Interkingdom sensing of fungal tyrosol promotes bacterial antifungal T6SS activity in the murine gut.

Type VI secretion systems (T6SSs) are molecular machines used by bacteria to release effectors that target either host cells, competing bacteria or fungi. Regulatory mechanisms underlying antifungal T6SS activity remain unexplored. Here we show, using mouse infection with wild-type and T6SS mutant bacteria, that T6SS activity of the enteropathogen, Yersinia pseudotuberculosis (Yptb), reduces fungal prevalence in the gut microbiota and has direct activity on Candida albicans. Screening of bacterial effector mutant strains, and structural and biochemical analyses identify TfeC as an antifungal chitinase T6SS effector that can kill C. albicans. In vivo experiments confirm that TfeC expression promotes Yptb colonization and reduces C. albicans abundance. We also show that Yptb senses the fungal quorum-sensing molecule, tyrosol, through the two-component system, EnvZ-OmpR, and responds by activating T6SS4. Our findings suggest that Yptb modulates its antifungal activities by detecting changes in fungal population density cues, revealing a mechanism of fungal-bacterial interkingdom communication mediated by fungal quorum-sensing molecules.