The temperature-dependent expression of type II secretion system controls extracellular product secretion and virulence in mesophilic SRW-OG1.

is a typical cold water bacterial pathogen that causes furunculosis in many freshwater and marine fish species worldwide. In our previous study, the pathogenic (SRW-OG1) was isolated from a warm water fish, was genomics and transcriptomics analyzed. Type II secretion system was found in the genome of SRW-OG1, while the expressions of , and were significantly affected by temperature stress. Also, sequence alignment analysis, homology analysis and protein secondary structure function analysis showed that , , and were highly conservative, indicating their biological significance. In this study, by constructing the mutants of , and , we investigated the mechanisms underlying temperature-dependent virulence regulation in mesophilic SRW-OG1. According to our results, , , and mutants presented a distinct reduction in adhesion, hemolysis, biofilm formation and motility. Compared to wild-type strain, inhibition of the expression of , , and resulted in a decrease in biofilm formation by about 23.66%, 19.63% and 40.13%, and a decrease in adhesion ability by approximately 77.69%, 80.41% and 62.14% compared with that of the wild-type strain. Furthermore, , , and mutants also showed evidently reduced extracellular enzymatic activities, including amylase, protease, lipase, hemolysis and lecithinase. The genes affecting amylase, protease, lipase, hemolysis, and lecithinase of SRW-OG1 were identified as , , , , , , , , and , which were notably affected by temperature stress and mutant of , and . All above, and regulate the virulence of SRW-OG1 by affecting biofilm formation, adhesion, and enzymatic activity of extracellular products, and are simultaneously engaged in temperature-dependent pathogenicity.