Characterization of a nosocomial outbreak caused by a multiresistant Acinetobacter baumannii strain with a carbapenem-hydrolyzing enzyme: High-level carbapenem resistance in A. baumannii is not due solely to the presence of beta-lactamases

From February to November 1997, 29 inpatients at Ramon y Cajal Hospital, Ma drid, Spain, were determined to be either colonized or infected with imipen em- and meropenem-resistant Acinetobacter baumannii (IMRAB) strains (MICs, 128 to 256 mu g/ml). A wide antibiotic multiresistance profile was observed with IMRAB strains. For typing IMRAB isolates, pulsed-field gel electropho resis was used. For comparative purposes, 30 imipenem- and meropenem-suscep tible A. baumannii (IMSAB) strains isolated before, during, and after the o utbreak were included in this study. The molecular-typing results showed th at the outbreak was caused by a single IMRAB strain (genotype A). By clonin g experiments we identified a class D beta-lactamase (OXA-24) encoded in th e chromosomal DNA of this IMRAB strain which showed carbapenem hydrolysis. Moreover, the outer membrane profile of the IMRAB strain showed a reduction in the expression of two porins at 22 and 33 kDa when compared with geneti cally related IMSAB isolates. In addition no efflux mechanisms were identif ied in the IMRAB strains. In summary, we report here the molecular characte rization of a nosocomial outbreak caused by one multiresistant A. baumannii epidemic strain that harbors a carbapenem-hydrolyzing enzyme. Although alt erations in the penicillin-binding proteins cannot be ruled out, the reduct ion in the expression of two porins and the presence of this OXA-derived be ta-lactamase are involved in the carbapenem resistance of the epidemic noso comial IMRAB strain.