Presence of genes for type III secretion system 2 in Vibrio mimicus strains

Background: Vibrios, which include more than 100 species, are ubiquitous in marine and estuarine environments, and several of them e. g. Vibrio cholerae, V. parahaemolyticus, V. vulnificus and V. mimicus, are pathogens for humans. Pathogenic V. parahaemolyticus strains possess two sets of genes for type III secretion system (T3SS), T3SS1 and T3SS2. The latter are critical for virulence of the organism and be classified into two distinct phylogroups, T3SS2 alpha and T3SS2 beta, which are reportedly also found in pathogenic V. cholerae non-O1/non-O139 serogroup strains. However, whether T3SS2-related genes are present in other Vibrio species remains unclear. Results: We therefore examined the distribution of the genes for T3SS2 in vibrios other than V. parahaemolyticus by using a PCR assay targeting both T3SS2 alpha and T3SS2 beta genes. Among the 32 Vibrio species tested in our study, several T3SS2-related genes were detected in three species, V. cholerae, V. mimicus and V. hollisae, and most of the essential genes for type III secretion were present in T3SS2-positive V. cholerae and V. mimicus strains. Moreover, both V. mimicus strains possessing T3SS2 alpha and T3SS2 beta were identified. The gene organization of the T3SS2 gene clusters in V. mimicus strains was fundamentally similar to that of V. parahaemolyticus and V. cholerae in both T3SS2 alpha- and T3SS2 beta-possessing strains. Conclusions: This study is the first reported evidence of the presence of T3SS2 gene clusters in V. mimicus strains. This finding thus provides a new insight into the pathogenicity of the V. mimicus species.