Effects of florfenicol on linezolid resistance in Enterococcus faecium from waterfowl farms.

This study aimed to investigate the antimicrobial resistance profiles of Enterococcus faecium (E. faecium) in waterfowl farms across selected provinces in China and to explore the impact of florfenicol on linezolid resistance in E. faecium. Between 2022 and 2025, 490 E. faecium strains were isolated from samples collected from duck and goose farms in 11 Chinese provinces. These isolates exhibited high resistance rates to multiple antibiotics, with a linezolid resistance rate of 69.2% (339/490). Transferable linezolid resistance genes were identified as the primary mechanism underlying linezolid resistance in E. faecium. Conjugation experiments revealed that subinhibitory concentrations of florfenicol increased (P < 0.05) the conjugation frequency of the optrA gene, thereby facilitating the dissemination of linezolid resistance. In vitro antibiotic induction experiments demonstrated that both florfenicol and linezolid could induce linezolid resistance in E. faecium. Mechanistic analyses indicated that florfenicol and linezolid triggered similar response patterns in the strains. Both antibiotic pressures induced mutations in domain Ⅴ of the 23S rRNA, primarily the widely reported major linezolid resistance mutations G2505A and G2576U, and in genes encoding ribosomal proteins, mainly consisting of an amino acid substitution at position 68 of the L4 protein; furthermore, they enhanced overall gene transcription levels of the strain, upregulating the expression of genes encoding multidrug efflux pumps, two-component systems, and stress response proteins. The expression levels of the optrA gene were elevated (P < 0.01) following induction by both florfenicol and linezolid. Additionally, induction by florfenicol and linezolid affected the resistance of E. faecium to various other antibiotics, such as erythromycin and ampicillin. In conclusion, this study provides novel insights into the emergence of resistance to linezolid, an antibiotic not approved for veterinary use, among E. faecium in waterfowl farms. Florfenicol can promote the generation and dissemination of linezolid resistance, ultimately compromising the clinical efficacy of linezolid.