ABSTRACTThe intracellular human pathogenShigella flexneriinvades the colon epithelium, replicates to high cell density within the host cell, and then spreads to adjacent epithelial cells. WhenS. flexnerigains access to the host cytosol, the bacteria metabolize host cytosolic carbon using glycolysis and mixed acid fermentation, producing formate as a by-product. We show thatS. flexneriinfection results in the accumulation of formate within the host cell. Loss of pyruvate formate lyase (PFL; ΔpflB), which converts pyruvate to acetyl coenzyme A (CoA) and formate, eliminatesS. flexneriformate production and reduces the ability ofS. flexnerito form plaques in epithelial cell monolayers. This defect in PFL does not decrease the intracellular growth rate ofS. flexneri; rather, it affects cell-to-cell spread. TheS. flexneriΔpflBmutant plaque defect is complemented by supplying exogenous formate; conversely, deletion of theS. flexneriformate dehydrogenase genefdnGincreases host cell formate accumulation andS. flexneriplaque size. Furthermore, exogenous formate increases plaque size of the wild-type (WT)S. flexneristrain and promotesS. flexnericell-to-cell spread. We also demonstrate that formate increases the expression ofS. flexnerivirulence genesicsAandipaJ. IntracellularS. flexneriicsAandipaJexpression is dependent on the presence of formate, andipaJexpression correlates withS. flexneriintracellular density during infection. Finally, consistent with elevatedipaJ, we show that formate altersS. flexneri-infected host interferon- and tumor necrosis factor (TNF)-stimulated gene expression. We propose thatShigella-derived formate is an intracellular signal that modulates virulence in response to bacterial metabolism.IMPORTANCEShigellais an intracellular pathogen that invades the human host cell cytosol and exploits intracellular nutrients for growth, enabling the bacterium to create its own metabolic niche. ForShigellato effectively invade and replicate within the host cytoplasm, it must sense and adapt to changing environmental conditions; however, the mechanisms and signals sensed byS. flexneriare largely unknown. We have found that the secretedShigellametabolism by-product formate regulatesShigellaintracellular virulence gene expression and its ability to spread among epithelial cells. We propose thatShigellasenses formate accumulation in the host cytosol as a way to determine intracellularShigelladensity and regulate secreted virulence factors accordingly, enabling spatiotemporal regulation of effectors important for dampening the host immune response.
Virulence gene deletion frequency is increased in Shigella flexneri following conjugation, transduction, and transformation