Genomic insights into mechanism underlying virulence variations between Vibrio mediterranei strains different in pathogenicity toward bivalves

Larval vibriosis, also known as bacillary necrosis, is a widespread bacterial disease that affects the larval development stages of mollusks. It often results in significant mortality among larvae and juvenile mollusks, leading to considerable economic losses. In 2019, during a mass mortality event of razor clam (Sinonovacula constricta) juveniles at a shellfish hatchery, Vibrio mediterranei was identified as the causative agent of larval vibriosis. Notably, the observed pathogenicity was not a species-specific characteristic of the Vibrio genus. To investigate the pathogenesis of V. mediterranei strains, we performed whole genome sequencing, genomic comparative analysis, and biological comparisons of strains with differing virulence, and explored the putative main virulence factors of V. mediterranei. Our findings showed that V. mediterranei strains exhibited similar GC content (∼44 %) and genomic compositions, including two chromosomes and varying numbers of plasmids. However, the high virulence strains have larger genome size, more genes, prophages and virulence-associated genes. Additionally, V. mediterranei strains with different virulence exhibited varying capacities for adhesion and biofilm formation on solid surfaces. Genomic comparisons revealed a strain-specific pathogenicity island (PAI), termed TCP-PAI, in high virulence strains. This PAI shared a similar genomic structure and amino acid identity with the key virulence factor, the TCP gene cluster, found in the human pathogen Vibrio cholerae. A comparative analysis of twenty-one V. mediterranei strains further confirmed the presence of TCP-PAI in different pathogenic strains, highlighting the role of TCP-PAI in the pathogenicity of V. mediterranei toward bivalves. This study is the first to identify a key virulence factor in V. mediterranei linked to the TCP gene cluster of V. cholerae, providing a basis for further investigation into the pathogenic mechanisms of vibriosis in bivalves.