MdrL, a major facilitator superfamily efflux pump of Listeria monocytogenes involved in tolerance to benzalkonium chloride.

Efflux pumps are recognized as an important mechanism for decreased susceptibility of benzalkonium chloride (BC) in Listeria monocytogenes. Previous studies showed that the efflux pump MdrL was overexpressed in L. monocytogenes exposed to BC. In the present work, we aimed to clarify the role of MdrL in tolerance to BC and environmental stresses including acid, alkali, osmotic, ethanol, and oxidative stresses, as well as resistance to other antimicrobial agents in L. monocytogenes EGD-e. In addition, regulation of the expression of mdrL by LadR was investigated. Gene deletion mutants were constructed by homologous recombination strategy. For the wild-type and mutant strains, minimum inhibitory concentrations (MICs) of antimicrobial agents were determined by the agar dilution, and the growth and survival analysis were also performed. LadR was expressed and the interaction between LadR and the mdrL promoter was investigated by electrophoretic mobility shift assay (EMSA). Compared to the wild-type strain, the growth of ΔmdrL deletion mutant strain was impaired in the presence of sublethal concentration of BC. Moreover, the mutant showed a lower level of survival than that of the wild-type strain in the presence of lethal concentration of BC. However, the deletion of mdrL had no impact on cefotaxime resistance and ethidium bromide efflux. BC could induce the expression of mdrL in L. monocytogenes and the mdrL expression was regulated by LadR instead of SigmaB. LadR was able to specifically bind to the mdrL promoter. The results showed that efflux pump MdrL was associated with BC tolerance in L. monocytogenes EGD-e. Moreover, our results also provided strong evidence that LadR negatively regulated the expression of mdrL. Since BC is commonly used in the food industry, efflux pump MdrL is beneficial for L. monocytogenes to survive this stress in food processing environments.