Colonization resistance against Clostridioides difficile is a graded, microbiota-intrinsic property of healthy human gut communities

Fecal microbiota transplantation cures approximately 90% of recurrent Clostridioides difficile infection, yet it remains unknown whether all healthy donor microbiota confer equivalent protection. We colonized germ-free C57BL/6 mice with stool microbiota from 30 healthy human donors and challenged them with C. difficile in the absence of antibiotic pretreatment. Donor microbiota conferred a spectrum of colonization resistance phenotypes: Resistant (no detectable colonization or toxin), Carrier (asymptomatic colonization with detectable toxin), Symptomatic (non-lethal diarrheal illness), and Susceptible (lethal infection). Of these, 8 conferred Resistant phenotypes, 12 Carrier, 6 mixed Resistant-Carrier outcomes, and 4 Symptomatic or Susceptible phenotypes. While 16S rRNA gene sequencing of donor stool did not distinguish phenotypes across any diversity or compositional metric tested, humanized mouse microbiomes exhibited clear phenotype-dependent differences after engraftment. Richness (observed amplicon sequence variants, Chao1) and diversity (Shannon and Faith's phylogenetic diversity) declined progressively from Resistant to Susceptible phenotypes, although substantial overlap was observed between groups. Differential abundance analysis identified taxa depleted across non-resistant phenotypes, including Lachnospiraceae taxa such as Hungatella and Sellimonas, and Bacteroides intestinalis. Shotgun metagenomics confirmed these associations and revealed coordinated depletion of biosynthetic and carbohydrate metabolism pathways in non-resistant phenotypes, consistent with broad loss of community metabolic capacity rather than loss of a single dominant function. These findings demonstrate colonization resistance is a graded, microbiota-associated ecological property, evident after host engraftment rather than being a binary trait encoded in donor stool. This has implications for donor screening in fecal microbiota transplantation and the rational design of microbiome-based therapeutics.