Ceftazidime-Induced Overexpression of MexJK-OprM Confers Resistance to Ceftolozane/Tazobactam in an Isolate of Pseudomonas aeruginosa.

OBJECTIVE

The emergence of resistance to ceftolozane/tazobactam (CTLZ/TAZ) in Pseudomonas aeruginosa poses a significant clinical challenge. This study investigates the mechanisms underlying CTLZ/TAZ resistance in a clinical isolate and examines the role of prior antibiotic exposure.

METHODS

A TAZ/CTLZ-resistant P. aeruginosa strain (Pa-R) was isolated from a patient with a lung abscess after six weeks of antibiotic therapy. Genetic relatedness between Pa-R and a pre-treatment susceptible strain (Pa-S) was assessed using PCR-based open reading frame typing (POT). Carbapenemase genes were detected via multiplex PCR. Sequencing and expression analyses of ampC, ftsI, oprD, and mex family genes were conducted. RNA interference targeting mexJK was performed to validate its role in resistance. In vitro antimicrobial exposure assays were conducted to evaluate resistance acquisition and the impact of prior ceftazidime (CAZ) exposure.

RESULTS

Pa-R and Pa-S were genetically identical and lacked carbapenemase genes. No mutations or overexpression of ampC were observed; however, mexJK expression was approximately 30-fold higher in Pa-R. RNA interference using mexJK-specific dsRNA restored CTLZ/TAZ susceptibility, confirming that high MexJK-OprM expression is the mechanism of resistance. In vitro assays demonstrated that CTLZ/TAZ alone did not induce resistance, whereas prior CAZ exposure led to CTLZ/TAZ resistance accompanied by mexJK overexpression.

CONCLUSION

This study demonstrated that overexpression of the MexJK-OprM efflux pump is a key mechanism underlying TAZ/CTLZ resistance. In addition, prior administration of CAZ contributed to the acquisition of resistance, highlighting the need to consider the risk of resistance acquisition when switching antibiotics. In such cases, the choice of carbapenems should be considered.