Hundreds of beneficial species compete for the same limited gut nutrients and attachment sites — that crowding is colonization resistance: an invader faces intense competition for a niche and struggles to establish.
Broad-spectrum antibiotics kill beneficial diversity as collateral damage, not just the targeted infection. A depleted, low-diversity gut has empty niches and weak resistance — exactly where C. difficile can take hold if exposed.
Treating the resulting infection with more antibiotics suppresses the pathogen short-term but deepens the diversity loss, so resistant spores persist and the population often rebounds. A fecal microbiota transplant instead reintroduces a large, diverse donor community directly, rapidly rebuilding competitive resistance and displacing C. difficile far more durably.
- Diversity — fraction of distinct beneficial bacterial groups still present and populous.
- Colonization resistance — how effectively the resident community can out-compete a newcomer for niche space.
- C. difficile population — share of gut niche space the opportunist has managed to occupy.