Carbapenem resistance in Acinetobacter baumannii: mechanisms and epidemiology

ABSTRACTThe increasing trend of carbapenem resistance in Acinetobacter baumannii worldwide is a concern since it limits drastically the range of therapeutic alternatives. Metallo-β-lactamases (VIM, IMP, SIM) have been reported worldwide, especially in Asia and western Europe, and confer resistance to all β-lactams except aztreonam. The most widespread β-lactamases with carbapenemase activity in A. baumannii are carbapenem-hydrolysing class D β-lactamases (CHDLs) that are mostly specific for this species. These enzymes belong to three unrelated groups of clavulanic acid-resistant β-lactamases, represented by OXA-23, OXA-24 and OXA-58, that can be either plasmid- or chromosomally-encoded. A. baumannii also possesses an intrinsic carbapenem-hydrolysing oxacillinase, the expression of which may vary, that may play a role in carbapenem resistance. In addition to β-lactamases, carbapenem resistance in A. baumannii may also result from porin or penicillin-binding protein modifications. Several porins, including the 33-kDa CarO protein, that constitute a pore channel for influx of carbapenems, might be involved in carbapenem resistance.