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Journal of Clinical Microbiology, September 2000, p. 3299-3305, Vol. 38, No. 9
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

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

Germán Bou,1,* Gonzalo Cerveró,1 M. Angeles Domínguez,2 Carmen Quereda,1 and Jesús Martínez-Beltrán1

Servicio de Microbiología, Hospital Ramón y Cajal, 28034 Madrid,1 and Servicio de Microbiología, Hospital de Bellvitge, CSUB, Barcelona,2 Spain

Received 1 February 2000/Returned for modification 3 April 2000/Accepted 22 June 2000

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 beta -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 beta -lactamase are involved in the carbapenem resistance of the epidemic nosocomial IMRAB strain.

* Corresponding author. Present address: Laboratory of Viral Immunology, Department of Immunology and Division of Infectious Diseases, 200 First St., Guggenheim 516 SW, Mayo Clinic, Rochester, MN 55905. Phone: (507) 284-9646. Fax: (507) 284-3757. E-mail: germanbou{at}mailcity.com.

Journal of Clinical Microbiology, September 2000, p. 3299-3305, Vol. 38, No. 9
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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