Nature of the Kanagawa phenomenon of Vibrio parahaemolyticus

Construction and characterization of different hemolysin gene deletion strains in Vibrio parahaemolyticus (ΔhlyA, ΔhlyIII) and evaluation of their virulence

Vibrio parahaemolyticus, a halophilic food-borne pathogen, possesses an arsenal of virulence factors. The pathogenicity of V. parahaemolyticus results from a combination of various virulence factors. HlyA and hlyIII genes are presumed to function in hemolysis, in addition to tdh and trh in V. parahaemolyticus. To confirm the hemolytic function of genes hlyA and hlyIII, ΔhlyA and ΔhlyIII strains of V. parahaemolyticus were separately constructed via homologous recombination. The cytotoxicity and pathogenicity of the ΔhlyA and ΔhlyIII strains were evaluated using a Tetrahymena-Vibrio co-culture model and an immersion challenge in Litopenaeus vannamei. Results indicated that the hemolytic activity of the ΔhlyA and ΔhlyIII strains decreased by approximately 31.4 % and 24.9 % respectively, compared to the WT strain. Both ΔhlyA and ΔhlyIII exhibited reduced cytotoxicity towards Tetrahymena. Then shrimp infection experiments showed LD50 values for ΔhlyA and ΔhlyIII of 3.06 × 108 CFU/mL and 1.23 × 108 CFU/mL, respectively, both higher than the WT strain's value of 2.57 × 107 CFU/mL. Histopathological observations revealed that hepatopancreas from shrimps challenged with ΔhlyA and ΔhlyIII exhibited mild symptoms, whereas those challenged with the WT strain displayed severe AHPND. These findings indicate that the ΔhlyA and ΔhlyIII strains are significantly less virulent than the WT strain. In conclusion, both hlyA and hlyIII are vital virulence genes involved in hemolytic and cytotoxic of V. parahaemolyticus.

Isolation and characterization of a Vibrio owensii phage phi50-12

Vibrio owensii is a widely distributed marine vibrio species that causes acute hepatopancreatic necrosis in the larvae of Panulirus ornatus and Penaeus vannamei, and is also associated with Montipora white syndrome in corals. We characterized V. owensii GRA50-12 as a potent pathogen using phenotypic, biochemical, and zebrafish models. A virulent phage, vB_VowP_phi50-12 (phi50-12), belonging to the N4-like Podoviridae, was isolated from the same habitat as that of V. owensii GRA50-12 and characterized. This phage possesses a unique sequence with no similar hits in the public databases and has a short latent time (30 min), a large burst size (106 PFU/infected cell), and a wide range of pH and temperature stabilities. Moreover, phi50-12 also demonstrated a strong lysis ability against V. owensii GRA50-12. SDS-PAGE revealed at least nine structural proteins, four of which were confirmed using LC–MS/MS analysis. The size of the phi50-12 genome was 68,059 bp, with 38.5% G + C content. A total of 101 ORFs were annotated, with 17 ORFs having closely related counterparts in the N4-like vibrio phage. Genomic sequencing confirmed the absence of antibiotic resistance genes or virulence factors. Comparative studies have shown that phi50-12 has a unique genomic arrangement, except for the well-conserved core regions of the N4-like phages. Phylogenetic analysis demonstrated that it belonged to a group of smaller genomes of N4-like vibrio phages. The therapeutic effect in the zebrafish model suggests that phi50-12 could be a potential candidate for application in the treatment of V. owensii infection or as a biocontrol agent. However, further research must be carried out to confirm the efficacy of phage50-12.

Two DsbA Proteins Are Important for Vibrio parahaemolyticus Pathogenesis

Bacterial pathogens maintain disulfide bonds for protein stability and functions that are required for pathogenesis. Vibrio parahaemolyticus is a Gram-negative pathogen that causes food-borne gastroenteritis and is also an important opportunistic pathogen of aquatic animals. Two genes encoding the disulfide bond formation protein A, DsbA, are predicted to be encoded in the V. parahaemolyticus genome. DsbA plays an important role in Vibrio cholerae virulence but its role in V. parahaemolyticus is largely unknown. In this study, the activities and functions of the two V. parahaemolyticus DsbA proteins were characterized. The DsbAs affected virulence factor expression at the post-translational level. The protein levels of adhesion factor VpadF (VP1767) and the thermostable direct hemolysin (TDH) were significantly reduced in the dsbA deletion mutants. V. parahaemolyticus lacking dsbA also showed reduced attachment to Caco-2 cells, decreased β-hemolytic activity, and less toxicity to both zebrafish and HeLa cells. Our findings demonstrate that DsbAs contribute to V. parahaemolyticus pathogenesis.

