Prevalence of antimicrobial resistance phenotypes and genes in stable fly- and manure-derived bacterial isolates from clinically relevant taxa in dairy settings.

Aims: This study aimed to characterize and compare the antimicrobial resistance profiles of clinically relevant bacterial taxa isolated from biting stable flies (Stomoxys spp.) and bovine manure samples collected at a dairy research facility over the course of an entire fly breeding season. The presence of extended-spectrum beta-lactamase and other antimicrobial resistance genes (ARGs) was also examined.

Methods and results: A total of 606 fly- and 180 manure-derived strains were tested via disk diffusion for susceptibility to commonly administered antibiotics used in veterinary and human medicine. A small percentage of Enterobacterales exhibited resistance to the tested antimicrobials, including ceftiofur and other beta-lactam antibiotics. Extended spectrum beta-lactamase genes (TEM, CTX, OXA, CMY) were detected by polymerase chain reaction (PCR) amplification in ceftiofur-resistant Escherichia coli, Klebsiella, and Enterobacter spp. isolates. We additionally identified pirlimycin-resistant Staphylococcus and Mammaliicoccus spp. isolates encoding lnuA, a lincosamide resistance gene found primarily on small mobilizable plasmids.

Conclusions: These findings highlight the significance of stable flies in the carriage of antimicrobial-resistant bacterial strains and plasmid-associated ARGs on dairy farms.