Failure of antisera to thermostable direct hemolysin and cholera enterotoxin to prevent accumulation of fluid caused by Vibrio parahaemolyticus

Contribution of Vibrio parahaemolyticus virulence factors to cytotoxicity, enterotoxicity, and lethality in mice

Vibrio parahaemolyticus, one of the human-pathogenic vibrios, causes three major types of clinical illness: gastroenteritis, wound infections, and septicemia. Thermostable direct hemolysin (TDH) secreted by this bacterium has been considered a major virulence factor of gastroenteritis because it has biological activities, including cytotoxic and enterotoxic activities. Previous reports revealed that V. parahaemolyticus strain RIMD2210633, which contains tdh, has two sets of type III secretion system (T3SS) genes on chromosomes 1 and 2 (T3SS1 and T3SS2, respectively) and that T3SS1 is responsible for cytotoxicity and T3SS2 is involved in enterotoxicity, as well as in cytotoxic activity. However, the relative importance and contributions of TDH and the two T3SSs to V. parahaemolyticus pathogenicity are not well understood. In this study, we constructed mutant strains with nonfunctional T3SSs from the V. parahaemolyticus strain containing tdh, and then the pathogenicities of the wild-type and mutant strains were evaluated by assessing their cytotoxic activities against HeLa, Caco-2, and RAW 264 cells, their enterotoxic activities in rabbit ileal loops, and their lethality in a murine infection model. We demonstrated that T3SS1 was involved in cytotoxic activities against all cell lines used in this study, while T3SS2 and TDH had cytotoxic effects on a limited number of cell lines. T3SS2 was the major contributor to V. parahaemolyticus-induced enterotoxicity. Interestingly, we found that both T3SS1 and TDH played a significant role in lethal activity in a murine infection model. Our findings provide new indications that these virulence factors contribute to and orchestrate each distinct aspect of the pathogenicity of V. parahaemolyticus.

Isolation and characterization of a toxic intracellular protein from Vibrio parahaemolyticus

A toxic factor released from disrupted cells of Vibrio parahaemolyticus was partially purified by gel filtration after precipitation with (NH4)2SO4 at 40% saturation. The factor, which was a thermostable protein of 63 kDa, lysed human erythrocytes at a concentration of 0.15 μg ml(-1). Its LD50 by intravenous injection into mice was 6.4 μg. Fluid accumulated in suckling mice force-fed with the toxic material (1 to 25 μg). Haemolytic activity, which occurred maximall at 37°C and pH 7.0 was enhanced by Ca(2+), Cu(2+) and Zn(2+), each at 1 mM. Anti-toxic-factor serum agglutinated V. parahaemolyticus cells. The factor may play a role in the pathogenesis of V. parahaemolyticus infections and in the host's defence mechanisms against infection by the microorganism.

Enterotoxigenicity of Vibrio parahaemolyticus with and without genes encoding thermostable direct hemolysin

Vibrio parahaemolyticus produces a thermostable direct hemolysin (TDH) that has been implicated in the pathogenesis of diarrheal disease caused by this organism. However, previous studies attempting to demonstrate the contribution of the hemolysin to virulence have been inconclusive. We investigated this putative virulence factor by using an isogenic TDH-negative (TDH-) strain constructed by specifically inactivating the two copies of the tdh gene encoding TDH. The enterotoxigenicities of the parent strain (AQ3815) and the mutant strain were tested by adding sterile culture supernatants to rabbit ileal tissue mounted in Ussing chambers. The culture filtrate of the parent strain produced a significant increase in short-circuit current (Isc), compared with the change induced by the TDH- mutant. The capacity of the culture filtrate of AQ3815 to increase the Isc was reduced by neutralization with anti-TDH serum, and the return of the cloned tdh gene to the TDH- mutant restored the ability to increase the Isc. These results were corroborated by rabbit ileal loop assays in which AQ3815 caused fluid accumulation but the TDH- mutant did not. No microscopic damage was seen in mucosal tissues exposed to the culture filtrate of either strain. These results indicate that TDH has an enterotoxigenic effect on rabbit small intestine and could be responsible for the watery diarrhea seen with V. parahaemolyticus.

The extracellular cytolysin of Vibrio vulnificus: inactivation and relationship to virulence in mice

The ca. 51-kDa extracellular cytolysin of Vibrio vulnificus has been proposed as a virulence factor. We inactivated the structural gene for cytolysin in fully virulent, clinical V. vulnificus strains by both transposon mutagenesis and marker exchange techniques. Inactivation of the cytolysin did not affect virulence in our mouse models. The 50% lethal dose of cytolysin-negative strains was comparable to that of the cytolysin-positive parent strains after intraperitoneal inoculation with and without iron loading. Similar results were obtained after intradermal injection: cytolysin-positive and -negative strains had the same 50% lethal dose and caused comparable tissue damage. While we cannot say that the cytolysin has no effect on the pathogenesis of V. vulnificus infections, its role appears to be of much less importance than are other factors, such as encapsulation.

Current perspectives on the epidemiology and pathogenesis of clinically significant Vibrio spp

Recent taxonomic advances have now implicated several different Vibrio species as human pathogens. While the most common clinical presentation of Vibrio infection continues to be gastroenteritis, an increasing number of extraintestinal infections are being reported, particularly in immunocompromised individuals. Detection of Vibrio infections requires a good clinical history and the use of appropriate isolation and identification procedures by the laboratory to confirm illnesses attributed to Vibrio species. Except for Vibrio cholerae O1 and Vibrio parahaemolyticus, there is little direct evidence linking the production of a myriad of cell-associated or extracellular factors produced by each species with human disease and pathogenesis. Many questions regarding pathogenic Vibrio species remain unanswered, including their frequency and distribution in environmental specimens (water, shellfish), infective doses, virulence potential of individual isolates, and markers associated with such strains.

Enzyme-linked immunosorbent assays for detection of thermostable direct hemolysin of Vibrio parahaemolyticus

Several systems for enzyme-linked immunosorbent assay (ELISA) of thermostable direct hemolysin (TDH) of Vibrio parahaemolyticus were tested, and single-antibody sandwich ELISA systems gave satisfactory results. ELISA was able to detect as little as several nanograms of purified TDH per milliliter. The method of De Jong (J. Clin. Microbiol. 17:928-930, 1983) and the glutaraldehyde method were successful for preparing conjugates of alkaline phosphatase and anti-TDH antibody. TDH in fluids in intestinal loops of experimental animals challenged with living V. parahaemolyticus was accurately detectable by ELISA.

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.