Comparative evaluation of a chromogenic agar medium-PCR protocol with a conventional method for isolation of Vibrio parahaemolyticus strains from environmental and clinical samples

Screening for pathogenic Vibrio parahaemolyticus has become routine in certain areas associated with food-borne outbreaks. This study is an evaluation of the CHROMagar Vibrio (CV) medium-PCR protocol and the conventional method (TCBS (thiosulfate-citrate-bile salts-sucrose) agar plus biochemical and Wagatsuma agar tests) for detection of V. parahaemolyticus in shrimp, water, sediment, and stool samples collected for biosurveillance in an endemic area of northwestern Mexico. A total of 131 environmental and clinical samples were evaluated. The CV medium-PCR protocol showed a significantly improved ability (P < 0.05) to isolate and detect V. parahaemolyticus, identifying isolates of this bacteria missed by the conventional method. Although some other bacteria, distinct from pathogenic V. parahaemolyticus, produced violet colonies similar to that of V. parahaemolyticus on CV medium, we were able to detect a superior number of samples of V. parahaemolyticus with the CV medium-PCR protocol than with the conventional method. The Kanagawa phenomenon is routinely determined on Wagatsuma agar for the diagnosis of V. parahaemolyticus (pathogenic) positive for thermostable direct hemolysin (TDH) in developing countries. In our results, Wagatsuma agar showed low sensitivity (65.4% at 24 h and 75.6% at 48 h) and specificity (52.4% at 48 h) for identifying V. parahaemolyticus positive for TDH. Overall, our data support the use of the CV medium-PCR protocol in place of the conventional method (TCBS-biochemical tests-Wagatsuma agar) for detection of pathogenic V. parahaemolyticus, both in terms of effectiveness and cost efficiency.

Insufficiency of the Kanagawa hemolytic test for detecting pathogenic Vibrio parahaemolyticus in Shanghai, China

We evaluated the Kanagawa hemolytic test and tdh gene test for accuracy in identifying pathogenic Vibrio parahaemolyticus isolates in Shanghai. One hundred and seventy-two V. parahaemolyticus isolates were collected from diarrhea patients, freshly harvested sea fish, or fresh water samples. Statistical data for the Kanagawa hemolytic test and tdh gene test were compared. There were 83.51% isolates (81/97) from patients and 22.22% isolates (10/45) from sea-fish positive for the tdh gene. However, none of 30 isolates from fresh water samples were tdh-positive. Positive Kanagawa hemolytic tests were obtained in 88.66%, 46.67%, and 76.67% of isolates, which were from patients, sea fish, and fresh water samples, respectively. Positive rates of the Kanagawa hemolytic tests and the tdh gene tests were significantly different in isolates from those 3 sources (P < 0.001). The tdh gene test showed higher specificity than the Kanagawa hemolytic test on identifying pathogenic V. parahaemolyticus isolates in Shanghai, China.

Development of two animal models to study the function of Vibrio parahaemolyticus type III secretion systems

