Risk factors for and role of OprD protein in increasing minimal inhibitory concentrations of carbapenems in clinical isolates of Pseudomonas aeruginosa.

Purpose: This study examined the risk factors for, and molecular mechanisms underlying, the increase in carbapenem minimum inhibitory concentrations (MICs) in clinical isolates of Pseudomonas aeruginosa.

Methodology: Consecutive clinical isolates of P. aeruginosa were collected. The MicroScan WalkAway system detected more than fourfold increases in the MICs of carbapenems in P. aeruginosa isolates serially recovered from some patients during their clinical course. The clinical risk factors associated with this increase were examined by multiple logistic regression analysis. Western blot analysis and nucleotide sequencing of the oprD gene of 19 clonally related and paired P. aeruginosa isolates from the same patients were undertaken to examine the mechanisms underlying the increase in MICs.

Results: The results showed that prior use of carbapenems (OR, 2.799; 95 % CI, 1.088-7.200; P=0.033) and the use of ventilators or tracheostomies (OR, 2.648; 95 % CI, 1.051-6.671; P=0.039) were risk factors for increased carbapenem MICs. Analysis of the underlying mechanisms revealed that loss of functional OprD protein due to mutation of the oprD gene tended to occur in P. aeruginosa isolates with imipenem MICs of more than 8 µg ml; a reduction in OprD expression was observed in P. aeruginosa isolates with imipenem MICs of 4 or 8 µg ml. This difference in the resistance mechanism was not correlated with the MICs of meropenem.

Conclusion: This difference in the resistance mechanism of P. aeruginosa indicates a critical breakpoint at an imipenem MIC of 8 µg ml, in accordance with EUCAST criteria. Reducing carbapenem use will prevent P. aeruginosa clinical isolates from developing resistance to carbapenems.