Microbiome and Human Health

Introduction to the Human Microbiome

The human microbiome—the collective community of microorganisms (bacteria, archaea, fungi, viruses, protists) inhabiting our bodies—comprises approximately as many microbial cells as human cells, encoding 150 times more genes than the human genome. The gut microbiome is the most studied body site compartment, but the skin, oral, vaginal, respiratory, and urinary microbiomes each have distinct compositions and functions. Advances in 16S rRNA amplicon sequencing and shotgun metagenomic sequencing have enabled culture-independent characterisation of these communities at unprecedented resolution.

The microbiome is increasingly recognised as a virtual organ with profound effects on host physiology. It trains the immune system during early life, metabolises dietary components and xenobiotics, synthesises vitamins, competes with pathogens, produces short-chain fatty acids and neuroactive compounds, and communicates with the brain through the gut-brain axis. Dysbiosis—alterations in microbiome composition or function—correlates with and in some cases causally contributes to inflammatory bowel disease, obesity, type 2 diabetes, cardiovascular disease, mental health disorders, and cancer.

Microbiome Development and Composition

Early Life Microbiome Establishment

The neonatal gut is colonised during and after birth—vaginal delivery exposes infants to maternal vaginal/faecal microbiota (rich in Lactobacillus); caesarean-born infants are initially colonised by skin and environmental bacteria (Staphylococcus, Cutibacterium). Breastfeeding provides human milk oligosaccharides (HMOs) selectively feeding Bifidobacterium longum and other beneficial species, together with maternal antibodies and milk microbiota. Antibiotic exposure in early life disrupts microbiome maturation. Epidemiological evidence links early-life dysbiosis to increased risk of allergic disease, asthma, obesity, and inflammatory bowel disease—the hygiene hypothesis extended to microbiome depletion.

Factors Shaping Adult Microbiome

Adult microbiome composition is influenced by diet (fibre promotes butyrate-producing Firmicutes; Western high-fat diet reduces diversity), medications (antibiotics, PPIs, metformin, statins shift composition), geography (traditional versus urbanised populations show dramatically different compositions), genetics (some host genetic loci including the FUT2 secretor gene that determines which ABO-related glycans coat the gut influence specific taxa), and age (diversity declines in frailty, with loss of Bifidobacterium). Diet modification can shift microbiome composition within days, while core stable components persist over years. Long-term dietary patterns (Mediterranean vs. Western) show the strongest associations with microbiome composition and diversity.

Microbiome-Immune Interactions

Microbiome and Immune Development

Germ-free mice raised without any microbiome have severely underdeveloped gut-associated lymphoid tissue (GALT), immature innate immune responses, and altered adaptive immune repertoire. Specific bacteria are required for Th17 cell development in the small intestine (segmented filamentous bacteria, SFB), for Foxp3+ Treg induction in the colon (Clostridia clusters IV and XIVa producing butyrate), and for IgA maturation. These findings demonstrate the microbiome's essential instructive role in shaping immune system development—a process that in humans occurs predominantly in the first years of life, with potentially lifelong consequences.

Metabolites as Microbiome-Host Signals

Short-chain fatty acids (SCFAs)—butyrate, propionate, acetate—produced by fermentation of dietary fibre by Firmicutes and Bacteroidetes are key microbiome-host communication molecules. Butyrate is the primary energy source for colonocytes and promotes epithelial barrier integrity; it also inhibits histone deacetylases promoting anti-inflammatory Treg differentiation. Propionate travels to the liver where it inhibits cholesterol synthesis. Bile acids modified by gut bacteria (secondary bile acid deoxycholic acid) signal through FXR and TGR5 receptors regulating lipid and glucose metabolism. Tryptophan metabolites produced by the microbiome include aryl hydrocarbon receptor ligands influencing intestinal immune homeostasis and indoles reinforcing barrier function.

Microbiome and Disease

Dysbiosis in IBD (Crohn's disease and ulcerative colitis) is characterised by reduced diversity, Faecalibacterium prausnitzii depletion (anti-inflammatory butyrate producer), and Proteobacteria expansion. Germ-free mouse models showed that IBD develops only with a microbiome—no microbiome, no colitis—and specific bacteria (adherent-invasive E. coli, Fusobacterium nucleatum) can initiate or exacerbate disease. Type 2 diabetes associations include Akkermansia muciniphila depletion (reduced in insulin resistance, metformin treatment partially restores it). Obesity microbiome transfers demonstrate causality—germ-free mice colonised with obese-donor microbiota gain more fat than those receiving lean-donor microbiota on identical diets.

Examples and Applications

Example 1: FMT for C. difficile

Clostridioides difficile infection (CDI) is precipitated by antibiotic disruption of normal gut microbiota, allowing this toxin-producing anaerobe to expand. Standard antibiotics (vancomycin, fidaxomicin) treat acute infection but recurrence rates reach 25-65%. FMT from a healthy donor—delivered via colonoscopy, nasogastric tube, or encapsulated frozen stool—restores microbiome diversity and competition, achieving over 90% resolution of recurrent CDI. FDA approved two FMT products (RBX2660, SER-109) in 2023. The mechanistic basis relates to restoring secondary bile acid metabolism by Lachnospiraceae that inhibits C. difficile spore germination.

