Phenotypic and genotypic characterization of antimicrobial resistance in Enterococcus spp. Isolated from the skin microbiota of channel catfish (Ictalurus punctatus) in Southeastern United States.

BACKGROUND: Aquaculture systems may contribute to the emergence and persistence of antimicrobial-resistant (AMR) bacteria, posing risks to animal, environmental, and human health. This study characterized the phenotypic and genotypic antimicrobial resistance profiles of Enterococcus spp. isolated from the skin microbiota of 125 channel catfish (Ictalurus punctatus) harvested from two earthen ponds in Alabama, USA. METHODS: Skin swabs from the body of channel catfish were enriched in Enterococcosel broth and cultured on Enterococcosel agar at 28 °C for 24 h. Isolates were confirmed using Biolog Gen III and VITEK(®)2, and antimicrobial susceptibility was determined using the Kirby-Bauer disk diffusion method. Thirty-five randomly sampled isolates underwent whole-genome sequencing for genotypic characterization. RESULTS: 36% of isolates exhibited multidrug resistance (resistance to ≥ 3 antimicrobial classes), with the highest resistance rates observed for ampicillin (44.8%), rifampicin (42.4%), and tetracycline (38.4%). The most prevalent resistance genes were aac(6')-Iid (65.7%), aac(6')-Ii (22.9%), efmA, and msr(C) (20.0% each). Plasmid replicons rep1 and repUS15 frequently co-occurred with resistance genes. Biofilm-associated genes, including efaA, fsrA, fsrB, sprE, ebpABC, ace, and scm, were commonly detected. Multivariate analyses (PERMANOVA, PCA) revealed no significant species-level differences in resistance burden or biofilm gene carriage, indicating similar resistance and virulence gene carriage across species in this dataset. CONCLUSIONS: The skin microbiota of pond-raised catfish harbors antimicrobial-resistant Enterococcus spp. with mobile resistance elements and biofilm-associated virulence factors, suggesting a potential role in AMR persistence within aquaculture settings. These findings support the need for targeted AMR surveillance in fish-associated microbiota as part of integrated One Health strategies.