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The microbiome in IBS

TL;DR — This is the largest mechanistic sub-literature in IBS (2,797 PubMed records for irritable bowel syndrome AND (microbiome OR microbiota) on 2026-09-02) and the one with the least clinical yield. Composition differences replicate at the level of taxa: Enterobacteriaceae, Lactobacillaceae and Bacteroides increased; uncultured Clostridiales I, Faecalibacterium (including F. prausnitzii) and Bifidobacterium decreased; diversity decreased or unchanged (Pittayanon 2019, PMID 30940523, 24 studies) — and a uniform re-analysis of 1,167 faecal 16S samples from 9 studies found significantly lower alpha diversity and 26 discriminating genera (Li 2024, PMID 39777150). What has not happened is translation. No microbiome test is used diagnostically; probiotic efficacy is strain-specific and rated low-to-very-low certainty by GRADE across almost every analysis of 82 trials and 10,332 patients (Goodoory 2023, PMID 37541528); and faecal microbiota transplantation has produced the single most spectacular positive trial in the field (response 89.1% with 60 g FMT vs 23.6% placebo; El-Salhy 2020, PMID 31852769) alongside two well-conducted negatives (Lahtinen 2020, PMID 32343000; Yau 2023, PMID 37667968), with meta-analyses splitting on whether the pooled effect exists at all (RR 1.44, 0.88–2.33, very low certainty — Lo 2024, PMID 39289768 — versus OR 0.46, 0.33–0.64 in a Bayesian network meta-analysis — Wu 2024, PMID 38999862). The causal question Pittayanon posed in 2019 — "whether these microbes are a product or cause of IBS" — is still open.

What replicates in composition

Direction in IBS Taxon Source
Increased Family Enterobacteriaceae (phylum Proteobacteria) Pittayanon 2019, PMID 30940523
Increased Family Lactobacillaceae Pittayanon 2019, PMID 30940523
Increased Genus Bacteroides Pittayanon 2019, PMID 30940523
Decreased Uncultured Clostridiales I Pittayanon 2019, PMID 30940523
Decreased Genus Faecalibacterium, incl. F. prausnitzii Pittayanon 2019, PMID 30940523
Decreased Genus Bifidobacterium Pittayanon 2019, PMID 30940523
Decreased or unchanged Alpha diversity Pittayanon 2019, PMID 30940523; significantly lower by random-effects model in Li 2024, PMID 39777150

Caveats attached to that table by its own authors: more than 40% of the included studies did not state whether cases and controls were comparable on sex and/or age (Pittayanon 2019, PMID 30940523). The 2024 pooled re-analysis added 26 genera and 12 predicted pathways with significant odds ratios and classification potential, and reported that Ruminococcaceae and Christensenellaceae correlated negatively with vitamin B6 metabolism, which was decreased in IBS (Li 2024, PMID 39777150). Mucosa-associated and faecal communities differ from each other and both differ from controls (Sundin 2020, PMID 31953505), so "the IBS microbiome" is at minimum two compartments. A further systematic review with meta-analysis has examined the immunological-status/dysbiosis interaction (Ahmed 2026, PMID 40903688).

None of these differences has been converted into a diagnostic. Eleven biomarkers "performed no better than symptom-based criteria" in the diagnostic accuracy literature (Sood 2015, PMID 26076071), and the 2026 review of serological and faecal markers reaches the same conclusion for the field as a whole (Burns 2026, PMID 41793943).

Probiotics: many trials, strain-level answers, low certainty

The definitive synthesis is 82 RCTs, 10,332 patients, of which only 24 were at low risk of bias across all domains (Goodoory 2023, PMID 37541528):

Outcome Evidence of benefit GRADE certainty
Global symptoms Escherichia strains Moderate
Global symptoms Lactobacillus strains; L. plantarum 299V Low
Global symptoms Combination probiotics, LacClean Gold S, Duolac 7s, Bacillus strains Very low
Abdominal pain Saccharomyces cerevisiae I-3856, Bifidobacterium strains Low
Abdominal pain Combination probiotics, Lactobacillus, Saccharomyces, Bacillus strains Very low
Bloating/distension Combination probiotics, Bacillus strains Very low
Adverse events (55 trials, >7,000 patients) No significant increase vs placebo —

