Fecal Microbiota Transplant (FMT)

Definition

Fecal microbiota transplant (FMT) is the transfer of processed stool from a healthy screened donor into the gastrointestinal tract of a recipient, with the goal of restoring normal gut microbial diversity and correcting dysbiosis.

Rationale / Mechanism

The gut microbiota provides colonization resistance - the normal anaerobic flora (particularly Bacteroides, Lachnospiraceae, Ruminococcaceae, Faecalibacterium) prevents pathogenic organisms from proliferating. In Clostridioides difficile infection (CDI), antibiotic exposure markedly reduces diversity and eliminates these protective species, creating a dysbiotic environment where C. difficile replicates unchecked. FMT dramatically reshapes this unhealthy microbiome to resemble that of the healthy donor, restoring colonization resistance and preventing further recurrences. - Harrison's Principles of Internal Medicine 22E, p. 3882

Primary Indication: Recurrent CDI

FMT is the most established therapeutic use of microbiome manipulation and is recommended for patients with multiple recurrences of CDI in whom antibiotic treatment has failed.

C. difficile is the leading cause of antibiotic-associated diarrhea and causes a growing worldwide epidemic
~15-30% of successfully treated CDI patients develop recurrent disease
Patients with recurrent CDI have a markedly less diverse, dysbiotic microbiota
FMT was first successfully used for severe CDI as far back as the 1950s
Efficacy for recurrent CDI ranges from 77% to 100%; multiple FMTs may be needed for full clinical response
In a landmark randomized trial, infusion of donor feces via nasogastric tube was significantly more effective for recurrent CDI than vancomycin alone
Sabiston Textbook of Surgery, p. block 22; Yamada's Textbook of Gastroenterology, 7e

Routes of Administration

Route	Notes
Colonoscopy (lower)	Most effective; direct instillation into colon
Rectal enema	Lower route, widely used
Nasogastric tube	Upper route; less effective than lower approaches
Oral frozen capsules	Convenient; studies show comparable efficacy

A direct comparison found lower (colonic) approaches more effective than upper routes. - Sabiston Textbook of Surgery

Antibiotic Treatment of CDI (Context for When FMT is Used)

From Sabiston's antibiotic treatment table:

Condition	Treatment	Duration
First episode	Oral vancomycin 125 mg QID or fidaxomicin 200 mg BID (metronidazole if unavailable, nonsevere only)	10 days
Fulminant (hypotension, shock, ileus, megacolon)	Vancomycin 500 mg QID (oral/NG) ± IV metronidazole	Until improvement
Recurrences	Fidaxomicin, vancomycin ± rifaximin; FMT for 2nd or subsequent recurrences	Varies

FMT is considered when antibiotics have failed for recurrent CDI.

Donor Selection and Safety

Donors must be systematically screened for infectious diseases and transmissible pathogens before use
Frozen screened samples from stool banks are now widely distributed
Estimated >10,000 FMTs are performed annually in the US
High-risk recipients (HIV/AIDS, recent bone marrow transplant, severe immunodeficiency, patients on high-dose steroids, anti-TNF agents, calcineurin inhibitors, chemotherapy) require great caution - these are ironically also the patients most susceptible to severe CDI
A serious adverse event was reported where two immunocompromised patients developed bacteremia with antibiotic-resistant E. coli from the same donor, with one fatality - reinforcing the importance of rigorous donor screening
Yamada's Textbook of Gastroenterology, 7e

Emerging / Investigational Uses

Beyond CDI, FMT is being explored for a wide range of conditions:

Domain	Evidence Status
Inflammatory bowel disease (IBD)	Active trials
Obesity and insulin resistance	Early positive signals
Graft-vs-host disease (GVHD)	Small prospective studies
Spondyloarthritis / autoimmune	Investigational
Immunotherapy resistance in melanoma	Promising pilot data (Woelk & Snyder, 2021)
Alcohol-associated hepatitis	Pilot: lower mortality vs historical controls
Autism	Active research (Kang et al., 2017)
Neurological/psychiatric disorders	Very early stage

FMT's success in CDI has made it the "poster child" for microbiome-based therapies potentially transforming management of diseases previously considered refractory. - Harrison's 22E

Adverse Effects

Generally infrequent for recurrent CDI; benefits outweigh risks in that indication
Transmission of antibiotic-resistant organisms from donor (serious, rarely fatal)
Post-FMT autoimmune phenomena reported: peripheral neuropathy, Sjogren's syndrome, ITP, rheumatoid arthritis (4/77 patients in one study)
Conversely, improvement in pre-existing arthritis and allergy also reported
As FMT use expands to more immunocompromised patients, complication rates may increase

Current Guidelines Summary

Not first-line for initial CDI
Recommended for patients with multiple recurrences where antibiotics have failed
Consider bezlotoxumab (monoclonal antibody against toxin B) as adjunct with antibiotics to prevent recurrence - especially in severe CDI, Ribotype 027, and immunocompromised patients
Surgery (subtotal colectomy or diverting ileostomy with colonic lavage) reserved for fulminant CDI with systemic toxicity, toxic megacolon, or perforation

Recent Evidence Update (2026): A 2026 systematic review and meta-analysis (PMID 41081988) directly compared FMT vs antibiotics for CDI, providing the most current head-to-head efficacy and safety data - worth reviewing for any exam or clinical application.

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