A programmed reset mechanism primes <i>Clostridioides difficile</i> virulence at the onset of infection.

Abstract

<i>Clostridioides difficile</i> is a major cause of antibiotic-associated diarrhea. This pathogen produces toxins essential for disease and flagella that promote intestinal colonization. Flagella and toxins are coregulated in an ON/OFF manner leading to a heterogeneous population of toxigenic, flagellated cells (<i>flg</i>-ON) and aflagellate cells with attenuated toxin production (<i>flg</i>-OFF). How selective pressures affect ON/OFF switching dynamics and overall population composition is unknown. Here, we use a mouse model of disease to evaluate the ON/OFF makeup of the <i>C. difficile</i> population in vivo. Two key results emerged: (i) Directional inversion of the <i>flg</i> switch to the "ON" orientation occurs during spore germination, and (ii) toxin-induced inflammation enriches for a <i>flg</i>-OFF population during infection. These findings reveal a cycle in which predominantly <i>flg</i>-OFF spores are disseminated into the environment, and spore germination in the next host resets <i>C. difficile</i> to a virulent population of motile, toxigenic cells at infection onset.