soy, MRS, and GAM broth were
supplemented with 50 mM daidzein. No daidzein metab-
olism was observed during the entire incubation period
when the fecal sample was incubated with MRS or HI.
Hence we decided to use GAM broth as a basic growth
medium in the studies that followed. By selecting
colonies from GAM agar plates on which serial dilutions
from the equol-producing fecal sample had grown, one
colony was retrieved that produced equol upon sub-
culturing. After several transfers to fresh growth
medium, the culture maintained its ability to convert
daidzein into equol.
Genetic analysis. Genomic DNA of the isolate was
extracted using a Wizardꢁ Genomic DNA Purification
Kit (Promega, Madison, WI, USA). The following
oligonucleotides were synthesized as primers for
amplification of the bacterial 16S rRNA gene: 9F, 50-
GAG TTT GAT CCT GGC TCA G-30; and 1510R, 50-
GGC TAC CTT GTT ACG A-30. Sequencing of the
16S rDNA fragments was performed using the ABI
PRISM 3100 Genetic Analyzer System (Applied Bio-
systems, Foster City, CA, USA). Analysis of 16S RNA
gene sequences and sequence homology searches were
completed with standard DNA sequencing programs
and the BLAST server of the National Center for

2662
S. YOKOYAMA and T. SUZUKI
Table 1. Morphological and Biochemical Properties of the Isolated
Bacterium
Pleomorphic bacilli
(0:7{0:8 ꢂ 2:0{4:0 mm)
Cell shape
Gram staining
Spore
+
ꢁ
Motility
ꢁ
Growth rate
Diameter
Color
Scanty
<1:0 mm
Cream
Round
Lens
Entire
Smooth
Opaque
Butyrous
+
Shape
Colony form
Uplift condition
Marginal
Surface
10 µm
Fig. 1. Microscopic Image of the Equol-Producing Bacterium Iso-
lated from Human Feces.
Transparency
Property
30
After gram-staining, the isolated bacterium was observed under a
light microscope (magnification, ꢂ2;000). The bold line below the
photograph indicates the size bar.
Growth temp.
(ꢀC)
45
+
Catalase
+
It did not utilize the other sugars tested, including
mannitol, lactose, saccharose, maltose, salicin, D-xylose,
L-arabinose, cellobiose, D-mannose, melezitose, raffi-
nose, D-sorbitol, L-rhamnose, and D-trehalose. It also
degraded gelatin, but not esculin.
Oxidase
ꢁ
+
Gelatin
Esculin
ꢁ
ꢁ
Urease
Glycerin oxidation
Glucose oxidation
+
Genetic properties of the isolated equol-producing
bacterium
ꢁ
The 1,469-basepair sequence of the 16S rRNA gene
(rDNA) of the isolated bacterium was sequenced. The
16S rDNA nucleotide sequence has been deposited in
DDBJ/GenBank/EMBL under accession no. AB379693.
The identity of the isolated strain was determined by
comparing the 16S rDNA sequence with a database. The
isolate had 93.3% sequence similarity with Eggerthella
hongkongensis HKU10 (accession no. AY288517). The
phylogenetic tree also indicated that the isolate was to be
placed in the same clusters as Eggerthella species, but
it was different from other equol-producing bacteria
(Fig. 2). Hence it was named strain YY7918, as one
species of the genus Eggerthella.
Mannitol,
Lactose,
Saccharose,
Maltose,
Salicin,
D-Xylose,
L-Arabinose,
Cellobiose,
D-Mannose,
Melezitose,
Raffinose,
D-Sorbitol,
L-Rhamnose,
D-Trehalose
O/F test
ꢁ
ꢃThese estimations were performed using GAM medium.
