Genetic Mechanisms of Antimicrobial Resistance of Acinetobacter baumannii

OBJECTIVE: To summarize published data identifying known genetic mechanisms of antibiotic resistance in Acinetobacter baumannii and the correlating phenotypic expression of antibiotic resistance. DATA SOURCES: MEDLINE databases (1966-July 15, 2010) were searched to identify original reports of genetic mechanisms of antibiotic resistance in A. baumannii. DATA SYNTHESIS: Numerous genetic mechanisms of resistance to multiple classes of antibiotics are known to exist in A. baumannii, a gram-negative bacterium increasingly implicated in nosocomial infections. Mechanisms may be constitutive or acquired via plasmids, integrons, and transposons. Methods of resistance include enzymatic modification of antibiotic molecules, modification of antibiotic target sites, expression of efflux pumps, and downregulation of cell membrane porin channel expression. Resistance to beta-lactams appears to be primarily caused by beta-lactamase production, including extended spectrum beta-lactamases (bla(TEM), bla(CTX-M), bla(KPC)), metallo-beta-lactamases (bla(IMP), bla(VIM), bla(SIM)), and most commonly, oxacillinases (bla(OXA)). Antibiotic target site alterations confer resistance to fluoroquinolones (gyrA, parC) and aminoglycosides (arm, rmf), and to a much lesser extent, beta-lactams. Efflux pumps (tet, ade, abe) contribute to resistance against beta-lactams, tetracyclines, fluoroquinolones, and aminoglycosides. Finally, porin channel deletion (carO, oprD) appears to contribute to beta-lactam resistance and may contribute to rarely seen polymyxin resistance. Of note, efflux pumps and porin deletions as solitary mechanisms may not render clinical resistance to A. baumannii. CONCLUSIONS: A. baumannii possesses copious genetic resistance mechanisms. Knowledge of local genotypes and expressed phenotypes for A. baumannii may aid clinicians more than phenotypic susceptibilities reported in large epidemiologic studies.