Vibrio parahaemolyticus is an emerging food- and waterborne pathogen that encodes two type III secretion systems (T3SSs). Previous studies have linked type III secretion system 1 (T3SS1) to cytotoxicity and T3SS2 to intestinal fluid accumulation, but animal challenge models needed to study these phenomena are limited. In this study we evaluated the roles of the T3SSs during infection using two novel animal models: a model in which piglets were inoculated orogastrically and a model in which mice were inoculated in their lungs (intrapulmonarily). The bacterial strains employed in this study had equivalent growth rates and beta-hemolytic activity based on in vitro assays. Inoculation of 48-h-old conventional piglets with 10(11) CFU of the wild-type strain (NY-4) or T3SS1 deletion mutant strains resulted in acute, self-limiting diarrhea, whereas inoculation with a T3SS2 deletion mutant strain failed to produce any clinical symptoms. Intrapulmonary inoculation of C57BL/6 mice with the wild-type strain and T3SS2 deletion mutant strains (5 × 10(5) CFU) induced mortality or a moribund state within 12 h (80 to 100% mortality), whereas inoculation with a T3SS1 deletion mutant or a T3SS1 T3SS2 double deletion mutant produced no mortality. Bacteria were recovered from multiple organs regardless of the strain used in the mouse model, indicating that the mice were capable of clearing the lung infection in the absence of a functional T3SS1. Because all strains had a similar beta-hemolysin phenotype, we surmise that thermostable direct hemolysin (TDH) plays a limited role in these models. The two models introduced herein produce robust results and provide a means to determine how different T3SS1 and T3SS2 effector proteins contribute to pathogenesis of V. parahaemolyticus infection.

Comparison of the pathogenic potentials of environmental and clinical vibrio parahaemolyticus strains indicates a role for temperature regulation in virulence

Although the presence of pathogenic Vibrio spp. in estuarine environments of northern New England has been known for some time (C. H. Bartley and L. W. Slanetz, Appl. Microbiol. 21: 965-966, 1971, and K. R. O'Neil, S. H. Jones, and D. J. Grimes, FEMS Microbiol. Lett. 60:163-167, 1990), their virulence and the relative threat they may pose to human health has yet to be evaluated. In this study, the virulence potential of 33 Vibrio parahaemolyticus isolates collected from the Great Bay Estuary of New Hampshire was assessed in comparison to that of clinical strains. The environmental isolates lack thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH), which are encoded by tdh and trh, respectively. Though not hemolytic, they do possess putative virulence factors, such type III secretion system 1, and are highly cytotoxic to human gastrointestinal cells. The expression of known and putative virulence-associated traits, including hemolysin, protease, motility, biofilm formation, and cytotoxicity, by clinical reference isolates correlated with increased temperature from 28°C to 37°C. In contrast, the environmental isolates did not induce their putative virulence-associated traits in response to a temperature of 37°C. We further identified a significant correlation between hemolytic activity and growth phase among clinical strains, whereby hemolysin production decreases with increasing cell density. The introduction of a tdh::gfp promoter fusion into the environmental strains revealed that they regulate this virulence-associated gene appropriately in response to temperature, indicating that their existing regulatory mechanisms are primed to manage newly acquired virulence genes.

Serogroup, virulence, and genetic traits of Vibrio parahaemolyticus in the estuarine ecosystem of Bangladesh

Forty-two strains of Vibrio parahaemolyticus were isolated from Bay of Bengal estuaries and, with two clinical strains, analyzed for virulence, phenotypic, and molecular traits. Serological analysis indicated O8, O3, O1, and K21 to be the major O and K serogroups, respectively, and O8:K21, O1:KUT, and O3:KUT to be predominant. The K antigen(s) was untypeable, and pandemic serogroup O3:K6 was not detected. The presence of genes toxR and tlh were confirmed by PCR in all but two strains, which also lacked toxR. A total of 18 (41%) strains possessed the virulence gene encoding thermostable direct hemolysin (TDH), and one had the TDH-related hemolysin (trh) gene, but not tdh. Ten (23%) strains exhibited Kanagawa phenomenon that surrogates virulence, of which six, including the two clinical strains, possessed tdh. Of the 18 tdh-positive strains, 17 (94%), including the two clinical strains, had the seromarker O8:K21, one was O9:KUT, and the single trh-positive strain was O1:KUT. None had the group-specific or ORF8 pandemic marker gene. DNA fingerprinting employing pulsed-field gel electrophoresis (PFGE) of SfiI-digested DNA and cluster analysis showed divergence among the strains. Dendrograms constructed using PFGE (SfiI) images from a soft database, including those of pandemic and nonpandemic strains of diverse geographic origin, however, showed that local strains formed a cluster, i.e., "clonal cluster," as did pandemic strains of diverse origin. The demonstrated prevalence of tdh-positive and diarrheagenic serogroup O8:K21 strains in coastal villages of Bangladesh indicates a significant human health risk for inhabitants.

