What is in a fecal transplant?
An FMT preparation is processed donor stool — filtered, suspended in saline (sometimes with a cryoprotectant), and dispensed as a liquid or as freeze-dried capsules. The active ingredient is the entire community of bacteria, archaea, fungi, viruses, and metabolites that populate a healthy donor's colon.
How donors are screened
Donors complete a detailed health questionnaire and are tested for blood-borne and stool pathogens, including HIV, hepatitis viruses, syphilis, multidrug-resistant organisms, parasites, and (since 2020) SARS-CoV-2. Most donors are turned away. In the US, screened stool is supplied through stool banks and a small number of specialist centres.
Delivery routes
- Colonoscopy — a high dose deposited in the right colon; allows simultaneous endoscopic inspection.
- Retention enema — a lower-cost, bedside-friendly route that avoids sedation but reaches only the left colon.
- Oral capsules — frozen or lyophilised stool in acid-resistant capsules, swallowed over 1–3 days.
- Nasoenteric tube — the original modern route; rarely used today because capsules deliver the same upper-gut dose without a tube.
How it works in the gut
FMT does not target a single pathogen the way an antibiotic does. It re-establishes a diverse microbial community that can outcompete invaders, restore short-chain fatty acid and bile-acid metabolism, reinforce the colonic mucus layer, and recalibrate immune signalling. Engraftment of donor strains is partial and patchy, and in many indications the response correlates with which donor was used — the so-called "super-donor" effect.
What FMT is not
- It is not a probiotic. Probiotics deliver one or a few strains; FMT delivers an ecosystem.
- It is not a cure-all. FDA recognition is limited to recurrent C. difficile; everything else is investigational.
- It is not safe to do at home. The FDA has documented serious infections — including a fatality — from unscreened material.