Prevalence of qnrS-positive Escherichia coli from chicken in Thailand and possible co-selection of isolates with plasmids carrying qnrS and trimethoprim-resistance genes under farm use of trimethoprim.

One hundred and twenty chicken samples from feces (n = 80), the carcass surface at slaughter at 2 meat chicken farms (n = 20), and retail chicken meat from 5 markets (n = 20) collected during 2018 and 2019 were examined for the prevalence of plasmid-mediated quinolone resistance (PMQR) in Escherichia coli. We detected qnrS-positive E. coli in a total of 74 samples from feces (n = 59), the carcass surface (n = 7), and retail meat (n = 8). These 74 qnrS-positive isolates were tested for antimicrobial susceptibility to determine the minimum inhibitory concentrations (MICs) of certain antimicrobials and genetically characterized. Ampicillin-resistance accounted for 71 of the 74 isolates (96%), followed by resistance to oxytetracycline (57/74; 77%), enrofloxacin (ERFX) (56/74; 76%), sulfisoxazole (SUL) (56/74; 76%), trimethoprim (TMP) (49/74; 66%), and dihydrostreptomycin (48/74; 65%). All farm-borne SUL-and TMP-resistant isolates except one were obtained from samples from farm...