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

Abstract

Simple Summary Aeromonas salmonicida is a bacterial pathogen that can cause severe disease and mortality in salmonid fish. However, its effects on Arctic grayling (Thymallus arcticus grubei) in China remain poorly understood. We isolated strain Ag001 from diseased Arctic grayling and examined its identity, pathogenicity, genome, and antimicrobial susceptibility. Multilocus sequence typing classified Ag001 as sequence type 2, linking it to a lineage associated with disease outbreaks in salmonid fish. Infection experiments showed that Ag001 was highly pathogenic to both Arctic grayling and rainbow trout (Oncorhynchus mykiss), causing severe damage to the spleen, liver, and kidney. Genome analysis identified two major protein-delivery systems involved in bacterial infection. The chromosome encoded a type II secretion system, whereas a plasmid carried a type III secretion system. Together with several toxin- and virulence-related genes, these systems may contribute to the high pathogenicity of Ag001. The strain was susceptible to enrofloxacin, neomycin, and doxycycline under laboratory conditions. This study provides the first report and comprehensive characterization of A. salmonicida from diseased Arctic grayling in China, expanding its known host range and providing useful information for disease surveillance and control in cold-water aquaculture.