Molecular epidemiology of hospital carbapenem resistant Pseudomonas aeruginosa

Pseudomonas aeruginosa is an important opportunistic pathogen, characterized by environmental versatility and ability to cause a wide range of human infections in susceptible individuals. The increasing frequency of multi-drug-resistant P. aeruginosa (MDRPA) strains in hospital environment is of major concern due to limited treatment options. The aims of the study were to investigate the underlying mechanisms that conferred the carbapenem resistance phenotype, and to examine the clonal relationships of isolates. The study included 38 P. aeruginosa strains, with reduced susceptibility to at least one of the carbapenems, recovered consecutively from various clinical specimens of separate patients at the Clinical Hospital Centre Rijeka over a period of six months (July to December 2009). To assess the mechanisms leading to resistance, all 38 isolates were analysed for expression of the chromosomal beta-lactamase (AmpC), the porin important for entry of carbapenems (OprD) and an efflux system known to extrude some beta-lactams. All isolates were genotyped by pulsed-field electrophoresis (PFGE). Up to 82% of the carbapenem-resistant isolates were multidrug-resistant. PCR analysis did not detect any MBLs or KPC genes. Carbapenem resistance was driven mainly by mutational inactivation of OprD (82% of strains), accompanied or not by hyperexpression of MexAB-OprM (44% of strains) or MexAB-OprJ (11% of strains). Pulsed-field gel electrophoresis (PFGE) revealed 35 distinct profiles distributed in 5 clusters. The largest pulsogroup designated C comprised 22 out of 38 isolates (58%). Molecular epidemiological studies have revealed that two clones, A and C, caused outbreaks in hospital ICUs. Our study showed oligoclonal outbreak of P. aeruginosa with several mechanisms involved in carbapenem resistance - the low outer membrane permeability combined with efflux system overexpression.