Rapid and specific detection of the thermostable direct hemolysin gene in Vibrio parahaemolyticus by loop-mediated isothermal amplification

Several investigators have reported that thermostable direct hemolysin (TDH) and TDH-related hemolysin are important virulence factors of Vibrio parahaemolyticus, but it has been difficult to detect these factors rapidly in seafood and other environmental samples. A novel nucleic acid amplification method, termed the loop-mediated isothermal amplification (LAMP), which amplifies DNA with high specificity and rapidity under isothermal conditions, was applied. In this study, we designed tdh gene-specific LAMP primers for detection of TDH-producing V. parahaemolyticus. The specificity of this assay was evaluated with 32 strains of TDH-producing V. parahaemolyticus, one strain of TDH-producing Grimontia hollisae, 10 strains of TDH-nonproducing V. parahaemolyticus, and 94 strains of TDH-nonproducing bacteria, and the sensitivity was high enough to detect one cell per test. Moreover, to investigate the detection of TDH-producing V. parahaemolyticus in oysters, the LAMP assay was performed with enrichment culture in alkaline peptone water of oyster samples inoculated with TDH-producing V. parahaemolyticus and TDH-nonproducing V. parahaemolyticus and V. alginolyticus after enrichment in alkaline peptone water. These results suggest that the LAMP assay targeting tdh gene has high sensitivity and specificity and is useful to detect TDH-producing V. parahaemolyticus in oyster after enrichment.

High potential of adhesion to abiotic and biotic materials in fish aquaculture facility by Vibrio alginolyticus strains

AIMS

The ability of Vibrio alginolyticus strains isolated from Sparus aurata and Dicentrarchus labrax nursery to adhere to epithelial cell lines (Hep-2 and Caco-2), fish mucus and their ability to form a biofilm on different surfaces (glass, polystyrene, polyethylene and polyvinyl-chloride) was investigated in this study.

METHODS AND RESULTS

The extracellular products were rich in enzymes and the strains were haemolytic on Wagatsuma agar and possessed several hydrolytic exoenzymes such as proteases, DNase and lipases. Most strains tested were multiresistant to the 17 antibiotics tested including those used in the farm to treat vibriosis.

CONCLUSIONS

These bacteria were able to form a biofilm on all the surfaces tested and the cell density was the highest on the PVC surface followed by that on the glass slides, polystyrene and the polyethylene surface. More than 50% of the tested strains were adhesive to the epithelial cell lines (Hep-2 and Caco-2).

SIGNIFICANCE AND IMPACT OF THE STUDY

These properties allow these bacteria to survive, proliferate and persist in all stages of fish rearing nursery even after seawater treatment with UV light.

Tetrameric Structure of Thermostable Direct Hemolysin from Vibrio parahaemolyticus Revealed by Ultracentrifugation, Small-angle X-ray Scattering and Electron Microscopy

The thermostable direct hemolysin (TDH) is a major virulence factor of Vibrio parahaemolyticus. We have characterized the conformational properties of TDH by small-angle X-ray scattering (SAXS), ultracentrifugation and transmission electron microscopy. Sedimentation equilibrium and velocity studies revealed that the protein is tetrameric in aqueous solvents. The Guinier plot derived from SAXS data provided a radius of gyration of 29.0 A. The elongated pattern with a shoulder of a pair distance distribution function derived from SAXS data suggested the presence of molecules with an anisotropic shape having a maximum diameter of 98 A. Electron microscopic image analysis of the negatively stained TDH oligomer showed the presence of C(4) symmetric particles with edge and diagonal lengths of 65 A and 80 A, respectively. Shape reconstruction was carried out by ab initio calculations using the SAXS data with a C(4) symmetric approximation. These results suggested that the tetrameric TDH assumes an oblate structure. The hydrodynamic parameters predicted from the ab initio model differed slightly from the experimental values, suggesting the presence of flexible segments.

