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APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Aug. 1996, p. 3023–3025 Vol. 62, No. 8
Copyright q 1996, American Society for Microbiology
Production of Thermostable Direct Hemolysin by Vibrio
parahaemolyticus Enhanced by Conjugated Bile Acids
Department of Bacteriology and Pathology, Kanagawa Prefectural Public
Health Laboratory, Asahi-ku, Yokohama 241, Japan
Received 26 February 1996/Accepted 15 May 1996
The effects of conjugated bile acids, glycocholic acid, and taurocholic acid (TC) on production of thermostable direct hemolysin (TDH) by Vibrio parahaemolyticus were determined by a reversed passive latex agglutination assay against TDH. The amount of TDH excreted in growth medium containing either glycocholic acid
or taurocholic acid (5 mM/liter) was, on a per-cell basis, 4- to 16-fold greater than that excreted in medium
without the bile acids. The amounts of TDH released from lysed cells grown with the bile acids (5 mM/liter)
were 4- to 32-fold greater than those from lysed cells grown without, suggesting that the bile acids enhanced
synthesis of TDH within bacterial cells. These data imply that the conjugated bile acids play a key role in the
pathogenicity of V. parahaemolyticus.
Vibrio parahaemolyticus is a halophilic bacterium widely distributed in estuarine and marine environments (8) and often
causes acute gastroenteritis in humans who consume raw or
improperly cooked seafood (1, 2). Past epidemiological studies
(11, 16) revealed a strong association of thermostable direct
hemolysin (TDH) produced by members of this species with its
etiology. TDH has been purified and characterized as a thermostable cytotoxic protein that exhibits cardiotrophism in rats
and mice (6, 7) and fluid-accumulating activity in the rabbit
ileal loop (12). Several in vitro studies demonstrated that production of TDH was influenced by concentrations of NaCl
(11), sugars (4), amino acids (3, 9), and phosphate (10) in
growth media. Little information on its ability to produce TDH
within the human intestine, however, is available.
In the intestine, the bacteria may encounter concentrations
of bile that has a strong detergent-like property. Noh and
Gilliland (13) have reported that the presence of 0.3% oxgall in
reaction buffer increased the cellular permeability of Lactobacillus acidophilus, thereby enhancing its enzymatic activity. Glycocholic acid (GC) and taurocholic acid (TC) are glycine- and
taurine-conjugated cholic acids, respectively, and the major, if
not only, constituents of human bile. A recent study (5) has
demonstrated that GC and TC at concentrations ranging from
0.03 to 0.3% enhanced excretion of bacterial b-glucuronidase
by Escherichia coli and Clostridium perfringens. We therefore
postulated that the bile acids may have similar effects on production of bacterial toxins, for example, TDH of V. parahaemolyticus. In this study, we aimed to determine whether the
presence of the bile acids enhances TDH production by V.
Three strains of V. parahaemolyticus, K-10100, K-10297, and
K-10300, isolated from feces of patients from past food poisoning outbreaks in Kanagawa Prefecture, Japan, were used.
As described in our previous report (15), K-10297 and K-10300
carry the TDH gene whereas K-10100 does not. Cultures of the
strains were maintained on heart infusion (HI) agar (Difco
Laboratories, Detroit, Mich.) supplemented with 2% NaCl
throughout the study. A reversed passive latex agglutination
assay (KAP-RPLA kit; Denka Seiken Co., Tokyo, Japan)
which employs polyvalent capture antibody against TDH (14)
was used to detect and quantify TDH produced in a growth
medium by the bacterium, according to the manufacturer’s
instructions. In this assay, a minimum of;1 to 2 ng/ml of TDH
can be detected at a titer of 1:2.
