Expression of the MexXY-OprM efflux system in Pseudomonas aeruginosa with discordant cefepime/ceftazidime susceptibility profiles Somvadee Laohavaleeson1, Karen Lolans2,3, John P Quinn2,3,4, Joseph L Kuti1, David P Nicolau11Center for Anti-Infective Research and Development, Hartford Hospital, Hartford, CT, USA; 2John Stroger Hospital, Chicago, IL, USA; 3Chicago Infectious Disease Research Institute, Chicago, IL, USA; 4Rush University Medical Center, Chicago, IL, USAAbstract: While MIC distributions and percent susceptibility for cefepime and ceftazidime are generally similar among Pseudomonas aeruginosa, we noted an increasing discordance in susceptibility favoring ceftazidime at our hospital. Quantitative reverse transcriptase-polymerase chain reaction was utilized to explore overexpression of the MexXY-OprM efflux as the mechanism for this phenotype profile. Thirteen of 15 (87%) randomly selected isolates had mexY gene expression levels of 5.8–40.8-fold relative to the wild-type reference strain. While mexY overexpression was noted in the majority of isolates, other resistance mechanisms appear to contribute to the observed phenotypic profile of the Pseudomonas aeruginosa studied. Clinicians must understand not only the magnitude of difference in the MIC profiles between agents, but also the mechanism(s) responsible for these observations if strategies (ie, pharmacodynamic dosing) are to be designed to optimize patient care outcomes in the face of increasing resistance.Keywords: Pseudomonas aeruginosa, efflux, MexXY-OprM, mexY, cefepime, resistance