Efflux pump contribution to multidrug resistance in clinical isolates of Pseudomonas aeruginosa.

Study Objective: To determine if increased expression of efflux pumps, mutations in the genes encoding regulatory proteins for efflux pumps, or the combination is associated with multidrug-resistant (MDR) Pseudomonas aeruginosa isolates.

Design: Microbiologic evaluation of prospectively collected Pseudomonas aeruginosa isolates.

Setting: University teaching hospital. ISOLATES: One hundred eight unique P. aeruginosa isolates-50 non-MDR and 58 MDR isolates-obtained from pulmonary or blood sources from patients admitted to the intensive care unit between January 1, 1999, and December 31, 2004.

Measurements And Main Results: Isolates were considered MDR if they were resistant to at least three of the following four drugs: ciprofloxacin, tobramycin, ceftazidime, or imipenem. Possible mutations in efflux regulatory genes mexR, nfxB, and mexZ were analyzed by using polymerase chain reaction amplification and DNA sequencing. Determination of the expression of outer membrane proteins OprM and OprJ was performed by using sodium dodecyl sulfate- polyacrylamide gel electrophoresis immunoblotting. Differences in regulatory gene mutations and outer membrane protein expression were compared between non-MDR and MDR isolates. Among the 108 P. aeruginosa isolates, the MDR isolates were more likely to overexpress OprM compared with non-MDR isolates (64% vs 2%, p<0.001). Mutations in mexR and mexZ were present in 64% and 26% of MDR strains, respectively, but were not associated with OprM overexpression or multidrug resistance. Expression of OprJ was not associated with MDR isolates (odds ratio [OR] 3.7, 95% confidence interval [CI] 0.7-18.5, p=0.11). Mutations in nfxB (12% of MDR strains) were also not associated with multidrug resistance (OR 3.5, 95% CI 0.7-17.8, p=0.13). Eight (100%) of 8 isolates with OprJ expression plus OprM overexpression, 12 (92%) of 13 isolates with combined mexR and mexZ mutations, 5 (100%) of 5 isolates with nfxB plus mexZ mutations, and 16 (100%) of 16 isolates with OprM overexpression plus mexZ mutations were MDR isolates.

Conclusion: The presence of one regulatory gene mutation or simple expression of a single outer membrane protein was not linked to multidrug resistance. However, OprM overexpression and multiple efflux regulatory gene mutations or efflux protein expression were associated with MDR P. aeruginosa isolates.