Concept · biology
Microbiome
Follow Microbiome — see important new research and changes in evidence.Change log
What changed
Dated edits to this page's evidence: studies added or removed from a claim, claims added or withdrawn, and new explanations tagged here. Rewordings are not listed.
Selective bacterial bile-salt hydrolases can change host bile-acid chemistry and weight-related phenotypes in colonization models—evidence that specific microbial enzymes, not only community membership lists, can matter for host physiology.
- Removed a supporting study: B. dorei rises before T1D autoimmunity
- Removed a supporting study: How much do gut bacterial gene contents differ?
- Added a scope qualifier: Antibiotics hurt bee gut and survival
- Added a scope qualifier: How much do gut bacterial gene contents differ?
Using Mendelian randomisation on gut-taxa genetic instruments, eleven stringent microbiome-to-cancer associations were reported (for example higher breast-cancer odds with Actinobacteria/Bifidobacteriales signals; Tyzzerella3 linked with higher lung adenocarcinoma but lower colorectal cancer risk).
- New claim
- Added a supporting study: Mendelian randomisation associates specific gut taxa with several cancers
- Added a scope qualifier: B. dorei rises before T1D autoimmunity
- Added a scope qualifier: Gut microbes, blood metabolites, and metabolic traits
In children who later developed type 1 diabetes autoimmunity, Bacteroides dorei became dominant in the gut community before persistent autoantibodies (mean diagnosis age about 16.8 months).
- New claim
- Added a supporting study: B. dorei rises before T1D autoimmunity
- Added a scope qualifier: Mendelian randomisation associates specific gut taxa with several cancers
- Added a scope qualifier: Gut microbes, blood metabolites, and metabolic traits
In middle-aged men, gut microbiota composition was associated with plasma metabolites and metabolic-syndrome–related traits, placing the microbiome alongside genetics and lifestyle as a correlated host factor.
- New claim
- Added a supporting study: Gut microbes, blood metabolites, and metabolic traits
- Added a scope qualifier: Selective bacterial BSH shifts host metabolism
- Added a scope qualifier: Mendelian randomisation associates specific gut taxa with several cancers
Even within the same named gut species, strain-level gene content varies substantially among healthy people—so species labels alone understate functional diversity.
- New claim
- Added a supporting study: How much do gut bacterial gene contents differ?
- Added a scope qualifier: Human body microbiome biogeography
- Added a scope qualifier: Is the gut microbiome the same along the intestine?
Among ~20 Bacteroidetes strains, some BSHs prefer steroid cores; BtΔ2086 loses deconjugation; colonization changes weight on high-fat diet vs WT.
- Claim withdrawn
Cases show B. dorei dominance prior to persistent autoantibodies (mean diagnosis age 16.8 months).
- Claim withdrawn
Strain-level gene content varies substantially among individuals for the same species in natural habitats.
- Claim withdrawn
Four of eight core gut species dropped; elevated mortality in treated bees; germ-free bees did not show the same antibiotic mortality spike.
- Claim withdrawn
Cancer MR, pediatric T1D timing, and adult metabolic-trait associations all involve gut microbiota, but they answer different disease and design questions. Treating them as one interchangeable “gut microbiome disease effect” collapses distinct estimands.
- New tension
- Added a supporting study: B. dorei rises before T1D autoimmunity
- Added a supporting study: Mendelian randomisation associates specific gut taxa with several cancers
- Added a supporting study: Gut microbes, blood metabolites, and metabolic traits
Human observational and MR studies do not automatically generalise to bee antibiotic/pesticide disruptions or to enzyme-focused colonization experiments. Animal and insect systems test different host–microbe questions; they limit how far a human association travels rather than falsifying it.
- New tension
- Added a supporting study: Selective bacterial BSH shifts host metabolism
- Added a supporting study: Antibiotics hurt bee gut and survival
- Added a supporting study: Mendelian randomisation associates specific gut taxa with several cancers
- Added a supporting study: Pesticides reshape honey bee gut microbes
Systems and scales differ across microbiome studies (species, tissues, methods), so mechanisms should not be over-generalised.
