mechanism of resistance and antibacterial susceptibility in extendedspectrum lactamase phenotype klebsiella pneumoniae and klebsiella oxytoca isolated between 2000 and 2010 in japan clinical isolates of klebsiella pneumoniae and klebsiella oxytoca 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 16 k pneumoniae and 19 of 359 53 k oxytoca were determined to be extendedspectrum lactamase esbl phenotype isolates and the identified esbls amongst the k pneumoniae isolates were ctxm2 3 14 and 15 and shv12 in contrast overproduction of chromosomal lactamase oxy2 which was due to a distinct mutation at the 10 promoter region of this gene conferred the esbl phenotype to all the k oxytoca isolates except one based on the clinical and laboratory standards institute breakpoints all the esbl phenotype k pneumoniae were susceptible to doripenem flomoxef moxalactam latamoxef cefmetazole and tazobactampiperacillin whereas the esbl phenotype k oxytoca were susceptible to ceftazidime and ceftibuten in addition to the above with the exception of tazobactampiperacillin amongst the oral antimicrobials ceftibuten was relatively effective against both esbl phenotype klebsiella species compared with levofloxacin and amoxicillinclavulanic acid