Background : Klebsiella oxytoca strain exhibiting an unusual inducible β-lactam resistance phenotype was isolated from a wound specimen of a patient at a university hospital in August 2002. The isolate was resistant to ampicillin, ampicillin-sulbactam, cephalothin, cefoxitin, and demonstrated reduced inhibition zone diameters for ceftazidime in combination with clavulanate versus those for ceftazidime when tested alone.

Methods : Antimicrobial susceptibilities were tested using the Etest and disk diffusion method. AmpC β-lactamase production was determined by modified Hodge test. The disk antagonism method was used to detect inducibility of β-lactamase. Conjugation experiments were performed by the filter mating method using the recipient Escherichia coli J53 Azi^r strain. PCR and DNA sequencing of DHA-specific PCR products were tested.

Results : The double disk synergy test was negative and the modified Hodge test was positive for the K. oxytoca isolate. Antagonism was observed between cefoxitin and oxyimino-cephalosporins. Sequence analysis of the DHA-specific PCR products revealed that they were identical to the amino acid sequence of the DHA-1 β-lactamase. Transfer of the resistance by conjugation experiments was successful.

Conclusions : We found a plasmid-mediated DHA-1 β-lactamase-producing K. oxytoca possessing an unusual inducible β-lactam resistance phenotype was found in a university hospital in Korea. The resistance phenotype was conferred by DHA-1 encoded by a self-transferable plasmid. (Korean J Clin Microbiol 2004;7(2):124-129)