- Tension withdrawn
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Mendelian randomisation proving a taxa–cancer association means the microbe’s mechanism in tumours is known.
- Added a misconception
B. dorei dominance before autoantibodies proves that B. dorei causes type 1 diabetes.
- Added a misconception
If two papers both study “the gut microbiome,” their disease conclusions can be pooled as one effect.
- Added a misconception
Microbiome findings from one model organism always transfer to humans.
- Removed a misconception
- Concept page published
The microbiome is the community of microorganisms living in or on a host (or in an environment), studied for composition, genes, and effects on host physiology. In this library it ranges from human gut taxa linked to metabolic traits and disease risk, through strain-level gene content and bile-salt enzymes, to experimental animal and bee systems that test how disruptions change the community.
Students meet microbiome claims that sound interchangeable—“gut bugs cause disease”—when the evidence is usually a specific host, assay, and outcome. The skill is saying what a study adds, where it applies, and what would change the position.
Evidence
What the evidence shows
Drawn from 11 studies in this library. Each finding starts with a plain-language takeaway, then the denser detail. Supports means evidence for a finding; Challenges means evidence against a stated position; Qualifies marks scope with a short note on each study’s contribution. Challenged positions are labeled — they are not findings.
Genetic instruments for gut taxa link a small set of microbiome–cancer associations under Mendelian randomisation.
Using Mendelian randomisation on gut-taxa genetic instruments, eleven stringent microbiome-to-cancer associations were reported (for example higher breast-cancer odds with Actinobacteria/Bifidobacteriales signals; Tyzzerella3 linked with higher lung adenocarcinoma but lower colorectal cancer risk).
- B. dorei rises before T1D autoimmunity— different disease and design — observational T1D timing, not cancer MR
- Gut microbes, blood metabolites, and metabolic traits— different outcome — metabolic traits/metabolites, not cancer
Study Role Design N Population Outcome Mendelian randomisation associates specific gut taxa with several cancers Supports Mendelian randomisationTwo-sample MR of MiBioGen gut taxa instruments against eight cancer GWAS summary sets 211 taxa instruments; cancer GWAS sample sizes vary by malignancy — no single primary N Gut microbiome and cancer GWAS summary statistics (IEU OpenGWAS/consortia) Causal microbiome–cancer associations (stringent and sensitivity analyses) B. dorei rises before T1D autoimmunity Qualifiesdifferent disease and design — observational T1D timing, not cancer MR CohortLongitudinal DIPP stool sequencing in HLA-risk children before T1D autoimmunity N=76 · 947 stool samples; 29 seroconverters vs 47 autoantibody-negative controls HLA-DQB1 moderate-to-high-risk children in DIPP Gut microbiota (B. dorei dominance) prior to persistent T1D autoantibodies Gut microbes, blood metabolites, and metabolic traits Qualifiesdifferent outcome — metabolic traits/metabolites, not cancer Cross-sectionalMETSIM subset microbiome–metabolite–metabolic trait association analysis N=531 · 531 middle-aged Finnish men from the METSIM cohort (parent cohort 10,197) Middle-aged Finnish men in METSIM Associations of gut microbiota with plasma metabolites and metabolic traits Before type 1 diabetes autoimmunity, Bacteroides dorei can dominate children’s gut communities.
In children who later developed type 1 diabetes autoimmunity, Bacteroides dorei became dominant in the gut community before persistent autoantibodies (mean diagnosis age about 16.8 months).
