First Report and Integrated Characterization of Aeromonas veronii Associated with the Protected Fish Diptychus maculatus in Xinjiang, China

Aeromonas veronii is a widely distributed opportunistic aquatic pathogen associated with diseases in freshwater fish. Despite the ecological and conservation significance of Diptychus maculatus, a protected cold-water fish inhabiting high-altitude ecosystems, information regarding its associated bacterial communities remains limited. This study aimed to isolate and characterize A. veronii recovered from D. maculatus and provide baseline information on its occurrence and phenotypic characteristics. Eight bacterial isolates were recovered from various tissues, including skin, gills, eye, intestine, dorsal fin, body kidney, gonad, and spleen of randomly sampled fish individuals from Xinjiang, China. Phenotypic and biochemical characterization, together with 16S rRNA sequencing, supported their identification as A. veronii, while gyrB analysis of a representative isolate provided additional species-level confirmation. Antimicrobial susceptibility testing revealed a consistent multidrug-resistance phenotype among all isolates. The isolates were susceptible to enrofloxacin, cefotaxime, ceftriaxone, and florfenicol. Intermediate responses were observed for ciprofloxacin, ofloxacin, doxycycline, oxytetracycline, and trimethoprim–sulfamethoxazole, whereas resistance was detected against norfloxacin, neomycin, penicillin, amoxicillin, tetracycline, and erythromycin. In vitro biofilm assays demonstrated weak to moderate biofilm-forming capacity among isolates. These findings provide baseline data for wildlife microbial surveillance and conservation-oriented monitoring of protected fish populations, supporting future investigations into environmental monitoring, genomic characterization, and host–microbe interactions.