The aim of this study is to investigate the trend of antimicrobial resistance of 696 Escherichia coli strains of chicken origin in China during 1970-2007, and to characterize the beta-lactamase determinants of these strains. Minimum inhibitory concentrations (MICs) of these strains against 11 antimicrobials were determined by the broth microdilution method. Quinolone-resistant, and first-generation cephalosporin-resistant strains have emerged since the 1990s; third-generation cephalosporin-resistant strains were not detected until 2003. Afterwards these resistant strains increased rapidly, and an obvious increase of isolates with decreased susceptibility to forfenicol was also observed. Three classes of beta-lactamase genes in E. coli strains were detected by PCR and confirmed by DNA sequencing. The detection rate of narrow-spectrum beta-lactamase genes decreased from 71.2% in the 1970s to 28.8% during 2004-2007. Extended-spectrum beta-lactamase (ESBL) genes did not emerge until 2004 and the detection rate was 18.5% between 2004 and 2007. The detection rate of bla(CMY-2), which emerged between 2000 and 2003, climbed to 6.6% during 2004-2007. bla(CTX-M) was the dominant genotype (75%) in ESBL gene-harboring E. coli strains. Fifty strains (17.6%), co-harboring multiple kinds of beta-lactamase genes, were detected in 284 beta-lactamase-producing strains. Southern hybridization revealed that two- or three kinds of ESBL and/or CMY-2 genes co-located on one plasmid in 9 strains. The rising trend of antimicrobial resistance and the high prevalence of beta-lactamase determinants in E. coli strains of chicken origin might be attributed to overuse of antimicrobials, especially beta-lactams, in poultry production.
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