Natural insetional inactivation of an outer membrane protein gene promotes carbapenem resistance in Acinetobacter baumannii

MUSSI MA; LIMANSKY AS; VIALE AM

ArmaÇao do Búzios. Brazil.

Simposio; III International Symposium on Myosin V; 2006

EMBO

The emergence of antibiotic resistance among both pathogenic and opportunistic microbes resident in hospitals represents a serious and recurrent problem for the treatment of infections. Certain strains of a particular species of the genus Acinetobacter, Acinetobacter baumannii (A. baumannii) now account for a large percentage of nosocomial infections, including pneumonia, bacteremia, skin and wound infections, and urinary tract infections. These strains are almost invariably multidrug resistant, having successfully resisted eradication attempts by the use of penicillins, aminoglycosides, cephalosporins, and even fluoroquinolones. The ultimate generation of b-lactam antibiotics, the carbapenems such as imipenem and meropenem, represent the last therapeutic option available to manage the multidrug resistant strains. Thus, the appearence of carbapenem-resistance renders the situation out of control. Attempts to reduce the dissemination of rapidly evolving antibiotic-resistant pathogens are best based on a detailed knowledge of the causes that promote these patterns of resistance. The outer membrane proteins responsible for the influx of carbapenems in A. baumannii are still poorly characterized. We found that resistance to both imipenem and meropenem in multidrug-resistant clinical strains of A. baumannii was associated with the loss of a heat-modifiable 29-kDa outer membrane protein, which we designated CarO. The chromosomal locus containing the carO gene was cloned and characterized. 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 b-barrel topology. The absence of CarO in the outer membrane of different carbapenem-resistant clinical strains of A. baumannii resulted from the disruption of carO by distinct insertion elements. The results presented here provide another example of the challenges that bacterial mobile elements pose to efforts directed toward the prevention of the emergence of antibiotic resistance. The in vitro selection of carbapenem resistant mutants in the laboratory resulted in bacteria lacking CarO in the outer membrane, but due to IS-independent, still unknown mechanisms. We also shown that CarO exists in the outer membrane of A. baumannii as an oligomer, a characteristic often encountered in bacterial outer membrane porins. No close homology between CarO and any member of the different families of outer membrane protein channels described so far could be detected at the sequence level. Notably, databases 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 among the class g-Proteobacteria. In fact, several species of this family are either known or suspected human pathogens. Whether they can eventually evolve carbapenem resistance by similar mechanisms represent a distressing possibility that requires further study.  The overall data thus support the notion that CarO participates in the influx of carbapenem antibiotics in A. baumannii.