nfxC-type cells of Pseudomonas aeruginosa that produce the MexEF-OprN efflux pump exhibit resistance to fluoroquinolones and chloramphenicol and hypersusceptibility to most classical beta-lactam antibiotics. We investigated the molecular mechanism of how the nfxC mutation causes beta-lactam hypersusceptibility. The MexAB-OprM extrusion pump transports and confers resistance to beta-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 beta-lactam hypersusceptibility in nfxC-type mutant cells is caused by MexT-mediated cancellation of C4-HSL-mediated enhancement of MexAB-OprM expression.