The mechanism of imipenem resistance in Pseudomonas aeruginosa isolates

Objective To investigate the possible contribution of OprD expression and metallo-β-lactamases (MBLs) to imipenem resistance in Pseudomonas aeruginosa. Methods Clinical imipenem-resistant strains were screened for MBLs production by a disk diffusion synergy test and subjected to PCR assays with primers specific for MBLs. Sequence analysis was conducted to identify the prevalence of MBLs gene. Biochemical properties of MBLs were determined by β-lactamase assays with crude preparation of β-lactamases. Expression of OprD2 was analyzed by quantitative RT-PCR and Western blot. Results Among the 34 imipenem-resistant P. aeruginosa strains, 2 (5.9%) and 3 (8.8%) were positive for VIM-2 and IMP-1 gene respectively. Crud extraction of β-lactamases showed imipenem-hydrolyzing activity. The activity was inhibited after treatment with EDTA. In these imipenem-resistant clinical isolates, OprD2 protein was low-expressed in 3 isolates (8.8%) and normally expressed in 3 isolates (8.8%), but not expressed in 28 isolates (82.4%). However, the 5 MBLs producing isolates (14.7%) were all lack of OprD2 expression. Conclusions Reduced or lack of OprD2 expression is the essential mechanism for imipenem-resistance in most P. aeruginosa isolates. VIM-2 and IMP-1 type MBLs are prevalent in P. aeruginosa clinical isolates in our institution.