Enantiomeric character of the equol produced by
strain YY7918
Morphological and biochemical properties of the
isolated equol-producing bacterium
To determine the enantiomeric character of the
biosynthesized equol, TLC-purified equol from the
bacterial culture was analyzed with a chiral column
HPLC-PDA detection system. The chiral column HPLC
data showed that there was a mono peak at a retention
time of 12.2 min at 270 nm (Fig. 3, panel B), while twin
peaks, corresponding to the S and R forms respectively,
were observed at retention times of 12.2 and 15.9 min
with commercial equol (Fig. 3, panel A). In order to
confirm the chirality, the purified equol was subjected to
CD spectrum analysis. The CD spectrum data showed
that positive (17 mdeg) and negative (ꢁ10 mdeg) peaks
The morphological and biochemical properties of the
isolated equol-producing bacterium are summarized in
Table 1. The strain grew on GAM agar plates as cream-
colored colonies with a smooth surface less than 1 mm
in diameter after 7 d of incubation at 37 ꢀC in an
anaerobic environment. Light microscopic observations
revealed that the cells were gram-positive pleomorphic
bacilli arranged in several chains (Fig. 1). The cells
were non-sporulating, non-motile obligate anaerobes.
The strain was catalase-positive, oxidase-negative, ure-
ase-negative, and indole-negative, and utilized glycerin.

A Novel Equol-Producing Bacterium from Human Feces
2663
Eggerthella sp. YY7918 (AB379693)*
TRICHOTOMY
Eggerthella hongkongensis HKU10T (AY288517)
516
Eggerthella lenta ATCC25559T (AF292375)
Eggerthella sinensis HKU14T (AY321958)
1000
919
SNU-Julong 732 (AY310748)*
1000
Gram-positive bacterium do03 (AB266102)*
909
974
Adlercreutzia equolifaciens FJC-B9T (AB306661)*
Collinsella aerofaciens JCM10188T
(AB011816)
Slackia exigua ATCC700122T (AF101240)
932
993
Slackia heliotrinireducens ATCC29202T (AF101241)
Actinomyces bovis NCTC11535T (X81061)
0.01
Fig. 2. Comparison of 16S rDNA Sequence from Strain YY7918 with Other Bacteria in the Database.
The 16S rDNA sequence was analyzed by extracting the DNA, performing PCR with universal 16S primers, and sequencing with the same
primers by standard methods. The resulting sequence of 1,469 bases was searched against Genbank (NCBI). The phylogenetic tree was
constructed using the FigTree program. Super-imposed T and asterisk represent the type strain and the equol-producing bacterium respectively.
Numbers at the branch points are bootstrap values based on 1,000 samplings. Parentheses indicate the accession numbers. Actinomyces bovis
NCTC11535 was used as the outgroup. The scale bar represents genetic distance.
occurred at 240 and 280 nm respectively (Fig. 3,
panel C), which was consistent with the report of Wang
et al.19) Consequently, based on these analyses, YY7918
was found to produce only S-equol from daidzein.
Eggerthella strains. This indicates that strain YY7918 is
closely related to the genus Eggerthella (Fig. 2). The
morphological and biochemical characteristics of the
strain were also similar to that of genus Eggerthella,25)
particularly the fact that no sugars were oxidized or
fermented by the bacterium (Table 1). These results
indicate that the bacterium belongs to the genus
Eggerthella. Strain YY7918 showed highest sequence
homology (93.3%) with E. hongkongensis HKU10
(Fig. 2). Generally, an object bacterial strain is identified
as the same species as a known one when there is over
97% sequence homology on 16S rDNA sequence analy-
sis, but no strain was found with 16S rDNA that was
over 97% homologous to strain YY7918. This suggests
that there is no reported species that is the same as strain
YY7918. In addition, strain YY7918 was morphologi-
cally different from E. hongkongensis. The cells of strain
YY7918 were pleomorphic bacilli (Fig. 1), while those
of E. hongkongensis were coccobacilli.25) Considering
the results of the 16S rDNA sequence homology search
and the morphological characteristics of the strain
together, strain YY7918 was identified as a novel
species of the genus Eggerthella.