In the first experiment, each bacterial strain was grown in HI
broth (Difco) supplemented with 2% NaCl, pH 7.8, and incubated at 378C for 6 h with agitation to obtain good midexponential growth. After incubation, the cells were harvested
by centrifugation (4,500 3 g, 20 min, 48C) and washed three
times with sterile HI broth. The suspension was then adjusted
to an optical density (OD) at 660 nm of 0.4 by using sterile HI
broth. Exactly 0.1 ml of the suspension was added to six different sterile broth media (100 ml each) as follows: (i) basal
medium (BM; pH 7.8) consisting of 2 g of Polypeptone
(Difco), 0.5 g of D-mannitol, and 5 g of NaCl; (ii) BM supplemented with 5 mM GC (Sigma Chemicals, St. Louis, Mo.),
referred to as BM1GC; (iii) BM supplemented with 5 mM TC
(Sigma), referred to as BM1TC; (iv) BM 1 5 mM cholic acid
(Sigma), referred to as BM1CH; (v) BM supplemented with 5
mM glycine (Sigma); and (vi) BM supplemented with 5 mM
taurine (Sigma). The initial concentration of bacteria in the
media was adjusted to ca. 2.0 3 105 CFU/ml. The bacterial
cells were then incubated at 378C for 18 h. It should be noted
that the use of BM was specifically recommended by the manufacturer of the KAP-RPLA kit for the maximum production
of TDH by the bacterium (14).
After incubation, the growth in each medium was measured
as the OD at 660 nm, and the OD was subsequently adjusted
to 0.1 by using sterile BM. After this OD adjustment, all preparations contained the same approximate number of CFU (ca.
1.0 3 108 CFU/ml), regardless of the presence of the bile acids
in the growth medium. Cells were then centrifuged (15,800 3
g, 20 min, 48C), and the supernatant was collected for a subsequent assay by KAP-RPLA. Since the KAP-RPLA kit was
provided with purified TDH (100 ng), the TDH was aseptically
dissolved in the above-described six media at a final concentration of 50 ng/ml (equivalent to an RPLA titer of 1:32), and
the mixtures were incubated at 378C for 18 h. After incubation,
the mixtures were assayed by KAP-RPLA for a possible effect
of the bile acids on TDH itself. All assays were performed in
* Corresponding author. Mailing address: Department of Bacteriology and Pathology, Kanagawa Prefectural Public Health Laboratory,
Nakao-cho 52, Asahi-ku, Yokohama 241, Japan.
Growth was significantly (P , 0.01, Student t test) inhibited
by the presence of the bile acids for all three strains tested, with
cholic acid being the most inhibitory (Fig. 1a). In spite of the
growth suppression, the amounts of TDH excreted per equivalent numbers of cells of K-10297 and K-10300 in the media
containing TC and GC increased 4- and 32-fold over those
excreted in the media without the bile acids (Fig. 1b). No such
increase, however, was observed for the strains grown in the
medium containing cholic acid (Fig. 1b). The presence of the
bile acids did not affect the purified TDH, in which the RPLA
titer remained constant (1:32) both before and after incubation
in any medium tested (Fig. 1b). Growth of K-10297 and
K-10300 and the RPLA titers measured in the spent BM supplemented with either glycine or taurine were comparable to
those in the unsupplemented spent BM (data not shown).
In the second experiment, the two TDH-producing strains,
K-10297 and K-10300, in HI broth supplemented with 2%
NaCl was inoculated into BM, BM1GC, and BM1TC (200 ml
each) at an initial CFU count of 2.0 3 105/ml and were incubated at 378C for 6 h with agitation. After incubation, the cells
were harvested by centrifugation (4,500 3 g, 10 min, 48C) and
washed three times in phosphate buffer (0.1 M KH2PO4 per
liter, pH 7.8) containing 5% NaCl. The bacterial pellet was
aseptically resuspended in 5 ml of the phosphate buffer to
obtain a dense bacterial suspension with an OD of 0.8 (CFU
ranging from 1.7 3 109 to 2.0 3 109 [Table 1]). The bacterial
cells thus prepared were lysed by sonication with cooling for 10
min, using a Bioruptor (Tosho Electric Co. Ltd., Tokyo, Japan). After sonication, the ODs of all cell suspensions decreased to less than 0.06, with a corresponding decrease in
CFU of less than 2.53 106 (Table 1), indicating that more than
99% of the cells were lysed by the sonication. The sonicated
aliquots were clarified by centrifugation (15,800 3 g, 20 min,
48C), and the supernatants were collected for a subsequent
assay by KAP-RPLA. The triplicate assays confirmed that the
amounts of TDH released from the lysed cells grown in
BM1GC and BM1TC increased 4- to 32-fold over those
grown in BM not containing the bile acids (Table 1).
The results of the present study demonstrate that the bile
acids enhance the production of TDH by V. parahaemolyticus.
