Oxazolidinone resistance genes in florfenicol-resistant enterococci from beef cattle and veal calves at slaughter.

Background: Linezolid is a critically important oxazolidinone antibiotic used in human medicine. Although linezolid is not licensed for use in food-producing animals, the use of florfenicol in veterinary medicine co-selects for oxazolidinone resistance genes.

Objective: This study aimed to assess the occurrence of , and in florfenicol-resistant isolates from beef cattle and veal calves from different herds in Switzerland.

Methods: A total of 618 cecal samples taken from beef cattle and veal calves at slaughter originating from 199 herds were cultured after an enrichment step on a selective medium containing 10 mg/L florfenicol. Isolates were screened by PCR for , and which are genes known to confer resistance to oxazolidinones and phenicols. One isolate per PCR-positive species and herd was selected for antimicrobial susceptibility testing (AST) and whole-genome sequencing (WGS).

Results: Overall, 105 florfenicol-resistant isolates were obtained from 99 (16%) of the samples, corresponding to 4% of the beef cattle herds and 24% of the veal calf herds. Screening by PCR revealed the presence of in 95 (90%) and in 22 (21%) of the isolates. None of the isolates contained . Isolates included for AST and WGS analysis were (.) ( = 14), ( = 12), ( = 1), ( = 2), ( = 1), (.) ( = 2), (.) ( = 1), and (.) ( = 1). Thirteen isolates exhibited phenotypic linezolid resistance. Three novel OptrA variants were identified. Multilocus sequence typing identified four ST18 belonging to hospital-associated clade A1. There was a difference in the replicon profile among and -harboring plasmids, with rep9 (RepA_) plasmids dominating in -harboring and rep2 (Inc18) and rep29 (Rep_3) plasmids in -carrying .

Conclusion: Beef cattle and veal calves are reservoirs for enterococci with acquired linezolid resistance genes and . The presence of ST18 highlights the zoonotic potential of some bovine isolates. The dispersal of clinically relevant oxazolidinone resistance genes throughout a wide variety of species including spp., , and the probiotic in food-producing animals is a public health concern.