- Mendelian randomisation associates specific gut taxa with several cancers— different disease and method — cancer MR in adults, not pediatric T1D timing
- Gut microbes, blood metabolites, and metabolic traits— different population — middle-aged metabolic traits, not autoimmunity onset
Study Role Design N Population Outcome B. dorei rises before T1D autoimmunity Supports CohortLongitudinal DIPP stool sequencing in HLA-risk children before T1D autoimmunity N=76 · 947 stool samples; 29 seroconverters vs 47 autoantibody-negative controls HLA-DQB1 moderate-to-high-risk children in DIPP Gut microbiota (B. dorei dominance) prior to persistent T1D autoantibodies Mendelian randomisation associates specific gut taxa with several cancers Qualifiesdifferent disease and method — cancer MR in adults, not pediatric T1D timing Mendelian randomisationTwo-sample MR of MiBioGen gut taxa instruments against eight cancer GWAS summary sets 211 taxa instruments; cancer GWAS sample sizes vary by malignancy — no single primary N Gut microbiome and cancer GWAS summary statistics (IEU OpenGWAS/consortia) Causal microbiome–cancer associations (stringent and sensitivity analyses) Gut microbes, blood metabolites, and metabolic traits Qualifiesdifferent population — middle-aged metabolic traits, not autoimmunity onset Cross-sectionalMETSIM subset microbiome–metabolite–metabolic trait association analysis N=531 · 531 middle-aged Finnish men from the METSIM cohort (parent cohort 10,197) Middle-aged Finnish men in METSIM Associations of gut microbiota with plasma metabolites and metabolic traits In middle-aged men, gut composition associates with metabolites and metabolic-syndrome traits.
In middle-aged men, gut microbiota composition was associated with plasma metabolites and metabolic-syndrome–related traits, placing the microbiome alongside genetics and lifestyle as a correlated host factor.
- Mendelian randomisation associates specific gut taxa with several cancers— different outcome — cancer odds via MR, not metabolic metabolites
- Selective bacterial BSH shifts host metabolism— mechanistic enzyme study — not a population metabolic association
Study Role Design N Population Outcome Gut microbes, blood metabolites, and metabolic traits Supports Cross-sectionalMETSIM subset microbiome–metabolite–metabolic trait association analysis N=531 · 531 middle-aged Finnish men from the METSIM cohort (parent cohort 10,197) Middle-aged Finnish men in METSIM Associations of gut microbiota with plasma metabolites and metabolic traits Mendelian randomisation associates specific gut taxa with several cancers Qualifiesdifferent outcome — cancer odds via MR, not metabolic metabolites Mendelian randomisationTwo-sample MR of MiBioGen gut taxa instruments against eight cancer GWAS summary sets 211 taxa instruments; cancer GWAS sample sizes vary by malignancy — no single primary N Gut microbiome and cancer GWAS summary statistics (IEU OpenGWAS/consortia) Causal microbiome–cancer associations (stringent and sensitivity analyses) Selective bacterial BSH shifts host metabolism Qualifiesmechanistic enzyme study — not a population metabolic association Animal / in-vitroBacteroidetes BSH selectivity screen plus B. thetaiotaomicron BT2086 deletion in gnotobiotic mice ~20 Bacteroidetes strains screened; monocolonization used 12 mice per Bt WT/KO group (8 GF controls) — no single primary N Human-gut Bacteroidetes strains and germ-free C57BL/6 mice Host metabolic phenotypes after selective bile-salt hydrolase loss Bacterial bile-salt enzymes can change host bile acids and weight-related phenotypes in models.
Selective bacterial bile-salt hydrolases can change host bile-acid chemistry and weight-related phenotypes in colonization models—evidence that specific microbial enzymes, not only community membership lists, can matter for host physiology.
- How much do gut bacterial gene contents differ?— different question — strain gene-content variation across people
- Antibiotics hurt bee gut and survival— different host — honey bees, not mammalian colonization models
Study Role Design N Population Outcome Selective bacterial BSH shifts host metabolism Supports Animal / in-vitroBacteroidetes BSH selectivity screen plus B. thetaiotaomicron BT2086 deletion in gnotobiotic mice ~20 Bacteroidetes strains screened; monocolonization used 12 mice per Bt WT/KO group (8 GF controls) — no single primary N Human-gut Bacteroidetes strains and germ-free C57BL/6 mice Host metabolic phenotypes after selective bile-salt hydrolase loss How much do gut bacterial gene contents differ? Qualifiesdifferent question — strain gene-content variation across people Cross-sectionalMetagenomic gene-deletion detection quantifying within-species gene content across gut communities N=207 · 252 fecal metagenomes from 207 individuals (HMP and European MetaHIT) Human gut metagenomes from public cohorts Inter-individual within-species gene-content variation Antibiotics hurt bee gut and survival Qualifiesdifferent host — honey bees, not mammalian colonization models Animal / in-vitroWorker honeybees fed tetracycline; 16S community and survival vs germ-free controls Cup cages of 30 bees × 15 replicates per condition; 16S profiles n≈14–15 per arm — no single primary N Worker honeybees with conventional or germ-free guts Core gut microbiota disruption and mortality after tetracycline Strain-level gene content varies a lot within the same gut species — species labels are coarse.
