Fecal Microbiota Transplantation (FMT)

Overview

Fecal microbiota transplantation (FMT) transfers a complete microbial community from a healthy screened donor to a recipient, aiming to restore microbial diversity, colonization resistance, and metabolic function. It represents the most direct form of microbiome intervention — a wholesale ecological reset rather than incremental modulation.

> Clinical disclaimer: FMT carries risks of pathogen transmission (including multidrug-resistant organisms and viruses), variable engraftment, and unknown long-term consequences. Currently FDA-approved only for recurrent C. difficile infection. All other indications are investigational.

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Mechanism of Action

FMT operates through ecological restoration rather than single-target pharmacology:

Colonization resistance: Donor anaerobes compete with pathogens for nutrients and attachment sites, re-establishing the competitive exclusion that antibiotics destroyed
SCFA restoration: Reintroduced Firmicutes (Faecalibacterium, Roseburia, Eubacterium) produce butyrate, propionate, and acetate — fueling colonocytes, maintaining barrier integrity, and modulating immune tolerance
Bile acid metabolism: Donor bacteria expressing bile salt hydrolase convert primary bile acids to secondary forms (deoxycholic, lithocholic acid) that inhibit C. difficile spore germination
Barrier repair: SCFA-driven colonocyte nutrition restores tight junction integrity, reducing translocation of LPS and microbial metabolites
Immune retraining: Diverse microbial antigens re-educate mucosal immune responses toward tolerance (Treg induction) rather than chronic inflammation

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Protocols and Administration

Parameter	Options	Notes
Delivery route	Colonoscopy, nasogastric tube, capsule (oral)	Colonoscopy has highest single-dose efficacy for CDI; capsules improving
Donor screening	Stool + blood panel (MDRO, HIV, HBV, HCV, parasites, C. diff)	Universal donor programs (stool banks) standardize screening
Donor selection	"Super-donors" with high diversity and SCFA production	Donor effects explain much of the outcome variance in UC trials
Dosing frequency	Single infusion (CDI) vs multi-session (UC, 5-40 infusions)	UC and other chronic conditions may require intensive protocols
FDA-approved products	Rebyota (rectal), Vowst (oral capsules)	For recurrent CDI only; standardized manufactured products

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Monitoring

Engraftment: 16S rRNA or metagenomic sequencing at baseline, 1 month, 3 months to assess donor microbiota colonization
Clinical response: Condition-specific outcome measures (CDI recurrence, UC Mayo score, PD UPDRS)
Adverse events: Monitor for fever, abdominal pain, bacteremia, new infections for 30 days post-FMT
Long-term: Metabolic and autoimmune screening annually (unknown long-term donor microbiota effects on recipient metabolism)

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Contraindications and Risks

Immunocompromised patients: Risk of bacteremia and invasive infections. Absolute contraindication in severe immunosuppression (neutropenia, uncontrolled HIV).
Pathogen transmission: Cases of ESBL-producing E. coli bacteremia (including one death) prompted enhanced donor screening requirements.
Unknown long-term effects: Donor microbiota may transmit metabolic phenotypes (obesity, autoimmune risk). Long-term registry data still accumulating.
Regulatory status: Investigational for all indications except recurrent CDI. IRB approval required for research use in other conditions.
Antibiotic exposure: Recent broad-spectrum antibiotics in the recipient reduce engraftment. Vancomycin taper-then-FMT protocol for CDI optimizes niche availability.

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