Microbial adhesion promotes Piezo1 activation to initiate innate immunity.

How mammals mount an effective immune response against infectious agents remains unresolved. Here we identify microbial adhesion to myeloid cells as a critical initiating event that precedes pattern recognition receptor (PRR) engagement. Using a skin infection model with pathogenic bacteria and fungi, we demonstrate that neutrophil recruitment occurs in two sequential phases. The early phase is PRR-independent and instead driven by microbial adhesion, which engages the mechanosensitive ion channel Piezo1 to promote leukotriene (LT)B4 production. Together with interleukin-1α, LTB4 induces CXCL1 release, triggering neutrophil infiltration via the same circuit at play during sterile inflammation. By contrast, the late phase is toll-like receptor (TLR)- and CXCL2-dependent, marking a transition to the canonical, pathogen-driven response. Our findings uncover microbial adhesion as a previously unrecognized danger signal that activates innate immunity via mechanotransduction, revealing a paradigm of how immune responses to infection are initiated.