Isolation, Pathogenicity, and Genomic Analysis of a Virulent Aeromonas salmonicida ST2 Strain from Thymallus arcticus grubei

Abstract

Aeromonas salmonicida, the causative agent of furunculosis, poses a serious threat to salmonid aquaculture, but its pathogenicity in rare cold-water fish species remains insufficiently characterized. This study reports the first isolation and characterization of a pathogenic A. salmonicida strain (Ag001) from diseased Thymallus arcticus grubei in China. Multilocus sequence typing (MLST), together with 16S rRNA, gyrB, and vapA gene analyses and phylogenetic characterization, identified the isolate as A. salmonicida subsp. salmonicida ST2. In intraperitoneal challenge assays using six bacterial dose groups and PBS controls, Ag001 showed high virulence toward T. arcticus grubei [LD50: 3.08 × 103 CFU/mL (95% CI: 9.42 × 102–1.01 × 104 CFU/mL), equivalent to 7.16 × 101 CFU/g (95% CI: 2.19 × 101–2.34 × 102 CFU/g)] and Oncorhynchus mykiss [LD50: 3.16 × 104 CFU/mL (95% CI: 1.19 × 104–8.38 × 104 CFU/mL), equivalent to 3.46 × 101 CFU/g (95% CI: 1.31 × 101–9.17 × 101 CFU/g)]. Comparative histopathological examination of spleen, liver, and kidney tissues from diseased and healthy fish revealed splenic necrosis, hepatic degeneration, and renal tubular damage in diseased fish. Genomic analysis identified complete T2SS and T3SS secretion systems, along with eight virulence-associated genes including aerA, hlyA, and vapA, suggesting that secretion systems and virulence factors may contribute to pathogenicity. Antimicrobial susceptibility testing showed susceptibility to enrofloxacin, neomycin, and doxycycline, but resistance to several other tested agents. These findings provide information on the host range, pathogenicity, genomic virulence features, and potential treatment options for A. salmonicida ST2 in cold-water aquaculture systems.