Substrate specificity of strain YY7918
TLC analysis indicated that the bacterium converted
equol (Rf ¼ 0:49) from daidzein and dihydrodaidzein
with almost 100% efficiency (50 mM equol/72 h of
culture) (Fig. 4, lanes 2 and 4). However, daidzin,
glysitein (a minor isoflavonoid of soy bean), genistein,
and formononetin, which is abundant in red clover, were
not metabolized by the bacterium (Fig. 4, lanes 3, 5, 6,
and 7 respectively).
Antimicrobial susceptibility of strain YY7918
The antibiotic susceptibility patterns of strain YY7918
are shown in Table 2. The strain was susceptible to
benzylpenicillin, ampicillin, piperacillin, imipenem,
meropenem, gentamicin, minocycline, and levofloxacin.
It was resistant to cefotiam, cefotaxime, cefoperazone/
sulbactam, ceftazidime, cefditoren, erythromycin, and
clindamycin.
Although three bacterial species (strains SNU-Julong
732 and do03, and Adlercreutzia equolifaciens) have
been reported as equol-producing bacteria,19–21) the
homologies of the 16S rDNA sequences from the newly
isolated bacteria with 16S rDNA from these strains were
Discussion
The 16S rDNA sequence of isolated strain YY7918
showed relatively high homology (92.9–93.3%) with

2664
S. YOKOYAMA and T. SUZUKI
differs in cell shape (coccobacilli) from strain
A
40
30
20
10
0
YY7918.21) Thus Eggerthella sp. YY7918 can be
distinguished from the other species that are already
known as equol producers. Since these bacteria all
belong to the family Coriobacteriaceae, this bacterial
family might play a pivotal role in isoflavonoid
metabolism within the mammalian intestine.
Several studies have indicated that equol has selective
affinity for estrogen receptors. In particular, S-equol has
a high affinity for estrogen receptor ꢀ, whereas R-equol
is relatively inactive.26–29) Since the enantiomeric char-
acter must relate to the bioavailability of the equol, it is
very important to clarify the enantiomeric character of
equol produced by these bacteria. In this study, strain
YY7918 was found to produce only S-equol. Strain
SNU-Julong 732 is known to produce S-equol from
dihydrodaidzein.19) Setchell et al. also established that
S-equol is the exclusive product of mammalian intestinal
bacterial synthesis from soy isoflavones using human
and rat urine collected after ingestion of soy foods and a
human fecal culture.29) These items of information
strongly suggest that only the S-form racemate of equol
is produced by intestinal bacteria.
5
10
15
20
25
30
35
–10
40
B
30
20
10
0
5
10
15
20
25
30
35
–10
The only legume isoflavonoids that Eggerthella sp.
YY7918 is able to metabolize are daidzein and dihy-
drodaidzein, the latter of which is thought to be the
middle metabolite of daidzein in intestinal bacteria.30)
Interestingly, daidzin, a glycoside form of daidzein, was
also scarcely converted to equol. These results suggest
that the bacterium has no ꢀ-glucosidase activity, at least
against daidzin. An examination of the biochemical data
indicated that Eggerthella sp. YY7918 has very few
sugar metabolism pathways. Although there has been no
report on isoflavonoid metabolism, E. hongkongensis
has been reported to retain ꢀ-glucosidase activity.25)
In this study, Eggerthella sp. YY7918 was susceptible
to ꢀ-lactam-, aminoglycoside-, tetracycline-, and new
quinolone-antibiotics, and was resistant to cefem, eryth-
romycin-, and clindamycin-antibiotics. These results are
not contradicted by reports on E. hongkongensis and
E. sinensis, which are sensitive to penicillin and metro-
nidazole and resistant to cefotaxime.25) It is possible that
equol-producing activity is reduced or lost when a
person takes ꢀ-lactam-, aminoglycoside-, tetracycline-,
or new quinolone-antibiotics. Atkinson et al. report that
when a human fecal culture is supplemented with
kanamycin (aminoglycoside-antibiotics) or rifampicin,
equol production is inhibited.31) By contrast, kanamycin
was virtually eliminated by equol production in the
plasma of cynomologus monkeys,32) and hence its
antimicrobial susceptibility effects might not be uni-
form. Because these effects were estimated in a complex
system using natural or artificial (fecal culture) micro-
flora (comprising many kinds of bacterial species),
antibiotics might be degraded or modified by resistant
bacteria in the flora. Equol production might also be
affected by the constitution of microfloral bias caused by
antibiotic prescription. Much more in vivo research is
Retention time (min)
C
20
10
Biosynthesized
S
–equol
0
–10
–20
240
260
280
300
320
Wave length (nm)
Fig. 3. Enantiomorphic Analysis of Bacterial-Synthesized Equol.
