[The roles of active efflux system overexpression and outer membrane protein OprD deficiency or loss in carbapenem resistance of Pseudomonas aeruginosa].

OBJECTIVE

To investigate the mechanisms of carbapenem resistance in Pseudomonas aeruginosa (PA).

METHODS

Forty-nine strains of PA were isolated from surgical intensive care unit during a period of 3 years. The levels of outer membrane protein OprD and OprN were measured by Western blotting. RT-PCR was used to measure the transcription levels of mexA gene. The metallo-beta-lactamase genes IMP and VIM and the negative regulator gene mexR for mexAB-OprM operon were amplified. The DNA fragments were sequenced by automated ABI PRISM 3700 sequencer.

RESULTS

42 of the 49 strains were resistant to carbapenem. 23 of the 42 strains showed loss of OprD and were all resistant to imipenem, but only one strain was resistant to meropenem. 18 of the 42 strains had a decreased OprD expression, 17 of which were resistant to Imipenem, and 3 were resistant to meropenem as well. 7 strains expressed OprD, all of which were sensitive to carbapenem. 27 strains overexpressed the mexAB-OprM. The resistant rate to imipenem of the mexAB-OprM overexpression group was 86.4%, not significantly different from that of the mexAB-OprM low expression group (81.5%, chi(2) = 0.005, P = 0.943). But the resistant rate to meropenem of the mexAB-OprM overexpression group was 44.4%, statistically higher than that of mexAB-OprM low expression group (13.6%, chi(2) = 5.417, P = 0.020). Nucleotide sequences and deduced amino acid sequences analysis revealed that eight strains overexpressed mexAB-OprM carried mutations in mexR gene, 7 of which had amino acid substitutions in MexR protein, and one of which had terminal code at the position of amino acid 32. 14 strains were found expressing OprN. Neither IMP gene nor VIM gene was found in the isolates.

CONCLUSION

In the clinical strains from SICU, the imipenem resistance is mainly mediated by OprD deficiency or loss. Overexpression of MexAB-OprM is the primary mechanism of meropenem resistance, which is upregulated by mutations in mexR gene. Metallo-beta-lactamases IMP and VIM are rarely seen.