Comparative Analysis of Phenotypic Characteristics and Antimicrobial Resistance Among Aeromonas veronii Strains Isolated from Different Farmed Freshwater Fish

Aeromonas veronii is an important zoonotic pathogen causing severe diseases and high mortality in freshwater aquaculture. In this study, five strains (LY-3, JY-1, G7, G8, and WL-3) were isolated from different diseased freshwater fish in Linyi City, Shandong Province, China, and identified as A. veronii using phylogenetic analysis based on 16S rRNA and gyr B sequences. Five strains grew at 20–40°C, pH 5.0–8.0, and 0–3% NaCl, demonstrating strong environmental tolerance. Their optimal temperature and pH were identical, but optimal NaCl concentration was strain-dependent. Despite harboring the hemolysin-related genes act , aer , and hlyA , all strains except JY-1 exhibited β-hemolytic activity. All strains exhibited distinct flagella-driven motility abilities. Specifically, only strain G8 possessed two polar flagella and exhibited the strongest swimming motility. Similarly, all strains formed biofilms, correlating with the production of N-acyl-homoserine lactone (AHL). Worryingly, all strains exhibited multi-drug resistance to three penicillin-class antibiotics, midecamycin, clindamycin, three carbapenems, vancomycin, and polymyxin B. Additionally, five strains carried bla TEM and bla CphA3 genes, and four strains (excluding LY-3) additionally harbored the bla OXA gene. Encouragingly, most of isolates remained susceptible to florfenicol and enrofloxacin. These results indicate considerable intra-specific heterogeneity among A. veronii strains from different geographical origins and host sources, suggesting an urgent need for precise treatments.