Isolation and molecular characterization of toxigenic Vibrio parahaemolyticus from the Kii Channel, Japan

Studies were conducted on the ecology of potentially pathogenic Vibrio parahaemolyticus in three coastal areas of Kii Channel, Tokushima, Japan. Seawater and seaweed samples were collected seasonally between June 2003 and May 2004. Total and toxigenic strains of V. parahaemolyticus were isolated using most probable number culture and colony blot hybridization. Toxigenic strains were serotyped and further characterized by random amplified polymorphic DNA (RAPD) and ribotyping. Six thousand strains of V. parahaemolyticus were isolated and 18 were found positive for tdh. V. parahaemolyticus were detected in all samples during summer and autumn, and from some samples during winter and spring. Among the toxigenic strains seven serotypes, five ribotypes and RAPD patterns were observed. Seven strains belonged to O3:K6 clone with identical ribotypes and RAPD patterns to that of a pandemic reference strain. The presence of toxigenic V. parahaemolyticus with pandemic potential might indicate a human health risk due to consumption of marine food sources.

Pandemic strains of O3:K6Vibrio parahaemolyticusin the aquatic environment of Bangladesh

A total of 1500 environmental strains of Vibrio parahaemolyticus, isolated from the aquatic environment of Bangladesh, were screened for the presence of a major V. parahaemolyticus virulence factor, the thermostable direct haemolysin (tdh) gene, by the colony blot hybridization method using a digoxigenin-labeled tdh gene probe. Of 1500 strains, 5 carried the tdh sequence, which was further confirmed by PCR using primers specific for the tdh gene. Examination by PCR confirmed that the 5 strains were V. parahamolyticus and lacked the thermostable direct haemolysin-related haemolysin (trh) gene, the alternative major virulence gene known to be absent in pandemic strains. All 5 strains gave positive Kanagawa phenomenon reaction with characteristic β-haemolysis on Wagatsuma agar medium. Southern blot analysis of the HindIII-digested chromosomal DNA demonstrated, in all 5 strains, the presence of 2 tdh genes common to strains positive for Kanagawa phenomenon. However, the 5 strains were found to belong to 3 different serotypes (O3:K29, O4:K37, and O3:K6). The 2 with pandemic serotype O3:K6 gave positive results in group-specific PCR and ORF8 PCR assays, characteristics unique to the pandemic clone. Clonal variations among the 5 isolates were analyzed by comparing RAPD and ribotyping patterns. Results showed different patterns for the 3 serotypes, but the pattern was identical among the O3:K6 strains. This is the first report on the isolation of pandemic O3:K6 strains of V. parahaemolyticus from the aquatic environment of Bangladesh.Key words: pandemic strains, Vibrio parahaemolyticus, aquatic environment.

Effect of Vibrio parahaemolyticus haemolysin on human erythrocytes

Haemolysin Kanagawa, a toxin from Vibrio parahaemolyticus, is known to trigger haemolysis. Flux studies indicated that haemolysin forms a cation channel. In the present study, channel properties were elucidated by patch clamp and functional significance of ion fluxes by fluorescence-activated cell sorting (FACS) analysis. Treatment of human erythrocytes with 1 U ml-1 haemolysin within minutes induces a non-selective cation permeability. Moreover, haemolysin activates clotrimazole-sensitive K+ channels, pointing to stimulation of Ca2+-sensitive Gardos channels. Haemolysin (1 U ml-1) leads within 5 min to slight cell shrinkage, which is reversed in Ca2+-free saline. Erythrocytes treated with haemolysin (0.1 U ml-1) do not undergo significant haemolysis within the first 60 min. Replacement of extracellular Na+ with NMDG+ leads to slight cell shrinkage, which is potentiated by 0.1 U ml-1 haemolysin. According to annexin binding, treatment of erythrocytes with 0.1 U ml-1 haemolysin leads within 30 min to breakdown of phosphatidylserine asymmetry of the cell membrane, a typical feature of erythrocyte apoptosis. The annexin binding is significantly blunted at increased extracellular K+ concentrations and by K+ channel blocker clotrimazole. In conclusion, haemolysin Kanagawa induces cation permeability and activates endogenous Gardos K+ channels. Consequences include breakdown of phosphatidylserine asymmetry, which depends at least partially on cellular loss of K+.

