The F pilus mediates a novel pathway of CDI toxin import

Contact‐dependent growth inhibition (CDI) is a widespread form of inter‐bacterial competition that requires direct cell‐to‐cell contact. CDI⁺ inhibitor cells express CdiA effector proteins on their surface. CdiA binds to specific receptors on susceptible target bacteria and delivers a toxin derived from its C‐terminal region (CdiA‐CT). Here, we show that purified CdiA‐CT⁵³⁶ toxin from uropathogenic Escherichia coli 536 translocates into bacteria, thereby by‐passing the requirement for cell‐to‐cell contact during toxin delivery. Genetic analyses demonstrate that the N‐terminal domain of CdiA‐CT⁵³⁶ is necessary and sufficient for toxin import. The CdiA receptor plays no role in this import pathway; nor do the Tol and Ton systems, which are exploited to internalize colicin toxins. Instead, CdiA‐CT⁵³⁶ import requires conjugative F pili. We provide evidence that the N‐terminal domain of CdiA‐CT⁵³⁶ interacts with F pilin, and that pilus retraction is critical for toxin import. This pathway is reminiscent of the strategy used by small RNA leviviruses to infect F⁺ cells. We propose that CdiA‐CT⁵³⁶ mimics the pilin‐binding maturation proteins of leviviruses, allowing the toxin to bind F pili and become internalized during pilus retraction.