Bacterial (Interferon)ce: Intracellular bacterial pathogens, such as Listeria monocytogenes , are detected in the cytosol of host immune cells, where they induce a host response that is often dependent on microbial secretion systems. Woodward et al. (p. 1703 , published online 27 May) now show that L. monocytogenes produce and release cyclic diadenosine monophosphate into the host cytosol, which induces the production of host type I interferon. Because a number of intracellular pathogens contain the protein machinery to generate this nucleotide and also activate this same innate immune pathway, a common molecular mechanism may exist for host detection of cytosolic bacterial pathogens.

Listeria monocytogenes is a facultative intracellular pathogen that induces a cytosolic signaling cascade resulting in expression of interferon (IFN)-β. Although type I IFNs are critical in viral defense, their role in immunity to bacterial pathogens is much less clear. In this study, we addressed the role of type I IFNs by examining the infection of L. monocytogenes in BALB/c mice lacking the type I IFN receptor (IFN-α/βR^−/−). During the first 24 h of infection in vivo, IFN-α/βR^−/− and wild-type mice were similar in terms of L. monocytogenes survival. In addition, the intracellular fate of L. monocytogenes in macrophages cultured from IFN-α/βR^−/− and wild-type mice was indistinguishable. However, by 72 h after inoculation in vivo, IFN-α/βR^−/− mice were ∼1,000-fold more resistant to a high dose L. monocytogenes infection. Resistance was correlated with elevated levels of interleukin 12p70 in the blood and increased numbers of CD11b⁺ macrophages producing tumor necrosis factor α in the spleen of IFN-α/βR^−/− mice. The results of this study suggest that L. monocytogenes might be exploiting an innate antiviral response to promote its pathogenesis.

Listeria monocytogenesinfection leads to robust induction of an innate immune signaling pathway referred to as the cytosolic surveillance pathway (CSP), characterized by expression of beta interferon (IFN-β) and coregulated genes. We previously identified the IFN-β stimulatory ligand as secreted cyclic di-AMP. Synthesis of c-di-AMP inL. monocytogenesis catalyzed by the diadenylate cyclase DacA, and multidrug resistance transporters are necessary for secretion. To identify additional bacterial factors involved inL. monocytogenesdetection by the CSP, we performed a forward genetic screen for mutants that induced altered levels of IFN-β. One mutant that stimulated elevated levels of IFN-β harbored a transposon insertion in the genelmo0052. Lmo0052, renamed here PdeA, has homology to a cyclic di-AMP phosphodiesterase, GdpP (formerly YybT), ofBacillus subtilisand is able to degrade c-di-AMP to the linear dinucleotide pApA. Reduction of c-di-AMP levels by conditional depletion of the di-adenylate cyclase DacA or overexpression of PdeA led to marked decreases in growth rates, bothin vitroand in macrophages. Additionally, mutants with altered levels of c-di-AMP had different susceptibilities to peptidoglycan-targeting antibiotics, suggesting that the molecule may be involved in regulating cell wall homeostasis. During intracellular infection, increases in c-di-AMP production led to hyperactivation of the CSP. Conditional depletion ofdacAalso led to increased IFN-β expression and a concomitant increase in host cell pyroptosis, a result of increased bacteriolysis and subsequent bacterial DNA release. These data suggest that c-di-AMP coordinates bacterial growth, cell wall stability, and responses to stress and plays a crucial role in the establishment of bacterial infection.IMPORTANCEListeria monocytogenesis a Gram-positive intracellular pathogen and the causative agent of the food-borne illness listeriosis. Upon infection,L. monocytogenesstimulates expression of IFN-β and coregulated genes dependent upon host detection of a secreted bacterial signaling nucleotide, c-di-AMP. Using a forward genetic screen for mutants that induced high levels of host IFN-β expression, we identified a c-di-AMP phosphodiesterase, PdeA, that degrades c-di-AMP. Here we characterizeL. monocytogenesmutants that express enhanced or diminished levels of c-di-AMP. Decreased c-di-AMP levels by conditional depletion of the diadenylate cyclase (DacA) or overexpression of PdeA attenuated bacterial growth and led to bacteriolysis, suggesting that its production is essential for viability and may regulate cell wall metabolism. Mutants lacking PdeA had a distinct transcriptional profile, which may provide insight into additional roles for the molecule. This work demonstrates that c-di-AMP is a critical signaling molecule required for bacterial replication, cell wall stability, and pathogenicity.