Example 2: Gut-Brain Axis

The gut-brain axis encompasses neural (vagal nerve, enteric nervous system), endocrine (gut hormone, HPA axis), and immune pathways connecting gut microbiome to brain function. Germ-free mice show altered anxiety, depression-like behaviour, and cognitive function normalised by specific microbiome colonisation. Lactobacillus rhamnosus JB-1 reduced anxiety behaviour in mice via vagal nerve; vagotomy abolished the effect. Clinical trials of probiotics for depression (Lactobacillus and Bifidobacterium combinations) show modest but consistent benefit. Fecal transplantation from depressed human donors induced depression-like behaviour in germ-free rats, suggesting microbiome causality in mood disorder.

Example 3: Microbiome and Immunotherapy Response

Multiple clinical studies found that gut microbiome composition correlates with anti-PD-1 immunotherapy response in melanoma, NSCLC, and renal cell carcinoma. Responders have higher abundance of Faecalibacterium prausnitzii, Akkermansia muciniphila, and Bifidobacterium; non-responders have different compositions. Germ-free mouse experiments confirmed microbiome causality—FMT from responders improved anti-PD-1 response in germ-free mice. Clinical trials of FMT from immunotherapy responders to non-responders are ongoing, with published case series showing FMT enabling responses in previously non-responding patients—a potentially transformative approach to improving immunotherapy efficacy.

Example 4: Probiotics in Clinical Practice

Probiotics—live microorganisms benefiting host health when consumed adequately—have established efficacy in antibiotic-associated diarrhoea prevention (Lactobacillus rhamnosus GG, Saccharomyces boulardii), acute infectious gastroenteritis duration reduction, and necrotising enterocolitis prevention in premature neonates (Lactobacillus acidophilus + Bifidobacterium infantis). Evidence for IBS (irritable bowel syndrome) improvement with specific strains is moderate. The substantial strain specificity—benefit from one Lactobacillus strain does not predict benefit from another—requires careful interpretation of aggregated data and illustrates the complexity of microbiome interactions.

Example 5: Akkermansia and Metabolic Health

Akkermansia muciniphila—a mucin-degrading bacterium constituting 1-3% of gut microbiota in healthy adults—is depleted in obesity, type 2 diabetes, and IBD. Supplementation of obese/diabetic mice with A. muciniphila (or its outer membrane protein Amuc_1100) reduced adiposity, improved glucose tolerance, and restored gut barrier function. Pasteurised (heat-killed) A. muciniphila supplementation in overweight human adults improved insulin sensitivity in a pilot clinical trial, providing clinical proof-of-concept. A. muciniphila exemplifies next-generation probiotic development based on mechanistic understanding of specific microbiome-host interactions.

Example 6: Microbiome in Mental Health

Depression and anxiety correlate with reduced microbiome diversity and specific compositional alterations (Lactobacillus and Bifidobacterium depletion; Bacteroidetes expansion). The Psychobiotic Trial showed dietary intervention increasing prebiotic fibre consumption shifted microbiome composition and reduced stress responses in healthy volunteers. MyNewGut study showed specific bacteria correlate with depression severity in population cohorts. Tryptophan conversion to serotonin is partly microbiome-regulated; gut bacteria produce GABA, dopamine precursors, and anti-inflammatory short-chain fatty acids. Clinical trials testing microbiome-targeted interventions (specific probiotics, prebiotics, FMT) for depression and anxiety are ongoing.

Example 7: Virome

The gut virome—predominantly bacteriophages targeting gut bacteria—is the least studied microbiome component but increasingly recognised as important. Phages regulate bacterial community composition through predation and horizontal gene transfer. The gut virome is altered in IBD, HIV infection, and other conditions. Bacteriophages engineered to target specific bacterial species (precision bacteriome editing) could selectively remove harmful bacteria while sparing beneficial ones—more precise than antibiotics. Some phage populations integrate as prophages in bacterial chromosomes; stress-induced prophage activation can rapidly alter bacterial community structure through lytic cycles.

Example 8: Microbiome and Drug Metabolism

Gut bacteria metabolise many orally administered drugs, affecting their bioavailability, efficacy, and toxicity. Irinotecan (colon cancer chemotherapy) is detoxified in the liver as irinotecan-glucuronide, but gut bacteria expressing beta-glucuronidase cleave the glucuronide reactivating the cytotoxic compound in the gut, causing severe diarrhoea. Beta-glucuronidase inhibitors (co-administered) reduced this toxicity without affecting antitumour efficacy in preclinical models. Levodopa for Parkinson's is decarboxylated by gut Enterococcus bacteria before absorption, reducing bioavailability—explaining highly variable inter-individual responses. Microbiome composition thus represents a major source of drug response variability requiring consideration in personalised pharmacotherapy.

Try it live

Everything above runs in your browser — open Gut Microbiome Ecology Simulator and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.