Acquisition of resistance to carbapenems in multidrug-resistant clinical strains of Acinetobacter baumannii: natural insertional inactivation of a gene encoding a member of a novel family of beta-barrel outer membrane proteins.

The outer membrane proteins responsible for the influx of carbapenem beta-lactam antibiotics in the nonfermentative gram-negative pathogen Acinetobacter baumannii are still poorly characterized. Resistance to both imipenem and meropenem in multidrug-resistant clinical strains of A. baumannii is associated with the loss of a heat-modifiable 29-kDa outer membrane protein, designated CarO. The chromosomal locus containing the carO gene was cloned and characterized from different clinical isolates. Only one carO copy, present in a single transcriptional unit, was found in the A. baumannii genome. The carO gene encodes a polypeptide of 247 amino acid residues with a typical N-terminal signal sequence and a predicted transmembrane beta-barrel topology. Its absence from different carbapenem-resistant clinical isolates of A. baumannii resulted from the disruption of carO by distinct insertion elements. The overall data thus support the notion that CarO participates in the influx of carbapenem antibiotics in A. baumannii. Moreover, database searches identified the presence of carO homologs only in species of the genera Acinetobacter, Moraxella, and Psychrobacter, disclosing the existence of a novel family of outer membrane proteins restricted to the family Moraxellaceae of the class gamma-Proteobacteria.