Production of thermostable direct hemolysin by Vibrio parahaemolyticus enhanced by conjugated bile acids

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.

Evaluation of two assay kits for thermostable direct hemolysin (TDH) as an indicator of TDH-related hemolysin (TRH) produced by Vibrio parahaemolyticus

Reversed passive latex agglutination (RPLA) or enzyme-linked immunosorbent assay kits with beads (Bead-ELISA) are commercially available in Japan to detect the thermostable direct hemolysin (TDH) produced by Vibrio parahaemolyticus isolates. We evaluated whether these kits can be used to assay the pathogenic toxin, TDH-related hemolysin (TRH), produced by some so-called Kanagawa phenomenon-negative V. parahaemolyticus strains isolated from patients with diarrhea. Our results showed that the two kits, RPLA and Bead-ELISA, can detect TRH, although they were originally developed for detection of TDH. This may be due to the use of polyclonal anti-TDH antisera that cross react with TRH. Although the sensitivity for TDH detection by RPLA and Bead-ELISA differed tenfold, that for TRH detection was essentially equal. The minimum concentration of TRH required for detection by the two assay kits was about 10 ng/ml.

Vibrio parahaemolyticus has a homolog of the Vibrio cholerae toxRS operon that mediates environmentally induced regulation of the thermostable direct hemolysin gene

In an effort to identify the regulatory gene controlling the expression of the tdh gene, encoding the thermostable direct hemolysin of Vibrio parahaemolyticus, we examined total DNA of AQ3815 (a Kanagawa phenomenon-positive strain) for sequences homologous to that of the toxR gene of Vibrio cholerae. The extracted DNA gave a weak hybridization signal under reduced-stringency conditions with a toxR-specific DNA probe. Cloning and sequence analysis of the probe-positive sequence revealed an operon (Vp-toxRS) which was highly similar to the toxRS operon of V. cholerae (Vc-toxRS) (52 and 62% similarities in the two genes, respectively). The deduced amino acid sequences of the Vp-toxRS gene products (Vp-ToxRS) contained regions similar to the proposed transmembrane and activity domains of the Vc-toxRS gene products (Vc-ToxRS). All clinical and environmental strains of V. parahaemolyticus examined possessed the Vp-toxRS genes. In the presence of Vp-ToxS, Vp-ToxR promoted expression of the tdh2 gene, one of two tdh genes (tdh1 and tdh2) carried by Kanagawa phenomenon-positive strains. The DNA sequence located 144 bp upstream of the tdh2 coding region was shown to be important for the Vp-ToxR-stimulated expression of the tdh2 gene in an Escherichia coli background. Comparative analysis of AQ3815 and its isogenic Vp-toxR null mutant gave the following results: (i) Vp-ToxR promoted, in an AQ3815 background, expression of the tdh gene to different degrees in various culture media, with KP broth (2% peptone, 0.5% NaCl, 0.03 M KH2PO4, pH 6.2) being most effective (12-fold); (ii) the promotion of tdh gene expression in KP broth was at the level of transcription; and (iii) Vp-ToxR was essential for demonstration of enterotoxic activity of AQ3815 in the rabbit ileal loop, a model previously used to demonstrate thermostable direct hemolysin-mediated enterotoxic activity of AQ3815. These results demonstrate that Vp-ToxR and Vc-ToxR share a strikingly similar function, i.e., direct stimulation at the transcriptional level of the gene encoding a major virulence determinant (enterotoxin) of a Vibrio species.

Production of monoclonal antibodies against thermostable direct hemolysin of Vibrio parahaemolyticus and application of the antibodies for enzyme-linked immunosorbent assay

