Comparative Genomics of Aeromonas hydrophila Secretion Systems and Mutational Analysis of hcp1 and vgrG1 Genes From T6SS

Abstract

Virulent <i>Aeromonas hydrophila</i> causes severe motile <i>Aeromonas</i> septicemia in warmwater fishes. In recent years, channel catfish farming in the U.S.A. and carp farming in China have been affected by virulent <i>A. hydrophila</i>, and genome comparisons revealed that these virulent <i>A. hydrophila</i> strains belong to the same clonal group. Bacterial secretion systems are often important virulence factors; in the current study, we investigated whether secretion systems contribute to the virulent phenotype of these strains. Thus, we conducted comparative secretion system analysis using 55 <i>A. hydrophila</i> genomes, including virulent <i>A. hydrophila</i> strains from U.S.A. and China. Interestingly, tight adherence (TaD) system is consistently encoded in all the vAh strains. The majority of U.S.A. isolates do not possess a complete type VI secretion system, but three core elements [<i>tssD</i> (<i>hcp</i>), <i>tssH</i>, and <i>tssI</i> (<i>vgrG</i>)] are encoded. On the other hand, Chinese isolates have a complete type VI secretion system operon. None of the virulent <i>A. hydrophila</i> isolates have a type III secretion system. Deletion of two genes encoding type VI secretion system proteins (<i>hcp1</i> and <i>vgrG1</i>) from virulent <i>A. hydrophila</i> isolate ML09-119 reduced virulence 2.24-fold in catfish fingerlings compared to the parent strain ML09-119. By determining the distribution of genes encoding secretion systems in <i>A. hydrophila</i> strains, our study clarifies which systems may contribute to core <i>A. hydrophila</i> functions and which may contribute to more specialized adaptations such as virulence. Our study also clarifies the role of type VI secretion system in <i>A. hydrophila</i> virulence.