Determining the importance of the stringent response for methicillin-resistant Staphylococcus aureus virulence in vivo

The stringent response is a stress signalling pathway with links to bacterial virulence. This pathway is controlled by the nucleotide alarmone (p)ppGpp, produced in Staphylococcus aureus by three synthetase enzymes. Here, we used a panel of synthetase mutants to examine the importance of this signalling network for S. aureus survival and virulence in vivo. Using a zebrafish larval infection model, we observed that infection with a (p)ppGpp null strain attenuated virulence. Zebrafish myeloid cell depletion restored the virulence during systemic infection, indicating that (p)ppGpp is important for phagocyte-mediated immune evasion. Primary macrophages infection studies, followed by in vitro tolerance assays and RNA-seq, revealed that (p)ppGpp is required to survive stressors found within the intracellular macrophage environment, with roles for each class of synthetase, and the linked transcription factor CodY, implicated. Taken together, these results define the importance of the stringent response and each class of synthetase for S. aureus infection.