Anti-S-layer monoclonal antibodies impact C. difficile physiology

Clostridioides difficile ( C. difficile ), a gram-positive anaerobic and spore-forming bacterium, is the leading cause of nosocomial antibiotic-associated diarrhea in adults and is characterized by high levels of recurrence and mortality. Surface-layer Protein A (SlpA), the most expressed protein on bacterial surface, plays a crucial role in the early stages of infection although its role in C. difficile physiology is yet to be fully understood. Anti-S-layer antibodies have been identified in the sera of convalescent patients and correlate with improved outcome of C. difficile infection (CDI). However, the precise mechanisms of how anti-S-layer antibodies can confer protection to the host remain unknown. In this study, we report the first monoclonal antibodies (mAbs) targeting S-layer of the reference strain 630. Characterization of these mAbs unravels important roles for S-layer protein in growth, toxin secretion, and biofilm formation with, surprisingly, opposite effects of different anti-SlpA mAbs on these functions. One anti-SlpA mAb impaired C. difficile growth and restored sensitivity to lysozyme-induced lysis. These findings suggest that anti-S-layer antibody responses may include protective and detrimental effects for the host and provide important insights for designing adequate S-layer-targeting therapeutics. ### Competing Interest Statement Unrelated to the submitted work, P.B. received consulting fees from the Regeneron Pharmaceuticals. The other authors have no conflicts of interest to declare.