Isolation, Identification, and Characteristics of Aeromonas salmonicida subsp. masoucida from Diseased Starry Flounder (Platichthys stellatus)

<i>Aeromonas salmonicida</i> is a predominant pathogen that infects fish. The pathogen <i>A. salmonicida</i> subsp. <i>masoucida</i> (ASM) was isolated for the first time from diseased starry flounders (<i>Platichthys stellatus</i>). Our study aimed to isolate, characterize, and investigate the pathogenicity of ASM. Bacterial species were identified using <i>16s rRNA</i>, <i>gyrB</i>, <i>dnaJ</i>, and <i>vapA</i> analyses. Phylogenetic tree analysis revealed that the ASM strains were clustered with the ASM ATCC strain and other strains isolated from black rockfish. In the antimicrobial susceptibility test, the three ASM strains were considered non-wild types for enrofloxacin, florfenicol, flumequine, oxolinic acid, and oxytetracycline susceptibility. Histopathological analysis revealed bacterial colonies in the secondary lamella and heart, indicating that ASM strains are highly virulent in fish. Comparative analysis and annotation via genome sequencing revealed that, among the 1156 factors, adherence factors were the most prevalent putative virulence determinants, followed by the effector delivery system and adherence. ASM was found to possess 43 type III secretion systems, 22 type VI secretion systems, 11 antimicrobial resistance genes, 3 stress genes, and prophage regions. These findings provide new insights into the virulence profile of ASM and highlight the risk posed by emerging pathogenic strains to starry flounders.