Total number, virulence genes, and heterogeneity of Vibrio parahaemolyticus in a single shellfish

Infection by Vibrio parahaemolyticus is established if seafood contaminated with pathogenic strains of this bacterium at a sufficiently high dose is consumed. We reported recently that V parahaemolyticus isolates from 67% of patients constituted a homogeneous population of a pathogenic strain and a heterogeneous population of V parahaemolyticus varying in the serotype and/or the virulence gene profile found in 33% of the patients. To assess the possibility that this result was due to the dominant presence of a homogeneous strain of V parahaemolyticus in shellfish, we enumerated and characterized V parahaemolyticus in shellfish samples including three kinds of molluscan bivalves. Using CHROMagar Vibrio isolation medium, presumed total Vibrio strains and presumed V parahaemolyticus in each of 52 shellfish samples were enumerated. The highest numbers of presumed total Vibrio strains and presumed V parahaemolyticus were detected in bloody clam and mussel, respectively. There was no correlation between the number of presumed total Vibrio strains and that of presumed V parahaemolyticus. Five to ten strains of V parahaemolyticus isolated from each shellfish sample were characterized. Virulence genes (tdh and trh) could be detected in none of the isolated strains. Diversities in the serotype and DNA fingerprints were confirmed among the isolated strains from a single shellfish sample representing six different shellfish groups. We therefore conclude that most of the single shellfish harbour a pathogenic strain-bearing heterogeneous population of V parahaemolyticus and the concentrations of pathogenic strains are low.