Characterization of Quinolone Resistance in Salmonella enterica from Farm Animals in China

This study was focused on the prevalence and antimicrobial susceptibilities of Salmonella directly isolated at animal clinics in Guangdong, People's Republic of China. The isolation rates from chickens, ducks, and pigs were 11.3% (11 of 97 samples), 15.4% (53 of 344 samples), and 3.0% (13 of 434 samples), respectively. Among the 77 Salmonella enterica isolates, the most predominant serovar was Typhimurium (81.8%, 63 isolates), followed by serovars Meleagridis (2.6%, 2 isolates) and Abaetetuba (1.3%, 1 isolate). Salmonella isolates were resistant to ciprofloxacin (16.9% of isolates) and nalidixic acid (66.2% of isolates), and 68 isolates (88.3%) were multidrug resistant, displaying resistance to three or more classes of antimicrobial agents. Eighteen isolates (23.4%) had at least one plasmid-mediated quinolone resistance gene, which was identified using PCR and DNA sequencing. The most prevalent plasmid-mediated quinolone resistance gene was aac(6')-Ib-cr, found in 14 isolates (18.2%), followed by oqxAB (9.1%) and qnrS (7.8%). Alterations in the gyrA gene were detected in 24 (57.1%) of 42 strains with a ciprofloxacin MIC of ≥0.25 μg/mL; the same level of susceptibility was found for enrofloxacin. Six types of mutations were found in the quinolone resistance determining regions of gyrA, and the predominant one (S83Y) was found singly in 15 (62.5%) of 24 isolates. We also found 22 different pulsed-field gel electrophoresis types among the Salmonella isolates. The Salmonella serovars and MICs of ciprofloxacin were similar within clusters, although individual differences were noted. This finding suggests that resistance plasmids were horizontally transmitted but also clonally spread.