Alterations of OprD in Carbapenem-Intermediate and -Susceptible Strains of Pseudomonas aeruginosa Isolated from Patients with Bacteremia in a Spanish Multicenter Study

Abstract

ABSTRACTWe investigated the presence of OprD mutations in 60 strains of metallo-ß-lactamase-negativePseudomonas aeruginosaintermediately susceptible (IS [n= 12]; MIC = 8 μg/ml) or susceptible (S [n= 48]; MICs ≤ 1 to 4 μg/ml) to imipenem and/or meropenem that were isolated from patients with bacteremia in order to evaluate their impact on carbapenem susceptibility profiles. The presence of mutations inoprDwas detected by sequencing analysis. OprD expression was assessed by both outer membrane protein (OMP) analysis and real-time PCR (RT-PCR). Fourteen (23%) isolates had an OprD identical to that of PAO1, and OprD modifications were detected in 46 isolates (77%). Isolates were classified as OprD “full-length types” (T1 [n= 40, including both wild-type OprD and variants showing several polymorphisms]) and OprD “deficient types” (T2 [n= 3 for OprD frameshift mutations] and T3 [n= 17 for premature stop codons inoprD]). RT-PCR showed that 5 OprD type T1 isolates presented reduced transcription ofoprD(0.1- to 0.4-fold compared to PAO1), whileoprDlevels increased more than 2-fold over that seen with PAO1 in 4 OprD type T1 isolates. A total of 50% of the isolates belonging to OprD “deficient types” were susceptible to both carbapenems, and 40% were susceptible to meropenem and intermediately susceptible to imipenem. Only one isolate (5%) within this group was intermediately susceptible to both carbapenems, and one (5%) was susceptible to imipenem and intermediately susceptible to meropenem. We concluded that OprD inactivating mutations in clinical isolates ofP. aeruginosaare not restricted only to carbapenem-resistant isolates but are also found in isolates with imipenem or meropenem MICs of only 0.06 to 4 μg/ml.