First detected OXA-50 carbapenem-resistant clinical isolates Pseudomonas aeruginosa from Bulgaria and interplay between the expression of main efflux pumps, OprD and intrinsic AmpC

Introduction. Carbapenems are often described as the most effective weapon against infections caused by multidrug-resistant bacteria especially those belonging to the group of non-fermenting bacteria such as Pseudomonas. The main mechanisms leading to resistance are the hyperexpression of certain efflux pumps belonging to the resisto-nodular division and the lower expression of the transmembrane porin OprD, sometimes in combination with excessive production of the intrinsic AmpC. Car-bapenemases are assumed to play a secondary role. Aim. The aim of this study was to determine the exact mechanisms of carbapenem resistance in Pseudomonas aeruginosa isolates from the largest Bulgarian University hospital 'St. George'-Plovdiv. Methodology. A total of 32 clinical isolates collected from different patients' samples resistant to imipenem and/or meropenem were examined via phenotypic and molecular-genetic tests. Results. No metallo-enzyme production was detected. Three isolates were positive for OXA-50-encoding genes in two of them in combination with other oxacillinases or the bla(VEB-1) gene. For the first time, OXA-50-producing P. aeruginosa have been reported in Bulgaria. The increased expression or hyperexpression of MexXY-OprM efflux pump was observed as the main mechanism of resistance. In most cases, it was combined with lower expression or lack of OprD with or without MexAB-OprM hyperexpression. No excessive production of AmpC was detected in comparison to the reference ATCC 27853 P. aeruginosa strain. Conclusion. The increased expression or overexpression of MexXY-OprM efflux pumps is the leading cause of carbapenem resistance in our isolates Pseudomonas, detected in 94% of the bacteria investigated.