Even within the same named gut species, strain-level gene content varies substantially among healthy people—so species labels alone understate functional diversity.
- Is the gut microbiome the same along the intestine?— different scale — mucosal vs luminal niches along the gut
- Human body microbiome biogeography— different scale — body-site biogeography, not within-species gene content
Open questions
Tensions and limits
Some items are genuine disagreements on the same question. Others mark different assays, populations, or outcomes — limits on how far one study travels — not a forced fight between papers.
Cancer MR, pediatric T1D timing, and adult metabolic-trait associations all involve gut microbiota, but they answer different disease and design questions. Treating them as one interchangeable “gut microbiome disease effect” collapses distinct estimands.
- Mendelian randomisation associates specific gut taxa with several cancers
- B. dorei rises before T1D autoimmunity
- Gut microbes, blood metabolites, and metabolic traits
Study Role Design N Population Outcome Mendelian randomisation associates specific gut taxa with several cancers Supports Mendelian randomisationTwo-sample MR of MiBioGen gut taxa instruments against eight cancer GWAS summary sets 211 taxa instruments; cancer GWAS sample sizes vary by malignancy — no single primary N Gut microbiome and cancer GWAS summary statistics (IEU OpenGWAS/consortia) Causal microbiome–cancer associations (stringent and sensitivity analyses) B. dorei rises before T1D autoimmunity Supports CohortLongitudinal DIPP stool sequencing in HLA-risk children before T1D autoimmunity N=76 · 947 stool samples; 29 seroconverters vs 47 autoantibody-negative controls HLA-DQB1 moderate-to-high-risk children in DIPP Gut microbiota (B. dorei dominance) prior to persistent T1D autoantibodies Gut microbes, blood metabolites, and metabolic traits Supports Cross-sectionalMETSIM subset microbiome–metabolite–metabolic trait association analysis N=531 · 531 middle-aged Finnish men from the METSIM cohort (parent cohort 10,197) Middle-aged Finnish men in METSIM Associations of gut microbiota with plasma metabolites and metabolic traits Human observational and MR studies do not automatically generalise to bee antibiotic/pesticide disruptions or to enzyme-focused colonization experiments. Animal and insect systems test different host–microbe questions; they limit how far a human association travels rather than falsifying it.
- Antibiotics hurt bee gut and survival
- Pesticides reshape honey bee gut microbes
- Selective bacterial BSH shifts host metabolism
- Mendelian randomisation associates specific gut taxa with several cancers
Study Role Design N Population Outcome Antibiotics hurt bee gut and survival Supports Animal / in-vitroWorker honeybees fed tetracycline; 16S community and survival vs germ-free controls Cup cages of 30 bees × 15 replicates per condition; 16S profiles n≈14–15 per arm — no single primary N Worker honeybees with conventional or germ-free guts Core gut microbiota disruption and mortality after tetracycline Pesticides reshape honey bee gut microbes Supports Animal / in-vitroIn-hive pesticide exposures across sites; 16S and ITS pyrosequencing of foragers and brood N=24 · 24 sequenced samples (4 treatments × 3 sites × 2 bee types), each pooled from five bees Honey bees (Apis mellifera) exposed to in-hive pesticides Gut bacterial and fungal community composition under pesticide treatments Selective bacterial BSH shifts host metabolism Supports Animal / in-vitroBacteroidetes BSH selectivity screen plus B. thetaiotaomicron BT2086 deletion in gnotobiotic mice ~20 Bacteroidetes strains screened; monocolonization used 12 mice per Bt WT/KO group (8 GF controls) — no single primary N Human-gut Bacteroidetes strains and germ-free C57BL/6 mice Host metabolic phenotypes after selective bile-salt hydrolase loss Mendelian randomisation associates specific gut taxa with several cancers Supports Mendelian randomisationTwo-sample MR of MiBioGen gut taxa instruments against eight cancer GWAS summary sets 211 taxa instruments; cancer GWAS sample sizes vary by malignancy — no single primary N Gut microbiome and cancer GWAS summary statistics (IEU OpenGWAS/consortia) Causal microbiome–cancer associations (stringent and sensitivity analyses)
Common misconceptions
Mendelian randomisation proving a taxa–cancer association means the microbe’s mechanism in tumours is known.
