Listeria monocytogenes multidrug resistance transporters activate a cytosolic surveillance pathway of innate immunity

To gain insight into the interaction of intracellular pathogens with host innate immune pathways, we performed an unbiased genetic screen of Listeria monocytogenes mutants that induced an enhanced or diminished host innate immune response. Here, we show that the major facilitator superfamily of bacterial multidrug resistance transporters (MDRs) controlled the magnitude of a host cytosolic surveillance pathway, leading to the production of several cytokines, including type I IFN. Mutations mapping to repressors of MDRs resulted in ectopic expression of their cognate transporters, leading to host responses that were increased up to 20-fold over wild-type bacteria, and a 20-fold decrease in bacterial growth in vivo. Mutation of one of the MDRs, MdrM, led to a 3-fold reduction in the IFN-beta response to L. monocytogenes infection, indicating a pivotal role for MdrM in activation of the host cytosolic surveillance system. Bacterial MDRs had previously been associated with resistance to antibiotics and other toxic compounds. This report links bacterial MDRs and host immunity. Understanding the mechanisms through which live pathogens activate innate immune signaling pathways should lead to the discovery of adjuvants, vaccines, and perhaps new classes of therapeutics. Indeed, we show that the mutants identified in this screen induced vastly altered type I IFN response in vivo as well.