BPS2026 - Bacterial flagellin gradients induce phenotypic switching in human neutrophils

Flagellar motility provides bacterial pathogens with a key evolutionary advantage in navigating heterogeneous host environments and evading immune clearance. Yet how motility influences bacterial spread in physically confined tissue-like spaces has remained unclear. Using a microfluidic platform that recapitulates narrow constrictions, we discovered that motile Escherichia coli penetrated deep into confined crevices, whereas non-motile mutants lacking flagella failed to colonize these regions. Co-incubation with human neutrophils revealed striking functional plasticity in the immune response. Against motile bacteria in confinement, neutrophils extended hyper-elongated pseudopodia (20–150 μm) to pursue them into narrow crevices. By contrast, in the absence of confinement, neutrophils produced neutrophil extracellular traps (NETs) against both motile and non-motile bacteria. Thus, the mechanical environment governed a switch between pseudopod response and NET release. This switch was further mediated by bacterial flagellin acting through Toll-like receptor 5 (TLR5): inhibition of TLR5 abolished the plasticity, inducing a default NET response. Perturbing host actin polymerization or myosin II contractility biased neutrophils toward one outcome or the other, implicating a balance between front-end actin extension and rear contractility in regulating the decision. Together, these findings show that bacterial motility enables penetration of confined physical barriers, while neutrophils integrate flagellin-derived cues with cytoskeletal regulation to toggle between exploratory pseudopodia and pro-inflammatory NETs. This work reveals a previously unrecognized form of host-pathogen crosstalk, in which flagellar propulsion influences bacterial access to various niches as well as the mode of neutrophil response.