Prevalence and molecular characterization of fluoroquinolone resistance in Escherichia coli isolates from dairy cattle with endometritis in China.

Fluoroquinolones are frequently used to treat infectious disease that is caused by Escherichia coli in dairy cattle. However, fluoroquinolone resistance occurs and is due either to chromosomal mutations in the bacterial topoisomerase genes and/or to plasmid-mediated resistance genes. The purpose of this study was to determine the prevalence and molecular characteristics of fluoroquinolone resistance determinants in E. coli strains (n=148) isolated from dairy cattle with bovine endometritis in Inner Mongolia (China). Analysis of the mutations in the quinolone resistance-determining regions of resistant E. coli isolates confirmed previously reported substitutions in the GyrA and ParE. However, we identified additional substitutions in the ParC and GyrB that have not been reported earlier. No plasmid-mediated quinolone resistance genes in any of the isolates were found. The number of point mutations found per isolate correlated with an increase in the minimum inhibitory concentration of ciprofloxacin. Overall, 45.5% of the isolates were positive for the class I integrase gene along with four gene cassettes that were responsible for resistance to trimethoprim (dfr1 and dfrA17) and aminoglycosides (aadA1 and aadA5), respectively. The prevalence of extended-spectrum β-lactamases (ESBLs) was 100%, and the blaTEM gene was predominant in all of the isolates. In conclusion, our results identify the mechanism of quinolone resistance for the first time and reveal the prevalence of integron and ESBLs in E. coli isolates from dairy cattle with bovine endometritis in China after 20 years of quinolone usage in cattle.