Analysis of pathogenicity and drug resistance of Nocardia seriolae from Micropterus salmoides based on whole genome sequencing

Micropterus salmoides is a crucial economic freshwater fish species in China. In recent years, the continuous expansion of its farming scale has been accompanied by increasingly prominent disease issues, which have inflicted significant economic losses on the aquaculture industry. In order to clarify the pathogeny of diseased M. salmoides in a culture base in Jingzhou, the main symptoms are bleeding of body surface ulcer and appearance of white nodules on the visceral surface.In this study, a dominant bacterium was isolated from the organs of diseased M. salmoides with white nodules. The isolated strain was identified through a combination of colony morphology, Gram staining, physiological and biochemical characteristics, as well as sequence analyses of the 16S rRNA gene, secA1 gene, and whole-genome sequencing. The pathogenicity of the isolated strain was assessed through virulence gene screening, experimental challenge, and histopathological examination. Its antimicrobial resistance was evaluated based on biofilm formation ability, resistance gene screening, and antimicrobial susceptibility testing.The dominant bacterium isolated from the diseased fish was identified as N. seriolae and designated as strain NS01. Comparative genomic analysis of the isolate NS01 revealed that N. seriolae NS01 from M. salmoides is more closely related to, and shares higher similarity with, strains originating from marine fish than to those from freshwater fish.The isolated strain NS01 showed a median lethal concentration (LD50) of 7.5×10⁷ CFU/mL in M. salmoides, with the presence of four virulence genes: pup, mig, mce 1A, and gapA. The isolated strain NS01 exhibited a moderate capacity for biofilm formation. It demonstrated susceptibility to eight antimicrobial agents, including gentamicin, neomycin, and chloramphenicol, while showing resistance to eleven others, such as penicillin, cefazolin, and norfloxacin. Analysis of resistance genes revealed that the isolated strain NS01 is closely related to several N. seriolae strains of marine fish origin, such as UTF1, KH-11, VT-45, and PY-31, at the genetic level of antimicrobial resistance.This study successfully isolated a strain of N. seriolae from the nodule-afflicted organs of diseased M. salmoides. The pathogenicity and antimicrobial resistance of this isolate were systematically analyzed, and its evolutionary history was elucidated through whole-genome sequencing. These findings provide a theoretical foundation and data support for investigating the pathogenic and resistance mechanisms of N. seriolae.