Clinical isolates of and collected from 20 Japanese medical facilities between 2000 and 2010 were analysed to evaluate the mechanisms of resistance and antibacterial susceptibilities to 14 antimicrobials. Overall, eight of 484 (1.6 %) and 19 of 359 (5.3 %) were determined to be extended-spectrum β-lactamase (ESBL) phenotype isolates, and the identified ESBLs amongst the isolates were CTX-M-2, -3, -14 and -15, and SHV-12. In contrast, overproduction of chromosomal β-lactamase OXY-2, which was due to a distinct mutation at the − 10 promoter region of this gene, conferred the ESBL phenotype to all the isolates except one. Based on the Clinical and Laboratory Standards Institute breakpoints, all the ESBL phenotype were susceptible to doripenem, flomoxef, moxalactam (latamoxef), cefmetazole and tazobactam/piperacillin, whereas the ESBL phenotype were susceptible to ceftazidime and ceftibuten in addition to the above, with the exception of tazobactam/piperacillin. Amongst the oral antimicrobials, ceftibuten was relatively effective against both ESBL phenotype species compared with levofloxacin and amoxicillin/clavulanic acid.


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