Nosocomial outbreak of a non-cefepime-susceptible ceftazidime-susceptible Pseudomonas aeruginosa strain overexpressing MexXY-OprM and producing an integron-borne PSE-1 betta-lactamase.

Cefepime (FEP) and ceftazidime (CAZ) are broad-spectrum cephalosporins that display similar MICs for wild-type Pseudomonas aeruginosa strains. Recently, P. aeruginosa isolates showing a discordance in susceptibility to CAZ and FEP have been noted at the Hospital de Bellvitge in Barcelona, Spain, and a clustering was suspected. During the study period (March to December 2007), 51 patients, particularly those in an intensive care units (ICUs) (n = 29 [57%]), infected or colonized with at least one P. aeruginosa non-FEP-susceptible and CAZ-susceptible (Fep(ns) Caz(s)) phenotype strain were detected. Twenty-three (45%) patients were infected, and the respiratory tract was the most frequent site of infection. Changes in the consumption of antimicrobials in the ICUs were observed over time: a progressive reduction in the levels of consumption of carbapenems (247 defined daily doses [DDD]/1,000 patient days to 66 DDD/1,000 patient days; P = 0.008), after restriction of its use in 2006, and an expected increase in the rate of piperacillin-tazobactam use (42 DDD/1,000 patient days in 2004 to 200 DDD/1,000 patient days in 2007; P < 0.001). Throughout the whole study period, only a single clone of a P. aeruginosa Fep(ns) Caz(s) phenotype strain was identified by pulsed-field gel electrophoresis analysis to be associated with the hyperexpression of MexXY-OprM and the production of an integron-borne PSE-1 ss-lactamase. In conclusion, we identified an epidemic P. aeruginosa clone of an Fep(ns) Caz(s) phenotype strain involving 51 patients, in particular, ICU patients. The combination of the overexpression of an efflux pump and PSE-1 ss-lactamase production is associated with the multidrug-resistant phenotype. The dominant use of a single class of antibiotics could have provided the selective pressure required for the emergence and spread of this P. aeruginosa strain.