Effect of single-dose carbapenem exposure on transcriptional expression of blaNDM-1 and mexA in Pseudomonas aeruginosa

The therapeutic option of a carbapenem antibiotic is compromised in Pseudomonas aeruginosa owing both to acquired and intrinsic resistance mechanisms. In recent years, New Delhi metallo-β-lactamase has been the focus as a predominant carbapenem resistance determinant. However, it is unclear which of the mechanisms might be adopted by a P. aeruginosa strain possessing both blaNDM-1 and an overexpressed MexAB-OprM system during carbapenem therapy. This study investigated the interplay of both mechanisms in clinical isolates of P. aeruginosa when exposed to meropenem. Five strains were used: (i) strain overexpressing MexAB-OprM but with no blaNDM-1; (ii) strain harbouring blaNDM-1 but expressing MexAB-OprM at basal level; (iii) strain possessing blaNDM-1 and overexpressing MexAB-OprM; (iv) P. aeruginosa PAO1; and (v) P. aeruginosa K2733-PAO1 (ΔMexAB-OprMΔMexCD-OprJΔMexEF-OprNΔMexXY-OprM) into which blaNDM-1 was cloned. Strains were incubated in Luria-Bertani broth with and without 1 μg/mL meropenem. Total RNA was isolated at 45-min intervals and was immediately reverse transcribed to cDNA. This was repeated for 6 h. Quantitative real-time PCR was performed for both resistance mechanisms. Meropenem exposure did not significantly elevate transcription of either the blaNDM-1 or mexA gene. However, an interesting finding was that upon single-dose exposure to carbapenem, the efflux pump system played a major role in bacterial survival compared with NDM-1. This study gives an insight into the bacterial response to carbapenem antibiotic when two different resistance mechanisms coexist. This type of study would be helpful in designing future antimicrobials.