Uncovering the Resistance Mechanisms in Extended-Drug-Resistant Pseudomonas aeruginosa Clinical Isolates: Insights from Gene Expression and Phenotypic Tests.

(1) Background: The purpose of the study was to describe the activity of mex efflux pumps in Multidrug-Resistant (MDR) clinical isolates of Pseudomonas aeruginosa and to compare the carbapenem-resistance identification tests with PCR; (2) Methods: Sixty MDR P. aeruginosa were analyzed for detection of carbapenemase by disk diffusion inhibitory method, carbapenem inactivation method and Modified Hodge Test. Endpoint PCR was used to detect 7 carbapenemase genes ( bla KPC , bla OXA48-like , bla NDM , bla GES-2 , bla SPM , bla IMP , bla VIM ) and mcr-1 for colistin resistance. The expression of mex A, mex B, mex C, mex E and mex X genes corresponding to the four main efflux pumps was also evaluated; (3) Results: From the tested strains, 71.66% presented at least one carbapenemase gene, with bla GES-2 as the most occurring gene (63.3%). Compared with the PCR, the accuracy of phenotypic tests did not exceed 25% for P. aeruginosa . The efflux pump genes were present in all strains except one. In 85% of the isolates, an overactivity of mex A, mex B and mostly mex C was detected. Previous treatment with ceftriaxone increased the activity of mex C by more than 160 times; (4) Conclusions: In our MDR P. aeruginosa clinical isolates, the carbapenem resistance is not accurately detected by phenotypic tests, due to the overexpression of mex efflux pumps and in a lesser amount, due to carbapenemase production.