Glutathione activates virulence gene expression of an intracellular pathogen

Intracellular pathogens are responsible for much of the world-wide morbidity and mortality due to infectious diseases. To colonize their hosts successfully, pathogens must sense their environment and regulate virulence gene expression appropriately. Accordingly, on entry into mammalian cells, the facultative intracellular bacterial pathogen Listeria monocytogenes remodels its transcriptional program by activating the master virulence regulator PrfA. Here we show that bacterial and host-derived glutathione are required to activate PrfA. In this study a genetic selection led to the identification of a bacterial mutant in glutathione synthase that exhibited reduced virulence gene expression and was attenuated 150-fold in mice. Genome sequencing of suppressor mutants that arose spontaneously in vivo revealed a single nucleotide change in prfA that locks the protein in the active conformation (PrfA*) and completely bypassed the requirement for glutathione during infection. Biochemical and genetic studies support a model in which glutathione-dependent PrfA activation is mediated by allosteric binding of glutathione to PrfA. Whereas glutathione and other low-molecular-weight thiols have important roles in redox homeostasis in all forms of life, here we demonstrate that glutathione represents a critical signalling molecule that activates the virulence of an intracellular pathogen. This study shows that glutathione, a ubiquitous antioxidant, is also a critical signalling molecule that allosterically activates the master virulence regulator in the intracellular pathogen Listeria monocytogenes . Glutathione signals promote Listeria pathogenicity To successfully colonize their hosts, intracellular pathogens must be able to sense their environment and modulate virulence gene expression. For instance, when Listeria monocytogenes infects host cells, it remodels its transcriptional program through activation of the master regulator PrfA. Previous work has suggested that PrfA is allosterically regulated by a small molecule activator, specific to the host intracellular environment, but the identity of this small molecule has proven elusive. Here Daniel Portnoy and colleagues show that bacterial and host-derived glutathione is essential for L. monocytogenes pathogenesis, but not via its canonical role in redox homeostasis. Rather, glutathione activates PrfA by acting as the previously predicted allosteric modulator.