A 2026 strain-specific meta-analysis of 32 articles covering 10 single strains found efficacy for Bifidobacterium longum (formerly B. infantis) 35624, Lactobacillus rhamnosus GG, Lactiplantibacillus plantarum 299v (DSM 9843), Saccharomyces cerevisiae CNCM I-3856 and Bacillus coagulans Unique IS2 (MTCC 5260); B. coagulans MTCC 5856 improved quality of life; results for Saccharomyces boulardii CNCM I-745 were conflicting; and it found no efficacy for Escherichia coli Nissle 1917, Lactobacillus gasseri BNR17 or Lactobacillus casei Shirota (Maslennikov 2026, PMID 41682832). Note the direct disagreement with Goodoory 2023, which rated Escherichia strains the highest-certainty benefit for global symptoms; the two reviews use different inclusion rules (single-strain-only versus all probiotics) and reach opposite conclusions about the same genus.

Earlier syntheses reached the same structural verdict from further back: "which particular combination, species or strains of probiotics are effective for IBS remains, for the most part, unclear," with sparse data for prebiotics and synbiotics (Ford 2018, PMID 30294792). A Bayesian network meta-analysis of 54 probiotic, 7 prebiotic/synbiotic and 6 FMT trials found probiotics favourable (OR 0.53, 0.48–0.59) and prebiotics/synbiotics not (Wu 2024, PMID 38999862).

The practical position: probiotics are safe, some strains probably help some patients, the evidence certainty is low, and no head-to-head trial identifies the best strain. Guidelines diverge accordingly (guidelines).

Gas-producing microtypes: the one compositional finding with a clinical correlate

The strongest link between composition and phenotype runs through gas metabolism rather than through taxa counts. Combining breath testing with 16S sequencing in IBS-C and IBS-D subjects drawn from two randomised trials (NCT03763175, NCT04557215), Villanueva-Millan and colleagues found (PMID 36114762):

Subtype Breath signature Microbial correlate
IBS-C Higher methane Higher microbial diversity; higher relative and absolute abundance of methanogens, predominantly Methanobrevibacter, including M. smithii; higher Ruminococcaceae and Christensenellaceae; metabolic pathways for methanogenesis and cofactor F420 biosynthesis enriched
IBS-D Higher hydrogen and hydrogen sulphide Lower microbial diversity; higher abundance of H₂S producers including Fusobacterium and Desulfovibrio; higher Enterobacteriaceae; sulfate-reduction pathways enriched

This is consistent with earlier work identifying Methanobrevibacter smithii as the predominant methanogen in constipation-predominant IBS with breath methane (Kim 2012, PMID 22573345; Ghoshal 2016, PMID 27458176), with methanogen overgrowth associating with delayed small-bowel and colonic transit on wireless motility capsule (Talamantes 2024, PMID 39068378), and with animal work in which hydrogen-sulphide producers drive a diarrhoea-like phenotype and a methane producer a constipation-like phenotype (Villanueva-Millan 2024, PMID 38060167). It also converges with the mechanistic finding inside a randomised FMT trial, where bloating improvement accompanied a fall in hydrogen-sulphide-producing pathways (Yau 2023, PMID 37667968).

Therapeutically this has generated one distinctive idea: targeting archaeal methanogenesis with statins, on the basis that lovastatin lactone inhibits enzymes in that pathway, with a modified-release formulation developed to reach intestinal methanogens (Gottlieb 2016, PMID 26559904; Muskal 2016, PMID 27347377; Hubert 2018, PMID 28989013). No phase 3 result was retrievable in this session's searches (searched 2026-09-02).

Metabolites: what is and is not altered

Analyte Finding Source
Blood serotonin (5-HT) Increased in IBS vs healthy controls, SMD 2.35 (95% CI 0.46–4.24), 5 studies Luo 2021, PMID 33407171
Total faecal short-chain fatty acids No significant difference, SMD −0.01 (−0.57 to 0.55) Luo 2021, PMID 33407171
Faecal acetate / propionate / butyrate −0.04 (−0.55 to 0.47) / 0.07 (−0.45 to 0.60) / −0.00 (−0.49 to 0.49) — all null Luo 2021, PMID 33407171
Faecal SCFAs across interventions Separately meta-analysed Ju 2024, PMID 38892659; Sun 2019, PMID 30762787
Colonic microbiome under low FODMAP Systematically reviewed and meta-analysed So 2022, PMID 35728042
Longitudinal omics Systematically reviewed Ng 2023, PMID 37110143

The null SCFA result matters because short-chain fatty acids are the usual mechanistic bridge between dietary fermentation and symptoms. The authors note their analysis did not differentiate IBS subgroups (Luo 2021, PMID 33407171), and the gas-microtype work above suggests why that may be fatal to the analysis: IBS-C and IBS-D appear to have opposite gas-metabolic profiles, so pooling them can cancel real differences.

