Virulence regulation of tryptophanase gene tnaA on mesophilic Aeromonas salmonicida SRW-OG1

Aeromonas salmonicida SRW-OG1 is a mesophilic aquatic pathogen, which can cause fish furunculosis and bring huge economic losses to aquaculture. The tnaA gene encodes tryptophanase, which converts L- tryptophan into indole, an important signaling molecule. In this study, the Delta tnaA and C-Delta tnaA strains of A. salmonicida were constructed to investigate the role of the tnaA gene. Compared with the wild-type strain (SRW-OG1), the deletion strain (Delta tnaA) showed obvious damage to swimming motility, extracellular enzyme activity and biofilm formation abilities, although its drug resistance was enhanced. The crude extract of Delta tnaA strain exhibited potent antibacterial activity. The Delta tnaA strain could not produce indole. The results of artificial infection showed that the virulence of Delta tnaA strain to Epinephelus coioides was weakened. There were 1024 differentially expressed genes (DEGs) identified, with 503 being upregulated and 521 downregulated in the SRW-OG1/Delta tnaA comparison. The transcriptomic analysis of SRW-OG1 and Delta tnaA strains revealed that the differentially expressed genes were primarily enriched in metabolism, immune response, and bacterial infection. The deletion of the tnaA leads to a decreased bacterial load in the spleen, liver, and head kidney of E. coioides. These results affirm that the tnaA gene is an important virulence gene in A. salmonicida.