A total nine hybridoma cell lines that produced monoclonal antibodies against thermostable direct hemolysin (Vp-TDH), a possible pathogenic toxin, of Kanagawa phenomenon-positive Vibrio parahaemolyticus was isolated and characterized. These monoclonal antibodies (mAbs) were divided into a minimum of five different specificity groups, including mAbs specific to Vp-TDH and common to Vp-TDH and Vp-TRH, a Vp-TDH-related hemolysin produced by Kanagawa phenomenon-negative V. parahaemolyticus. An enzyme-linked immunosorbent assay (ELISA) using mAb-1-D, a mAb specific for Vp-TDH, was developed for specific detection of Vp-TDH. On the other hand, the ELISA using mAb-9-D, and mAb common to both Vp-TDH and Vp-TRH, could be used for detection of both Vp-TDH and Vp-TRH. Thus, by combining these two ELISAs differential detection of Vp-TDH and Vp-TRH can be performed. Hence, the two ELISAs were applied for various strains of V. parahaemolyticus and it was found that most Kanagawa phenomenon-positive and -negative clinical isolates produced Vp-TDH and Vp-TRH, respectively, but all environmental strains, that were Kanagawa phenomenon-negative, produced neither toxin.

Hemolytic activity of and lethal toxin production by environmental strains of Vibrio parahaemolyticus

Repeated subculturing of Kanagawa-negative strains of Vibrio parahaemolyticus on Wagatsuma agar induced the production of a hemolysin which was not the thermostable direct hemolysin. Crude hemolysin exhibited a 30 to 40% lethal toxicity in mice after intraperitoneal injection. A 21-kilodalton protein band was observed with all the environmental isolates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Results suggested that a certain percentage of environmental strains of V. parahaemolyticus is responsible for pathogenesis.

Comparison of the modified Elek test and Wagatsuma agar for determination of the Kanagawa phenomenon of Vibrio parahaemolyticus

The modified Elek test and Wagatsuma agar were compared for their ability to detect the Kanagawa activity of 142 strains of Vibrio parahaemolyticus. The performance of the modified Elek test was on a par with that of the Wagatsuma agar as far as positivity was concerned, and the test was far superior to Wagatsuma agar in eliminating doubtful results. The results of the modified Elek test were not unduly influenced by the different types of agar used.

Detection of the thermostable direct hemolysin gene and related DNA sequences in Vibrio parahaemolyticus and other vibrio species by the DNA colony hybridization test

A specific gene probe for the Vibrio parahaemolyticus thermostable direct hemolysin gene was constructed and used to examine the presence or absence of the thermostable direct hemolysin gene or related DNA sequences in V. parahaemolyticus and other vibrios by the DNA colony hybridization method. The gene probe consisted of a 406-base-pair, completely internal fragment covering 71% of the structural gene with PstI linkers added to the ends. Six copies of this 415-base-pair PstI fragment were cloned into plasmid pBR322, which yielded large amounts of the probe DNA. One hundred forty-one V. parahaemolyticus strains were tested with the gene probe, and the results were compared with those of phenotypic assays for the thermostable direct hemolysin. All Kanagawa phenomenon-positive strains were gene positive. However, 86% of the strains that exhibited weak Kanagawa phenomenon and 16% of Kanagawa phenomenon-negative strains also reacted with the gene probe. Immunological methods for the detection of the thermostable direct hemolysin (modified Elek test, enzyme-linked immunosorbent assay) showed better correlation with gene probe results. All gene-positive strains produced hemolysin detectable in the enzyme-linked immunosorbent assay, although occasional strains showed weak reaction. The modified Elek test was slightly less sensitive than the enzyme-linked immunosorbent assay. All gene-negative strains were also negative in these immunological assays. One hundred twenty-one strains of Vibrio spp. other than V. parahaemolyticus were tested with the gene probe; only Vibrio hollisae strains reacted with the probe under stringent conditions.

Nucleotide sequence of the thermostable direct hemolysin gene of Vibrio parahaemolyticus

The gene encoding the thermostable direct hemolysin of Vibrio parahaemolyticus was characterized. This gene (designated tdh) was subcloned into pBR322 in Escherichia coli, and the functional tdh gene was localized to a 1.3-kilobase HindIII fragment. This fragment was sequenced, and the structural gene was found to encode a mature protein of 165 amino acid residues. The mature protein sequence was preceded by a putative signal peptide sequence of 24 amino acids. A putative tdh promoter, determined by its similarity to concensus sequences, was not functional in E. coli. However, a promoter that was functional in E. coli was shown to exist further upstream by use of a promoter probe plasmid. A 5.7-kilobase SalI fragment containing the structural gene and both potential promoters was cloned into a broad-host-range plasmid and mobilized into a Kanagawa phenomenon-negative V. parahaemolyticus strain. In contrast to E. coli, where the hemolysin was detected only in cell lysates, introduction of the cloned gene into V. parahaemolyticus resulted in the production of extracellular hemolysin.

