The CspC pseudoprotease regulates germination of Clostridioides difficile spores in response to multiple environmental signals
The gastrointestinal pathogen, Clostridioides difficile, initiates infection when its metabolically dormant spore form germinates in the mammalian gut. While most spore-forming bacteria use transmembrane germinant receptors to sense nutrient germinants, C. difficile uses the soluble pseudoprotease, CspC, to detect bile salt germinants. To gain insight into CspCs unique mechanism of action, we solved its crystal structure. Guided by this structure, we identified CspC mutations that confer either hypo- or hyper-sensitivity to bile salt germinant. Surprisingly, hyper-sensitive CspC variants exhibited bile salt-independent germination as well as increased sensitivity to amino acid and/or calcium co-germinants. Since the mechanism by which C. difficile spores sense co-germinants is unknown, our study provides the first evidence that CspC senses distinct classes of co-germinants in addition to bile salts. Since we observed that specific residues control CspCs responsiveness to these different signals, CspC is critical for regulating C. difficile germination in response to multiple environmental signals.