Defensive effects of human intestinal antimicrobial peptides against infectious diseases caused by Vibrio mimicus and V. vulnificus

Of human pathogenic Vibrio species, V. mimicus causes gastroenteritis whereas V. vulnificus causes fatal septicemia after consumption of contaminated seafood. These two pathogens produce hemolytic toxins termed V. mimicus hemolysin (VMH) and V. vulnificus hemolysin (VVH), respectively. These toxins elicit the cytolysis of various eukaryotic cells, as well as erythrocytes. The human intestine secretes cationic antimicrobial peptides (AMPs) to prevent infectious diseases. Paneth cells in the small intestine secrete α-defensin 5 (HD-5) and epithelial cells in the large intestine produce LL-37. In the present study, we examined the bactericidal activities of AMPs against V. mimicus and V. vulnificus. Although HD-5 showed no bactericidal activity, LL-37 revealed significant activity against both Vibrio species, suggesting that neither V. mimicus nor V. vulnificus can multiply in the large intestine. We also tested whether AMPs had the ability to inactivate the hemolytic toxins. Only HD-5 was found to inactivate VMH, but not VVH, in a dose-dependent manner through the direct binding to VMH. Therefore, it is considered that V. mimicus cannot penetrate the small intestinal epithelium because the cytolytic action of VMH is inactivated by HD-5.

A novel extracellular protease of Vibrio mimicus that mediates maturation of an endogenous hemolysin

Vibrio mimicus, a human pathogen that causes gastroenteritis, produces an enterotoxic hemolysin as a virulence factor. The hemolysin is secreted extracellularly as an inactive protoxin and converted to a mature toxin through removal of the N-terminal propeptide, which comprises 151 amino acid residues. In this study, a novel protease having the trypsin-like substrate specificity was purified from the bacterial culture supernatant. The N-terminal amino acid sequence of the purified protein was identical with putative trypsin VMD27150 of V. mimicus strain VM573. The purified protease was found to cause maturation of the protoxin by cleavage of the Arg(151)-Ser(152) bond. Deletion of the protease gene resulted in increased amounts of the protoxin in the culture supernatant. In addition, expression of the hemolysin and protease genes was detected during the logarithmic growth phase. These findings indicate that the protease purified may mediate maturation of the hemolysin.

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, T3SS2α and T3SS2β, 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 T3SS2α and T3SS2β 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 T3SS2α and T3SS2β 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 T3SS2α- and T3SS2β-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.