Vibrio parahaemolyticus and related halophilic Vibrios

Approximately 30 years have elapsed since Dr. Fujino's original discovery that Vibrio parahaemolyticus (then termed Pasteurella parahemolytica) was the cause of "summer diarrhea" in Japan. Since that finding, V. parahaemolyticus has been established as a cause of gastroenteritis in numbers and places approaching global proportions. It has been isolated in marine and estuarine areas almost worldwide and despite its halophilic nature, V. parahaemolyticus has been isolated from saline-free waters. The relationship of this organism to the environment reveals a close association with other marine organisms especially copepods on which the Vibrios depend for survival in winter months and growth in summer months. There is a uniquely provocative disparity between human strains of V. parahaemolyticus which are Kanagawa phenomenon (KP) positive and the environmental strains which to a large extent are KP negative, the significance being that pathogenicity is measured according to the Kanagawa phenomenon (hemolytic activity) reaction. The hemolysin of the pathogenic strains is a thermostable, cardiotoxic protein, which thus far has not been implicated in the mechanism(s) which causes human gastroenteritis. The interest in this organism has been widened in recent years by the finding that similar organisms, V. alginolyticus, lactose positive vibrios and group F vibrios also cause serious disease in humans.

Effect of pH on the production of the Kanagawa hemolysin by Vibrio parahaemolyticus

Production of the Kanagawa hemolysin by patient strains of Vibrio parahaemolyticus was found to respond to the pH rather than to the type of carbohydrate present in the growth medium. Regardless of the carbohydrate present, hemolysin production in peptone broth cultures occurred only when the pH was between 6.5 and 5.5. Mannitol, the sugar used in the Wagatsuma agar, lowered the pH to within this range, thus providing optimal conditions for hemolysin production. Glucose and mannose, although readily metabolized, lowered the pH below this range, inhibiting growth and hemolysin production. Alkaline cultures either without carbohydrates or containing non-metabolizable sugars showed little hemolytic activity because the pH always remained alkaline. In pH-stat cultures maintained at pH 6.2, higher hemolysin yields were produced irrespective of the presence or absence of mannitol. We conclude that the production of the Kanagawa hemolysin is under pH control. Marine strains of V. parahaemolyticus, which are Kanagawa negative, did not express detectable amounts of hemolysin under those conditions shown to stimulate hemolysin production by Kanagawa-positive strains.

Immunological methods for detection of Kanagawa phenomenon of Vibrio parahaemolyticus

A modified Elek test and an immuno-halo test were developed for detection of the thermostable direct hemolysin produced by Kanagawa phenomenon-positive strains of Vibrio parahaemolyticus. The sensitivity of the modified Elek test was almost the same as that of the Kanagawa phenomenon on Wagatsuma medium, and the results of both reactions correlated very well. The immuno-halo test was a little less sensitive than the modified Elek test. Since the Kanagawa phenomenon on Wagatsuma medium sometimes give false-positive results due to instability of the blood used in the medium, we recommend the immunological techniques described here as reproducible methods for identification of the thermostable direct hemolysin by V. parahaemolyticus.

Isolation of cryptic plasmid deoxyribonucleic acid from Kanagawa-positive strains of Vibrio parahaemolyticus

Twelve strains of Vibrio parahaemolyticus was examined for plasmid deoxyribonucleic acid (DNA) by dye-buoyant gradient centrifugation. Four Kanagawa-positive strains, all isolated from the same outbreak of gastroenteritis, contained multiple plasmid species of cryptic function. However, three Kanagawa-negative strains and five Kanagawa-positive strains were not found to contain demonstrable plasmid DNA. R-plasmids were successfully transferred from Escherichia coli to V. parahaemolyticus.

Effect of carbohydrate source on Kanagawa-type hemolysis of Vibrio parahaemolyticus

Almost all strains of Vibrio parahaemolyticus produced Kanagawa-type hemolysis on media of high salt content in the presence of fermentable carbohydrates.