The McbR transcription factor links the intracellular folate pool to virulence in enterohemorrhagic <i>Escherichia coli</i>

ABSTRACT Bacterial pathogens coordinate the expression of metabolically expensive virulence programs during host colonization. While the impact of environmental cues on virulence has been extensively studied, direct sensing of metabolic intermediates to inform virulence expression is less understood. Competition with the resident microbiota in the case of an infection has a massive impact on the bacteria’s central metabolism, which can be used as a cue for the timely expression of virulence factors. Here, we identify how fluctuations in the intracellular folate pool can affect the expression of virulence factors in the enteric pathogen enterohemorrhagic E. coli (EHEC). Using transcriptomics, mutagenesis, and ligand-binding assays, we demonstrate that the EHEC’s type III secretion system, which is a syringe-like apparatus employed by multiple pathogens to infect and manipulate host cell function, is repressed when the bacteria’s folate biosynthetic pathway is pharmacologically disrupted, and that this effect is mediated by two regulatory proteins: PurR and McbR. Additionally, we identify dihydrofolate as a ligand of the previously orphaned McbR transcription regulator. It is notable that folate (vitamin B9) is an essential building block for DNA synthesis, cell formation, and division throughout the tree of life. IMPORTANCE Host colonization requires enteric pathogens to tightly regulate their virulence program in response to a multitude of factors. In this work, we show that the bacterial physiological state directly informs pathogenic behavior in enterohemorrhagic E. coli (EHEC). Through the study of folate-starved bacterial cultures, we identified dihydrofolate as the endogenous ligand of the McbR transcription factor and demonstrated that McbR is directly linking one-carbon metabolism to virulence gene expression. These findings reveal how metabolites can act as regulatory signals controlling horizontally acquired pathogenicity islands.