Emergence of Multidrug-Resistant Bacterial Pathogens in Cultured Rainbow Trout (Oncorhynchus mykiss): Phenotypic and Molecular Insights

Abstract

The emergence of antimicrobial resistance in aquaculture poses a significant threat to both fish health and public safety. In this study, rainbow trout (Oncorhynchus mykiss) from the Kokernag trout-rearing unit were collected to isolate and characterize bacterial pathogens, assess antimicrobial susceptibility, and detect antibiotic resistance genes in the fish. Based on phenotypic and genotypic characterization, six bacterial isolates, including Aeromonas sobria, Escherichia coli, Flavobacterium psychrophilum, Pseudomonas spp., Staphylococcus spp., and Yersinia ruckeri, were identified. Antimicrobial susceptibility testing revealed variable resistance patterns, with high resistance observed against ampicillin, oxytetracycline, and erythromycin, whereas florfenicol, gentamicin, ceftazidime, and meropenem remained effective. Molecular screening confirmed the presence of sul1 and blaTEM antibiotic resistance genes among the isolates. These findings highlight the prevalence of multidrug-resistant bacteria in trout aquaculture, underscoring the importance of prudent antibiotic use, continuous surveillance, and integrated disease management strategies to safeguard both aquaculture productivity and public health.