Battle beyond membrane: flagella as a conduit for phage DNA entry and a trigger for bacterial defense in <i>Yersinia enterocolitica</i>

Bacterial flagella are known for facilitating motility to support nutrient acquisition and predator evasion, but can also serve as receptors for phages. Here, we characterize a dual role of flagella during infection of Yersinia enterocolitica by phage X1: functioning as a conduit for phage DNA entry and as a trigger for bacterial defense. X1 employs a "contraction-driven" mechanism, using its contractile tail to inject DNA into Yersinia flagella. Unlike other characterized flagellotropic phages that rely exclusively on flagella or less efficiently on secondary surface receptors, X1 can infect cells via lipopolysaccharides with even higher efficiency. Furthermore, certain Yersinia strains can detect X1 invasion and activate a flagellum-dependent toxin-antitoxin (Flag-TA) system, which requires the flagellar motor stator proteins MotAB to activate the abortive infection response. Our findings reveal a novel phage infection strategy and uncover a new bacterial defense that are both based on the flagellar machinery.