Mechanisms of Carbapenem Resistance in Multidrug-Resistant Clinical Isolates of Pseudomonas aeruginosa from a Croatian Hospital

Pseudomonas aeruginosa is an opportunistic pathogen, one of the leading causes of nosocomial infections such as pneumonia, urinary tract infections, and bacteraemia. The bacterial resistance to structurally unrelated antibiotics and its spread within hospitals limits the efficient antimicrobial options and patients' outcome. Carbapenems are important agents for the therapy of infections due to multidrug-resistant (MDR) P. aeruginosa; hence, the development of carbapenem resistance severely hampers effective therapeutic options. The aim of this investigation was to examine mechanisms of carbapenem resistance and genomic diversity in carbapenem-resistant MDR strains of P. aeruginosa, which caused an outbreak among patients in Clinical Hospital Rijeka. Most of the isolates showed decreased expression of porin that is important for the entry of carbapenems (oprD). Overexpression of MexAB-OprM, MexCD-OprJ, and MexEF-OprN efflux systems was observed in many of the isolates. Production of metallo--lactamases was not detected. Typing by pulsed-field gel electrophoresis discriminated the isolates into five clusters. The clonal distribution of the strains was related to the location of hospital departments where the isolates were collected, which implies that most of the infections were caused by spread of the epidemic strains within the hospital.