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

Results: Single or associated imipenem resistance mechanisms were identified among the 109 strains. Seven isolates (6.4%) were found to produce a metallo-b-lactamase (one VIM-1, four VIM-2, one VIM-4 and one IMP29). 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 (n1⁄41), MexXY (n1⁄49) and MexEF-OprN (n1⁄43). 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.