Enhancement of the mexAB - oprM Efflux Pump Expression by a Quorum-Sensing Autoinducer and Its Cancellation by a Regulator, MexT, of the mexEF - oprN Efflux Pump Operon in Pseudomonas aeruginosa

Abstract

ABSTRACT nfxC -type cells of Pseudomonas aeruginosa that produce the MexEF-OprN efflux pump exhibit resistance to fluoroquinolones and chloramphenicol and hypersusceptibility to most classical β-lactam antibiotics. We investigated the molecular mechanism of how the nfxC mutation causes β-lactam hypersusceptibility. The MexAB-OprM extrusion pump transports and confers resistance to β-lactam antibiotics. Interestingly, expression of the mexAB-oprM operon reached the highest level during the mid-stationary growth phase in both wild-type and nfxC -type mutant strains, suggesting that expression of the mexAB-oprM operon may be controlled by cell density-dependent regulation such as quorum sensing. This assumption was verified by demonstrating that exogenous addition of the quorum-sensing autoinducer N -butyryl- l -homoserine lactone (C4-HSL) enhanced the expression of MexAB-OprM, whereas N -(3-oxododecanoyl)- l -homoserine lactone had only a slight effect. Furthermore, this C4-HSL-mediated enhancement of mexAB-oprM expression was repressed by MexT, a positive regulator of the mexEF-oprN operon. It was concluded that β-lactam hypersusceptibility in nfxC -type mutant cells is caused by MexT-mediated cancellation of C4-HSL-mediated enhancement of MexAB-OprM expression.