Mechanism of imipenem resistance in AmpC β-lactamases-producing Pseudomonas aeruginosa

Objective To investigate the molecular mechanism of imipenem (IPM) resistance in AmpC β-lactamases-producing P. aeruginosa.Methods Two hundred and twenty strains of P. aeruginosa were isolated from inpatients between 2003 and 2007. Three-dimensional test was used to identify the P. aeruginosa producing β-lactamases (including AmpC β-lactamases). PCR and Multiplex PCR were performed to detect oprD2 genes and plasmid-mediated AmpC β-lactamase genes,respectively. The expression of chromosomal AmpC β-lactamases and oprD2 genes in P. aeruginosa were analyzed by real-time reverse transcriptase PCR.Results Production of β-lactamases was found in 40 isolates. AmpC β-lactamases,extended spectrum β-lactamases (ESBLs),metallo-β-lactamases (MBL) and an unknown type of β-lactamases were identified in 62.5% (25/40),20% (8/40),7.5% (3/40) and 10% (4/40) of these isolates,respectively. Of the 25 isolates expressing increased AmpC β-lactamases,the plasmid-mediated AmpC β-lactamase gene DHA was identified in only 1 isolate. Diminished expression of oprD2 was found in 12 isolates. These strains were resistant to imipenem. The other 13 isolates had normal oprD2 expression. Five of these 13 strains were resistant to imipenem. The AmpC lactamase crude extract (not MBL) from 7 isolates could weakly hydrolyze imipenem. Five of these 7 isolates also showed lower expression of oprD2 gene.Conclusions Some AmpC β-lactamases produced by P. aeruginosa can weakly hydrolyze imipenem. This hydrolytic activity is dependent on the expression level of AmpC β-lactamases. The mechanism of imipenem resistance in P. aeruginosa is a result of the interplay between diminished production of oprD2 and increased activity of AmpC β-lactamase.