Resource-access breadth marks protective strains against toxigenic Clostridioides difficile

Antibiotic disruption of gut microbiota creates opportunities for toxigenic Clostridioides difficile. One strategy is to occupy this niche with a non-toxigenic or weakly virulent strain, but even strains that cause little disease may fail to compete during pathogen challenge. We screened 21 sequence type 1 (ST1) isolates for their ability to protect mice co-colonized with toxigenic VPI10463, separating disease potential from protection. Some strains, such as ST1-75, caused little disease and protected, whereas others, such as ST1-68, caused little disease but failed to protect. Metabolomics showed that ST1-75 depleted more proline than ST1-68, whereas another protective strain, ST1-6, had a different metabolic profile; both protective strains nevertheless responded to many substrates. Substrate-utilization profiling revealed that protection across the 21 isolates correlated with amino-acid overlap with VPI10463, but not overall nutrient overlap or shared genome-encoded protein families. Relative amino-acid access explained variation beyond mono-colonization disease potential, and a consumer-resource model showed how nutrient overlap together with additional resource access can support persistence during competition. Additional experiments support that ST1-mediated protection occurs even in the absence of mature B and T cells. These findings distinguish low disease potential from protection, link protection to strain competition, and identify relative resource access as an informative phenotype for prioritizing strains for biotherapeutic testing.