Experimental human colonisation with non-toxigenic Clostridioides difficile: a placebo-controlled randomised clinical trial.

Clostridioides difficile infections remain a major global healthcare burden, underscoring the need for novel therapies. Human colonisation models provide mechanistic insight into C. difficile colonisation and facilitate identification of novel intervention targets. We conducted a placebo-controlled, randomised clinical trial (NCT05693077) administering non-toxigenic C. difficile (NTCD) capsules to healthy participants to assess safety and colonisation as primary endpoints, and microbiota susceptibility as a secondary endpoint. A total of 69 healthy participants (18-45 years), not previously colonised with C. difficile and without recent antibiotic use, were enrolled following a health assessment. NTCD capsules administered for five consecutive days at low or high dose, was safe with no dose-response relationship in colonisation outcomes. Vancomycin pretreatment induced colonisation success: with 5% colonisation without, 32% after one day, and 84% after five days vancomycin pretreatment. Some participants that cleared vancomycin rapidly acquired non-challenge C. difficile strains prior to NTCD challenge. Microbiota profiling (using shotgun metagenomics) revealed reduced α-diversity and pronounced community restructuring. These findings highlight the impact of antibiotic-mediated microbiota disruption, the widespread environmental presence of C. difficile, and the feasibility of meaningful microbiota assessment in small-scale intervention trials, thereby providing a robust tool to investigate this globally impactful infection.