The strain with the most direct evidence: Bifidobacterium longum (formerly infantis) 35624

Study Design Result
O'Mahony 2005, PMID 15765388 77 IBS patients randomised to Lactobacillus salivarius UCC4331, B. infantis 35624 (both 1×10¹⁰ cells in a malted milk drink) or the drink alone, 8 weeks B. infantis 35624 reduced composite and individual scores for abdominal pain/discomfort, bloating/distension and bowel-movement difficulty versus placebo for most treatment weeks; bowel frequency and consistency did not differ. Baseline IL-10/IL-12 ratio was abnormal (proinflammatory Th-1 pattern) and was normalised by B. infantis 35624 alone
Whorwell 2006, PMID 16863564 362 primary-care IBS patients, any subtype, randomised to placebo or encapsulated B. infantis at 1×10⁶, 1×10⁸ or 1×10¹⁰ cfu/mL for 4 weeks Only the 1×10⁸ dose beat placebo — on the primary endpoint of abdominal pain and on composite score, bloating, bowel dysfunction, incomplete evacuation, straining and gas passage; global symptom assessment exceeded placebo by >20% (p<0.02). Neither 1×10⁶ nor 1×10¹⁰ differed from placebo, and the highest dose had formulation problems
Yuan 2017, PMID 28166427 Meta-analysis of B. infantis 35624 trials Pooled synthesis
Ringel-Kulka 2017, PMID 27845337 Multicentre RCT in non-patients with abdominal discomfort and bloating Extends the strain beyond diagnosed IBS
Maslennikov 2026, PMID 41682832 Strain-specific meta-analysis B. longum (formerly B. infantis) 35624 among the strains with demonstrated efficacy

The Whorwell dose–response is the single most instructive probiotic result in this literature: the same organism was effective at one dose and indistinguishable from placebo at doses one hundred-fold above and below it. The authors' own conclusion — that clinical data are needed "in the final dosage form and dose" before products are used in practice — is routinely ignored by the commercial probiotic market. Broader probiotic reviews reach compatible conclusions (Brenner 2009, PMID 19277023; Charbonneau 2013, PMID 23549409).

Faecal microbiota transplantation: the field's sharpest contradiction

Trial n Design Primary outcome Result
El-Salhy 2020, PMID 31852769 (NCT03822299) 165 Placebo (own faeces) vs 30 g vs 60 g FMT from one well-characterised donor, via gastroscope ≥50-point IBS-SSS reduction at 3 months Response 23.6% / 76.9% (p<0.0001) / 89.1% (p<0.0001); dose–response; improvements in fatigue and QoL; mild self-limiting adverse events
Lahtinen 2020, PMID 32343000 (NCT03561519) 49 Autologous vs allogenic FMT via colonoscopy Sustained ≥50-point IBS-SSS reduction Not achieved in either group. Transient reduction at 12 weeks in the FMT arm vs baseline (p=0.01); microbiota shifted toward donor and stool water content fell; conclusion: single colonoscopic infusion "cannot be recommended"
Yau 2023, PMID 37667968 (NCT03125564) 56 (IBS-D) FMT vs placebo, duodenal route, baseline and week 4 >50-point IBS-SSS reduction at week 12 57.1% vs 46.4% (p=0.42) — negative. Bloating improved more with FMT (72% vs 30%, p=0.005), accompanied by reduced hydrogen-sulphide-producing pathway and Ruminococcus gnavus, enrichment of Lawsonibacter

Meta-analysis has not resolved this. Twelve RCTs, 615 participants: no significant difference for clinical response (RR 1.44, 0.88–2.33), IBS-SSS (SMD −0.31, −0.72 to 0.09) or IBS-QOL (SMD 0.30, −0.09 to 0.69); subgroup analysis restricted to low-risk-of-bias studies using endoscopic, nasojejunal or rectal-enema delivery did show benefit; trial sequential analysis judged the evidence inconclusive and GRADE certainty very low (Lo 2024, PMID 39289768). A separate Bayesian network meta-analysis put FMT at OR 0.46 (0.33–0.64) in favour (Wu 2024, PMID 38999862). Other syntheses of the same trial set exist (Halkjær 2023, PMID 37346153).