FIG. 1. Effects of the bile acids on growth of three strains of V. parahaemolyticus (a) and corresponding RPLA titers of TDH excreted in the medium per equivalent
numbers of cells (OD at 660 nm, 0.1 or ca. 1.0 3 108 CFU/ml) over 18 h (b). Error bars indicate standard errors of the means. p, P , 0.05. RPLA titers are the results
of triplicate tests. THD 332, purified TDH whose KAP-RPLA titer is known to be 1:32. CH, cholic acid.
TDH is known to be a dimer of polypeptides which can be
fragmented by treatment with sodium dodecyl sulfate (SDS) to
yield two subunits, each of 21 kDa (17). Since the salts of GC
and TC have a strong detergent or surfactant property (18), we
initially considered the possibility that, like SDS, the salts
might split TDH into fragments, thereby increasing its total
immunoreactivity. This was, however, not the case since the
purified TDH sustained its original immunoreactivity even after a prolonged incubation with the bile salts.
It has been demonstrated for E. coli and C. perfringens that
GC and TC enhance the extracellular secretion of bacterial
b-glucuronidase possibly because of their biochemical property which increases cellular permeability (5). We thus presumed that the bile acids acted similarly on V. parahaemolyticus
cells, with a resultant increase of TDH in the spent medium.
Alternatively, we considered the possibility that the observed
TDH increase may simply be caused by active cell lysis of the
strains due to a strong detergent effect of the bile acids. However, the observed increase of TDH is more likely to reflect
enhanced synthesis of TDH within the bacterial cell rather
than increased cellular permeability or active cell lysis since the
lysate of intact V. parahaemolyticus cells grown with the bile
acids had a TDH level at least severalfold greater than that of
cells grown without. It has been reported for L. acidophilus
that a fresh bile solution made of oxgall increased the cellular
permeability of the bacterium, allowing more substrate to enter the cells (13). A similar course of events may apply to
TDH-producing V. parahaemolyticus, and we postulated that
glycine and taurine that can be derived from the conjugated
bile acids might be utilized by the bacterium as essential substrates for TDH synthesis. This possibility was, however, unlikely since the supplementation of the growth media with
amino acids failed to enhance TDH production. The evidence
provided in the present study suggests that the bile acids enhance synthesis of TDH within the cell through a mechanism
yet to be defined, with a tendency of TC having greater effect
than GC.
This article is the first report that conjugated bile acids
enhance TDH production by V. parahaemolyticus. This in turn
suggests that the bile acids play a key role in the pathogenicity
of V. parahaemolyticus during its natural infection of the human intestine. A similar effect may apply to other enterotoxinproducing bacteria, such as Vibrio cholerae, enterotoxigenic
E. coli, and C. perfringens. Further studies are currently in
progress to test this possibility.
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activity of intestinal bacteria. Lett. Appl. Microbiol. 22:271–274.
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10. McCarter, L. L., and M. Silverman. 1987. Phosphate regulation of gene
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15. Osawa, R., T. Okitsu, H. Morozumi, and S. Yamai. 1996. Occurrence of
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TABLE 1. Effects of GC and TC on amount of TDH produced by two V. parahaemolyticus strainsa
Strain Growthmedium
Value before sonication Value after sonication
K-10297 BM 0.80 (1.86 0.2) 3 109 0.05 6 0.01 (1.5 6 0.3) 3 106 1:16
BM1GC 0.80 (2.0 6 0.1) 3 109 0.05 6 0.01 (1.4 6 0.2) 3 106 1:64
BM1TC 0.80 (2.1 6 0.1) 3 109 0.05 6 0.02 (1.8 6 0.3) 3 106 1:128
K-10300 BM 0.80 (1.96 0.9) 3 109 0.05 6 0.01 (2.1 6 0.3) 3 106 1:8
BM1GC 0.80 (2.1 6 0.3) 3 109 0.06 6 0.01 (2.5 6 0.4) 3 106 1:256
BM1TC 0.80 (1.7 6 0.9) 3 109 0.06 6 0.02 (2.1 6 0.3) 3 106 1:256
aWhere applicable, GC and TC were used at a concentration of 5 mM/liter. TDH titers were determined by KAP-RPLA. All values are the results of triplicate tests.
VOL. 62, 1996 NOTES 3025

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