C-di-AMP accumulation disrupts glutathione metabolism in Listeria monocytogenes.

Abstract

C-di-AMP homeostasis is critical for bacterial stress response, cell wall integrity, and virulence. Except for osmotic stress response, the molecular mechanisms underlying other processes are not well defined. A Listeria monocytogenes mutant lacking both c-di-AMP phosphodiesterases, denoted as the ΔPDE mutant, is significantly attenuated in the mouse model of systemic infection. We utilized the ΔPDE mutant to define the molecular functions of c-di-AMP. RNAseq revealed that the ΔPDE mutant is significantly impaired for the expression of virulence genes regulated by the master transcription factor PrfA, which is activated by reduced glutathione (GSH) during infection. Subsequent quantitative gene expression analyses revealed that the ΔPDE strain is defective for PrfA-regulated gene expression both at the basal level and upon activation by GSH. We further found the ΔPDE strain to be significantly depleted for cytoplasmic GSH and impaired for GSH uptake. The ΔPDE strain was also deficient in GSH under conditions that activate GSH synthesis by the synthase GshF and upon constitutive expression of gshF, suggesting that c-di-AMP accumulation inhibits GSH synthesis activity or promotes GSH catabolism. A constitutively active PrfA* variant restored virulence gene expression in ΔPDE in broth cultures supplemented with GSH but did not rescue virulence defect in vivo. Therefore, virulence attenuation at high c-di-AMP is likely associated with defects outside of the PrfA regulon. For instance, the ΔPDE strain was sensitive to oxidative stress, a phenotype exacerbated in the absence of GshF. Our data reveal GSH metabolism as another pathway that is regulated by c-di-AMP.IMPORTANCEC-di-AMP regulates both bacterial pathogenesis and interactions with the host. Although c-di-AMP is essential in many bacteria, its accumulation also attenuates the virulence of many bacterial pathogens. Therefore, disrupting c-di-AMP homeostasis is a promising antibacterial treatment strategy and has inspired several studies that screened for chemical inhibitors of c-di-AMP phosphodiesterases. However, the molecular functions of c-di-AMP are still not fully defined, and the underlying mechanisms for attenuated virulence at high c-di-AMP levels are unclear. Our analyses in Listeria monocytogenes indicate that virulence-related defects are likely outside of the virulence gene regulon. We found c-di-AMP accumulation to impair L. monocytogenes virulence gene expression and disrupt GSH metabolism. Further studies are necessary to establish the relative contributions of these regulations to virulence and host adaptation.