Co-resistance and plasmid-mediated co-dissemination of florfenicol and azithromycin resistance in non-O157 Shiga toxin-producing Escherichia coli from cattle in Xinjiang, China

Florfenicol is a critical antibiotic used in cattle to manage respiratory infections, while azithromycin is recommended for treating early-stage Shiga toxin-producing Escherichia coli (STEC) infections. This study focused on examining the co-resistance and mechanisms of co-dissemination of resistance to florfenicol and azithromycin in 367 non-O157 STEC strains isolated from cattle in Xinjiang between 2015 and 2023. Among these isolates, 16.3% showed resistance, with drug resistance rates ranging from 5.4 to 10.6% for tetracycline, ampicillin, cefotaxime, streptomycin, sulfamethoxazole, florfenicol, aztreonam, azithromycin, gentamicin, ceftazidime, and piperacillin. Resistance to cefepime, ampicillin-sulbactam, amikacin, amoxicillin-clavulanic acid, levofloxacin, ciprofloxacin, and piperacillin-tazobactam was lower, varying from 0.3 to 3.5%. Notably, 9.0% (n = 33) isolates exhibited multiple drug resistance, while 8.4% (n = 31) were resistant to florfenicol and/or azithromycin. Conjugation experiments demonstrated that resistance genes floR and/or mph(A) were transferred to the recipient strain Escherichia coli J53, leading to phenotypic resistance to florfenicol and/or azithromycin in the transconjugants. Whole genome sequencing (WGS) of STEC W24 identified that floR and mph(A) were located on a 77,016 bp IncA/C type plasmid, designated as W24-floR-mph(A). A genetic structure with three IS26 and one IS6100 sequences flanking the multi-drug resistance region was detected, including two IS26 copies adjacent to floR, tet(A), aph(6)-Id_1, aph(3'')-Ib_5, sul2 and mph(A). The findings highlight the presence of florfenicol and azithromycin resistance in bovine non-O157 STEC in Xinjiang, with floR and mph(A) genes residing on plasmids capable of horizontal transfer. Plasmids, transposons, and insertion sequences may mediate the dissemination of antimicrobial resistance genes.