Characterization of antimicrobial resistance and related resistance genes in Escherichia coli strains isolated from chickens in China during 2007-2012

In the present study, the prevalence of antimicrobial-resistant chicken Escherichia coli strains and the resistance genes in E. coli was investigated. For this purpose, 1002 chicken E. coli strains isolated from layer and broiler flocks in Shaanxi, Henan and Gansu provinces in China during 2007-2012 were examined. Antimicrobial susceptibility of these E. coli strains against 18 antimicrobials was determined by the Kirby-Bauer disk diffusion method. Eight out of the twenty antimicrobial resistance genes were detected by polymerase chain reaction (PCR). The sequences of the resistance genes in chicken E. coli strains were compared with the previously published sequences. Our results revealed that the antimicrobial resistance prevalence of E. coli strains in western China to ampicillin, doxycycline, tetracycline and nalidixic acid were consistently kept at 62-100%. The E. coli resistance to nalidixic acid and ciprofloxacin had an increasing trend, as high as 100% for nalidixic acid while the resistance prevalence to gentamicin had a decreasing trend. The detection rates of the genes for tetA, tetB, blaTEM, and aac(3)-II in chicken E. coli strains were positively correlated with their antimicrobial resistance (P <0.01) during 2007-2012. Among 1002 chicken E. coli strains tested, all E. coli strains were resistant to more than three kinds of antimicrobials. Our results revealed that 499 of the 1002 (49.8%) chicken E. coli strains were resistant to more than eight kinds of antimicrobials. Considering all the 1002 isolates, the detection prevalence of the genes for tetA, tetB, blaTEM in chicken E. coli strains were constantly over 88.9%. The detection prevalence of the genes for floR, sul-I and cmlA in chicken E. coli strains increased, while aac(3)-II declined from 75.0 to 28.6%.   Key words: Escherichia coli, antibiotic resistance, antibiotic resistance genes, polymerase chain reaction (PCR), chicken.