Abstract
Broad-spectrum cephalosporin (BSC) resistance has increased in Escherichia coli isolates from broiler chickens in Japan since 2004. The purpose of this study was to understand the epidemiology of BSC-resistant E. coli in livestock animals. Among 3274 E. coli isolates from 1767 feces of apparently healthy animals on 1767 farms between 2004 and 2009, 118 ceftiofur (CTF)-resistant isolates (CTF MIC ≥4 μg/mL) were identified on 74 farms. After elimination of apparently clonal isolates from a single animal, 75 selected CTF-resistant isolates (62 isolates from 61 broiler chickens, 10 isolates from 10 layer chickens, two isolates from two cows, and one isolate from a pig) were characterized. The bla CMY-2 gene was most frequently detected in 50 isolates, followed by bla CTX-M (CTX-M-2: six isolates; CTX-M-14: four isolates; CTX-M-25: two isolates; CTX-M-1: one isolate) and bla SHV (SHV-12: seven isolates; SHV-2, SHV-2a, SHV-5: one isolate each). In particular, 42 of 62 broiler chicken isolates harbored bla CMY-2. Pulsed-field gel electrophoresis analyses using XbaI revealed divergent profiles among the BSC-resistant isolates. The incompatibility groups of bla CMY-2 plasmids from 34 of the 42 broiler chicken isolates belonged to IncIγ (10 isolates), IncA/C (nine isolates), IncB/O (seven isolates) and IncI1 (six isolates), or were nontypeable (two isolates). Co-transmission of resistance to non-β-lactam antibiotics was observed in transconjugants with IncA/C plasmids, but not with IncI1, IncIγ, and IncB/O plasmids except for one isolate with IncB/O. Our findings suggest that the bla CMY-2 gene is a key player in BSC-resistant E. coli isolates and that coselection is unlikely to be associated with the abundance of bla CMY-2 plasmids, except for IncA/C plasmids.
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