MR supports a genetic-instrument association under its assumptions; the cancer MR paper notes unproven experimental pathways and limited strain resolution from 16S-based taxa.
B. dorei dominance before autoantibodies proves that B. dorei causes type 1 diabetes.
The finding is a temporal observational association before autoimmunity; it does not by itself establish causation.
If two papers both study “the gut microbiome,” their disease conclusions can be pooled as one effect.
Cancer odds, T1D autoimmunity timing, and metabolic metabolites are different outcomes in different designs. Pooling them as a single microbiome effect erases those limits.
Exam-style questions
Short-answer questions that ask you to explain or compare, not recall.
A classmate says the MR cancer paper and the B. dorei T1D paper disagree about whether the microbiome is harmful. How do you respond?
They are not answering the same question. One reports genetic-instrument associations with specific cancers; the other reports a temporal dominance pattern before T1D autoimmunity. Different diseases and designs are a scope limit, not a forced disagreement about a single “harm” effect.
What would most strengthen a causal position that a named gut taxon increases a cancer’s risk?
Convergent evidence beyond MR assumptions—such as strain-resolved exposures, experimental models that recover the pathway, and replication outside the original ancestry GWAS—while keeping the cancer endpoint specific.
Why does strain-level gene-content variation qualify species-level microbiome claims?
People can share a species name while differing in gene content, so functions (for example enzyme capacity) may not travel with the species label alone.
The studies
11 studies in this library bear on Microbiome, ordered by citations. The first 8 are shown.
- Human body microbiome biogeography
Healthy human body sites host distinct bacterial communities across a massive 16S survey.
- Antibiotics hurt bee gut and survival
Tetracycline shrinks the honeybee gut microbiome and elevates mortality after hive return.
- Mendelian randomisation associates specific gut taxa with several cancers
Genetic instruments for gut microbiota taxa showed causal associations with breast, lung, colorectal, prostate, gastric, and head/neck cancers—sometimes in opposing directions for the same genus.
- Selective bacterial BSH shifts host metabolism
Bacteroides bile salt hydrolase BT2086 selectively deconjugates bile acids and alters host metabolism.
- Gut microbes, blood metabolites, and metabolic traits
In 531 METSIM men, gut microbiota associate with plasma metabolites and metabolic-syndrome-relevant traits.
- B. dorei rises before T1D autoimmunity
Bacteroides dorei dominates the gut microbiome before autoimmunity in high-risk Finnish children.
- How do gut-bacteria vesicles enter host cells?
B. thetaiotaomicron outer-membrane vesicles are taken up within minutes via dynamin-dependent endocytosis/macropinocytosis and traffic to Golgi/ER/lysosomes.
- Infant diet shapes gut–host immunity
Breast- vs formula-fed infants differ in gut microbiota and host immune gene expression.
Show 3 more studiesShow fewer studies
- Pesticides reshape honey bee gut microbes
In-hive pesticide exposures change honey bee gut bacterial and fungal community composition.
- How much do gut bacterial gene contents differ?
Same gut bacterial species can differ widely in gene content across people, shaping functional individuality.
- Is the gut microbiome the same along the intestine?
Swine mucosal and luminal microbiomes differ by gut section, with distinct diversity patterns and bile-acid/SCFA environments.
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