Overexpression of AmpC and Efflux Pumps in Pseudomonas aeruginosa Isolates from Bloodstream Infections: Prevalence and Impact on Resistance in a Spanish Multicenter Study

Abstract

ABSTRACTThe prevalence and impact of the overexpression of AmpC and efflux pumps were evaluated with a collection of 190Pseudomonas aeruginosaisolates recovered from bloodstream infections in a 2008 multicenter study (10 hospitals) in Spain. The MICs of a panel of 13 antipseudomonal agents were determined by microdilution, and the expressions ofampC,mexB,mexY,mexD, andmexFwere determined by real-time reverse transcription (RT)-PCR. Up to 39% of the isolates overexpressed at least one of the mechanisms.ampCoverexpression (24.2%) was the most prevalent mechanism, followed bymexY(13.2%),mexB(12.6%),mexF(4.2%), andmexD(2.2%). The overexpression ofmexBplusmexY, documented for 5.3% of the isolates, was the only combination showing a significantly (P= 0.02) higher prevalence than expected from the frequencies of the individual mechanisms (1.6%). Additionally, all imipenem-resistant isolates studied (25 representative isolates) showed inactivating mutations inoprD. Most of the isolates nonsusceptible to piperacillin-tazobactam (96%) and ceftazidime (84%) overexpressedampC, whilemexB(25%) andmexY(29%) overexpressions gained relevance among cefepime-nonsusceptible isolates. Nevertheless, the prevalence ofmexYoverexpression was highest among tobramycin-nonsusceptible isolates (37%), and that ofmexBwas highest among meropenem-nonsusceptible isolates (33%). Regarding ciprofloxacin-resistant isolates, besides the expected increased prevalence of efflux pump overexpression, a highly significant link toampCoverexpression was documented for the first time: up to 52% of ciprofloxacin-nonsusceptible isolates overexpressedampC, sharply contrasting with the 24% documented for the complete collection (P< 0.001). In summary, mutation-driven resistance was frequent inP. aeruginosaisolates from bloodstream infections, whereas metallo-β-lactamases, detected in 2 isolates (1%) producing VIM-2, although with increasing prevalences, were still uncommon.