Under our experimental conditions, REP-PCR had a higher discriminatory power than AP-PCR, with PFGE as reference technique. The REP-PCR technique is a useful and expeditious method for the epidemiologic characterization of A. baumannii nosocomial outbreaks, the results being comparable to those obtained with the PFGE technique.

From February to November 1997, 29 inpatients at Ramón y Cajal Hospital, Madrid, Spain, were determined to be either colonized or infected with imipenem- and meropenem-resistant Acinetobacter baumannii (IMRAB) strains (MICs, 128 to 256 μg/ml). A wide antibiotic multiresistance profile was observed with IMRAB strains. For typing IMRAB isolates, pulsed-field gel electrophoresis was used. For comparative purposes, 30 imipenem- and meropenem-susceptible A. baumannii (IMSAB) strains isolated before, during, and after the outbreak were included in this study. The molecular-typing results showed that the outbreak was caused by a single IMRAB strain (genotype A). By cloning experiments we identified a class D β-lactamase (OXA-24) encoded in the 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 genetically related IMSAB isolates. In addition no efflux mechanisms were identified in the IMRAB strains. In summary, we report here the molecular characterization of a nosocomial outbreak caused by one multiresistant A. baumannii epidemic strain that harbors a carbapenem-hydrolyzing enzyme. Although alterations in the penicillin-binding proteins cannot be ruled out, the reduction in the expression of two porins and the presence of this OXA-derived β-lactamase are involved in the carbapenem resistance of the epidemic nosocomial IMRAB strain.

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A clinical strain of Escherichia coli (strain Ec 41553) that was resistant to ceftazidime produced a TEM-type beta-lactamase with a pI of 5.4. Clavulanic acid restored the ceftazidime activity, thus suggesting an extended spectrum beta-lactamase (ESBL). The gene encoding ESBL was located in a plasmid of 57 kb. After cloning and sequencing, the ESBL (TEM-29B) showed one amino acid replacement with respect to the TEM-1 sequence, Arg-164 to His. This change increased mainly the rate of hydrolysis of ceftazidime but not of cefotaxime and aztreonam. The relevance of this substitution in the increase of ceftazidime MIC is thus stressed.

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