Complexity of resistance mechanisms to imipenem in intensive care unit strains of Pseudomonas aeruginosa

Pseudomonas aeruginosa can become resistant to carbapenems by both intrinsic (mutation-driven) and transferable (-lactamase-based) mechanisms. Knowledge of the prevalence of these various mechanisms is important in intensive care units (ICUs) in order to define optimal prevention and therapeutic strategies. A total of 109 imipenem-non-susceptible (MIC 4 mg/L) strains of P. aeruginosa were collected in June 2010 from the ICUs of 26 French public hospitals. Their resistance mechanisms were characterized by phenotypic, enzymatic, western blotting and molecular methods. Single or associated imipenem resistance mechanisms were identified among the 109 strains. Seven isolates (6.4) were found to produce a metallo--lactamase (one VIM-1, four VIM-2, one VIM-4 and one IMP-29). Porin OprD was lost in 94 (86.2) strains as a result of mutations or gene disruption by various insertion sequences (ISPa1635, ISPa1328, IS911, ISPs1, IS51, IS222 and ISPa41). Thirteen other strains were shown to be regulatory mutants in which down-regulation of oprD was coupled with overexpressed efflux pumps CzcCBA (n1), MexXY (n9) and MexEF-OprN (n3). The lack of OprD was due to disruption of the oprD promoter by ISPsy2 in one strain and alteration of the porin signal sequence in another. Imipenem resistance in ICU P. aeruginosa strains may result from multiple mechanisms involving metallo--lactamase gene acquisition and genetic events (mutations and ISs) inactivating oprD, turning down its expression while increasing efflux activities or preventing insertion of porin OprD in the outer membrane. This diversity of mechanisms allows P. aeruginosa, more than any other nosocomial pathogen, to rapidly adapt to carbapenems in ICUs.