Infectious Disease · Medical Literature
Experimental human colonisation with non-toxigenic Clostridioides difficile: a placebo-controlled randomised clinical trial.
Abstract
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.
Clinical relevance
In healthy adults without prior C. difficile colonisation or recent antibiotic exposure, is oral administration of non-toxigenic Clostridioides difficile (NTCD) capsules safe, and what host and microbiota factors determine successful intestinal colonisation? __SNAPSHOT__ {"studyDesign":"Placebo-controlled, randomised clinical trial (Phase 1/human colonisation model; ClinicalTrials.gov NCT05693077)","setting":"Not reported (academic/clinical research centre; trial registration suggests European institutional setting)","sampleSize":"n=69 healthy participants; allocation by dose group (low-dose NTCD, high-dose NTCD, placebo) not specified in abstract","population":"Healthy adults aged 18–45 years, without prior C. difficile colonisation and without recent antibiotic use; screened via formal health assessment","intervention":"Oral NTCD capsules administered for five consecutive days at low or high dose; subsets received vancomycin pretreatment for one day or five days prior to NTCD challenge","comparator":"Placebo capsules (design implies parallel placebo arm; allocation ratio not reported in abstract)","primaryOutcome":"Safety (adverse events) and colonisation success (detection of NTCD in stool) as co-primary endpoints; microbiota susceptibility (via shotgun metagenomics) as secondary endpoint","followUp":"Not reported","fundingSource":"Not reported"}
Key findings
NTCD capsules were safe across both dose levels with no dose-response relationship observed in colonisation outcomes; specific adverse event rates and severity grading are not reported in the abstract. Colonisation success was strongly contingent on prior gut microbiota disruption by vancomycin: without vancomycin pretreatment, colonisation occurred in only 5% of participants; after one day of vancomycin pretreatment, colonisation rate rose to 32%; after five days of vancomycin pretreatment, colonisation rate reached 84%. No relative risk, odds ratio, confidence intervals, or p-values for these colonisation comparisons are reported in the abstract. NNT for colonisation could not be calculated in the absence of randomised comparator-group event rates for each vancomycin stratum. A notable mechanistic observation was that some participants who cleared vancomycin rapidly acquired non-challenge (environmental or endogenous) C. difficile strains before NTCD administration, suggesting rapid ecological niche re-opening following antibiotic cessation. Shotgun metagenomic profiling revealed reduced alpha-diversity (the abstract is truncated at this point; full quantitative metagenomics results are not available from the abstract alone).
Practice implications
This trial establishes, in a controlled human model, that oral NTCD administration is safe in healthy young adults and that colonisation is feasible but is critically dependent on prior antibiotic-induced disruption of the gut microbiota. The dose-escalation finding (5% vs. 32% vs. 84% colonisation without, after one day, and after five days of vancomycin pretreatment, respectively) provides the first human quantitative evidence for a duration-dependent relationship between vancomycin-induced dysbiosis and C. difficile colonisation permissiveness. This has direct mechanistic relevance to why CDI risk rises with prolonged antibiotic courses. The absence of a dose-response relationship between NTCD dose and colonisation success suggests that host microbiota susceptibility—rather than inoculum size—is the dominant determinant of colonisation. The observation that rapid vancomycin clearers acquired non-challenge C. difficile strains highlights the narrow and dynamic nature of the colonisation window. These findings inform the design of future NTCD-based prophylaxis trials targeting patients with CDI being treated with vancomycin. Critically, this was a healthy volunteer study (ages 18–45, no recent antibiotics, no prior CDI); findings may not directly generalise to elderly, immunocompromised, or hospitalised patients—the populations most at risk for CDI—who have fundamentally different baseline microbiota and comorbidity profiles. The trial did not assess protection against subsequent toxigenic C. difficile challenge, so no efficacy conclusion against clinical CDI can be drawn. Only an abstract was available for review; full safety data, statistical analyses, metagenomics results, and follow-up duration are not assessable from the abstract alone. __LIMITATIONS__ Healthy volunteer population aged 18–45 years limits generalisability to elderly or immunocompromised patients who bear the highest CDI burden.,No toxigenic C. difficile challenge was performed; colonisation success is a surrogate endpoint, and protection against clinical CDI was not evaluated.,The abstract is truncated; full metagenomics results, complete adverse event data, confidence intervals, p-values, and follow-up duration are not available for critical appraisal.,Sample size (n=69) is small, limiting statistical power for subgroup analyses; allocation ratios across dose groups are not reported.,Vancomycin pretreatment was used deliberately in a subset, creating non-randomised subgroups for the colonisation comparison; confounding by baseline microbiota composition cannot be excluded.,No dose-response relationship in colonisation was observed, but this may reflect insufficient inoculum range or the dominance of microbiota permissiveness as a confounder rather than a true null dose effect.,Rapid clearance of vancomycin by some participants introduced competitive colonisation with non-challenge strains, a protocol-related confound that may not be controllable in future studies.,Funding source is not reported; potential commercial interests in NTCD as a live biotherapeutic product cannot be assessed.,Follow-up duration is not reported in the abstract, limiting assessment of colonisation persistence and late adverse events. __BOTTOM_LINE_CATEGORY__ promising_but_preliminary __BOTTOM_LINE__ In healthy adults, oral NTCD capsules are safe but achieve clinically meaningful gut colonisation only when the microbiota is sufficiently disrupted by vancomycin pretreatment, with colonisation rates rising from 5% (no pretreatment) to 84% (five-day pretreatment). This human colonisation model provides mechanistic proof-of-concept that host microbiota susceptibility—not NTCD dose—governs colonisation success, informing the design of future prophylaxis trials in at-risk CDI patient populations. Efficacy against clinical CDI was not tested; results should not yet alter clinical practice but meaningfully define parameters for subsequent phase 2/3 prophylaxis trials.