The most plausible reading of the discordance is that FMT effect depends on donor, dose and route rather than on FMT as a category — which is El-Salhy's own explanation ("utilising a well-defined donor with a normal dysbiosis index and favourable specific microbial signature is essential"; PMID 31852769) and is exactly the kind of claim that a positive single-donor trial cannot itself establish. Targeted PubMed and registry searches on 2026-09-02 found donor-versus-autologous designs but no IBS trial randomising recipients among multiple allogenic donors.

Antibiotic modulation

Rifaximin is the one antibiotic with replicated efficacy in non-constipated IBS (RR of symptoms persisting 0.84, 0.79–0.90, across five similarly designed trials; Ford 2018, PMID 30294792). Because its interpretation is entangled with the contested SIBO construct, it has its own page: rifaximin-and-the-sibo-question. Faecal bacterial composition has been reported to predict rifaximin response in IBS-D (Li 2020, PMID 32470562) — an unreplicated result that, if it held, would be the field's first actionable microbiome test.

Why the causal question is still open

  1. No human dose–response outside a single trial. El-Salhy's 30 g/60 g gradient (PMID 31852769) is the only dose–response evidence, from one donor, in one centre.
  2. Compositional differences are confounded by diet and transit. Low-FODMAP diet itself changes the microbiome (Staudacher 2021, PMID 33183883; Cox 2020, PMID 31586453), and transit time is a major determinant of stool composition — so cross-sectional differences between patients and controls cannot be read as aetiological.
  3. The successful interventions do not map onto the observed deficits. Bifidobacterium is depleted in IBS (Pittayanon 2019, PMID 30940523), yet Bifidobacterium supplementation has only low-certainty evidence for abdominal pain and none for global symptoms (Goodoory 2023, PMID 37541528).
  4. A positive-response mechanism has been proposed but not tested prospectively: reduction in hydrogen-sulphide-producing bacteria accompanied the bloating benefit in the Hong Kong FMT trial (Yau 2023, PMID 37667968).

What guidelines do with this

ACG and BSG treat probiotics cautiously and do not recommend routine microbiome testing; FMT is not recommended outside trials in the guidance retrieved for guidelines. No regulator has approved a microbiome-directed product for IBS as of 2026-09-02 (searched this session; absence stated as of that date).

Open questions

  • Is dysbiosis cause or consequence? Posed explicitly in 2019 (Pittayanon 2019, PMID 30940523) and unanswered in 2026.
  • Why does one FMT trial report an 89% response rate and two others report none (El-Salhy 2020, PMID 31852769; Lahtinen 2020, PMID 32343000; Yau 2023, PMID 37667968)? Donor, dose and route are candidate explanations; as of a targeted PubMed and registry search on 2026-09-02, no IBS trial had randomised recipients among multiple allogenic donors.
  • Do Escherichia strains help or not? Goodoory 2023 (PMID 37541528) rates them the highest-certainty probiotic benefit; Maslennikov 2026 (PMID 41682832) finds E. coli Nissle 1917 ineffective.
  • Can faecal composition predict rifaximin response (Li 2020, PMID 32470562)? A single unreplicated report.
  • Is the depleted-Bifidobacterium/supplement-Bifidobacterium mismatch informative about mechanism, or does it simply mean transient colonisation does not restore function?
  • What does the hydrogen-sulphide pathway signal mean (Yau 2023, PMID 37667968)? It is the only mechanistic mediator identified inside a randomised IBS microbiome trial.
  • Does the methane/hydrogen-sulphide microtype framework generalise beyond the Cedars-Sinai trial populations (Villanueva-Millan 2022, PMID 36114762), and can it be used to allocate treatment?
  • Do statins targeting archaeal methanogenesis work in IBS-C? The rationale is developed (Gottlieb 2016, PMID 26559904) and no phase 3 result was retrievable this session (searched 2026-09-02).
  • Why are faecal short-chain fatty acids not different in IBS when fermentation is the presumed mechanism of dietary therapy (Luo 2021, PMID 33407171)? Is the null an artefact of pooling opposite subtypes?
  • Why did B. infantis 35624 work at 1x10^8 cfu but not at 1x10^6 or 1x10^10 (Whorwell 2006, PMID 16863564)? A hundred-fold dose window with no monotonic relationship is difficult to reconcile with a simple colonisation model.
  • Would a microbiome-stratified trial (treat only the dysbiotic) beat unstratified treatment? Never attempted.

References

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