HPLC elution profiles on a Sumichiral OA-7000 column of
commercial equol (racemic-mixture) (panel A) and equol purified
from daidzein-containing GAM broth cultured with Eggerthella sp.
YY7918 under anaerobic conditions (panel B). CD spectra of the
bacterial synthesized equol (panel C).
92.1% (accession no. AY310748), 92.0% (accession
no. AB266102), and 92.2–92.4% (accession nos.
AB306660–AB306663) respectively, lower than that of
E. hongkongensis HKU10. In addition, strain do03 is an
isolate from a non-human resource (rat cecum).20) Strain
SNU-Julong 732, which was isolated from human feces,
required dihydrodaidzein as the single substrate for
equol production, and no conversion was observed with
daidzein.19) A. equolifaciens, which was also isolated
from human feces and converted daidzein to equol,

A Novel Equol-Producing Bacterium from Human Feces
2665
S.F.
Equol
Genistein
Dihydrodaidzein
Daidzein
Ori.
1
2
7
3
4
5
6
Fig. 4. Bioconversion of Various Isoflavonoids by Eggerthella sp. YY7918.
Culture samples supplemented with various isoflavonoids (50 mM) were treated as described in ‘‘Materials and Methods,’’ developed in
toluene:acetone (2:1), and exposed to iodine vapor. Lane 1, isoflavonoid standards; lane 2, daidzein; lane 3, daidzin; lane 4, dihydrodaidzein;
lane 5, glycitein; lane 6, genistein; lane 7, formononetin.
intestinal microflora. Since equol is considered to be
beneficial in a variety of diseases, it might have some
Table 2. Antimicrobial Susceptibility of Eggerthella sp. YY7918
Antimicrobial agent
MICꢃ (mg/ml)
potential for use as a pharmaceutical agent. Because the
genus Eggerthella (E. lenta is listed as a representative)
belongs to biosafety level 1 according to the ‘‘Manual for
Biosafety Relating to Pathogenic Bacteria’’ (Japanese
Society for Bacteriology), strain YY7918 can be used in
the industrial production of S-equol. Hence a study to
determine ways of improving bacterial production of
S-equol in the near future is desirable.
Benzylpenicillin
Ampicillin
Piperacillin
0.5
50:25
52
Ampicillin/Sulbactam
Cefotiam
Cefotaxime
50:25=0:12
>8
16
Cefoperazone/Sulbactam
Ceftazidime
Cefditoren
16/8
>16
1
Imipenem
Meropenem
Gentamicin
4
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51
>2
>4
0.5
2
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isolated in this study was identified as a novel species, it
belongs to the same family as other equol-producing
bacteria reported previously.19–21) Hence it is thought that
these bacteria share common genotypic or phenotypic
features in the equol production process. More detailed
studies of strain YY7918 should yield much basic
knowledge of the mechanisms of daidzein metabolism
and the relationships between equol-producing bacteria,
the host, and other bacterial communities within the

2666
S. YOKOYAMA and T. SUZUKI
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