Molecular aspects of high-level resistance to sulbactam-cefoperazone in Klebsiella oxytoca clinical isolates

Nine Klebsiella oxytoca strains which demonstrated resistance to the combination of sulbactam and cefoperazone were isolated from geographically separate hospitals in Japan in 1995. Among them, K. oxytoca SB23 showed high- level resistance to sulbactam-cefoperazone (MIC, > 128 μg/ml) and aztreonam (MIC, 128 μg/ml). The sulbactam-cefoperazone resistance was not transferred from strain SB23 to Escherichia coli CSH2 by conjugation. β-Lactamase RbiA, produced by strain SB23, was purified, and the molecular mass was estimated to be 29 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Kinetic parameters for RbiA revealed that cefoperazone and aztreonam were hydrolyzed efficiently by this enzyme. Moreover, ceftazidime and imipenem were also hydrolyzed weakly by RbiA, although strain SB23 did not show any resistance to these agents. Clavulanate, sulbactam, and tazobactam failed to block the hydrolysis of cefoperazone by RbiA. The structural gene of RbiA (bla(RB1)) was cloned and sequenced, and the deduced amino acid sequence of RbiA demonstrated high-level similarities to those of the β-lactamases found in K. oxytoca D488, E23004, and plasmid-mediated MEN-1, which have been classified into Bush functional group 2be. Although RbiA demonstrates high- level molecular similarity to the enzymes in group 2be, from an enzymological point of view, this enzyme might be differentiated from the enzymes in that group. Hybridization analysis revealed that β-lactamase genes highly similar to bla(RB1) were generally encoded on the chromosome of the sulbactam- cefoperazone-resistant clinical isolates of K. oxytoca tested in the study, despite their different derivations. This observation suggests that sulbactam-cefoperazone-resistant K. oxytoca strains which produce RbiA-type β-lactamases have been proliferating in many hospitals in Japan.