The AraC-family regulator GadX enhances multidrug resistance in Escherichia coli by activating expression of mdtEF multidrug efflux genes.

Multidrug efflux pumps contribute to the resistance of Escherichia coli to many antibiotics and biocides. Here, we report that the AraC-family regulator GadX increases multidrug resistance in E. coli through activation of the MdtEF efflux pump. Screening of random fragments of genomic DNA for ability to increase beta-lactam resistance led to the isolation of a plasmid containing gadX, which codes for the regulator of acid resistance. When overexpressed, gadX significantly increased the resistance of the E. coli strain to oxacillin, cloxacillin, nafcillin, erythromycin, rhodamine 6G, and sodium dodecyl sulfate. The increase in drug resistance caused by gadX overexpression was completely suppressed by deleting the multifunctional outer membrane channel gene tolC. TolC interacts with different drug efflux pumps. Quantitative real-time polymerase chain reaction (PCR) showed that GadX activated the expression of mdtEF but none of the other drug efflux pumps in E. coli. Deletion of mdtEF completely suppressed GadX-mediated multidrug resistance. Our results indicate that the GadX regulator, in addition to its role in acid resistance, increases multidrug resistance in E. coli by activating the MdtEF multidrug efflux pump.