including Listeria monocytogenes, Mycobacterium tuberculosis, Legionella pneumophila, Francisella tularensis, group B Streptococcus, and Brucella abortus, it appears that induction of type I IFN is via a novel cytosolic pathway and is independent of TLRs. It has been suggested that these pathogens trigger the cytosolic DNA-sensing pathway, but neither the host sensors nor the bacterial ligands that trigger cytosolic induction of type I IFNs by these pathogens have been identified. Intriguingly, evidence from L. monocytogenes suggests that bacterial multidrug efflux pumps are required for induction of type I IFN , and raises the possibility that a small molecule substrate of these pumps, rather than DNA, is the bacterial trigger of type I IFN gene expression.