Mucus layer
A carbohydrate-rich coat some specialists eat, others merely hold. A thinned coat is a research theme, not a self-score.
The small intestine is faster and harsher; the colon is a fermentation vat. Obligate anaerobes dominate where oxygen is scavenged.
Metagenomes (NIH Human Microbiome Project, 2007 onward) showed most members were never grown as named pets. Sequence is not a care plan.
Parts
Lumen fermenters
Bacteroides, Firmicutes guilds and others break fibre. Names shift; the function is older than the brand.
Short-chain fatty acids
Acetate, propionate, butyrate feed colon cells and signal beyond the gut. Amounts follow fibre, not a capsule slogan.
Bile-tolerant specialists
High-fat, low-fibre patterns can enrich bile-resistant lineages. Pattern, not a villain species.
Gut archaea
Methanobrevibacter can consume hydrogen. They are residents, not pathogens.
Phage layer
Bacterial viruses outnumber cells in stool. They edit who blooms.
Life cycle
1Arrive and assemble
The first years recruit from family, place and milk. There is no universal starter pack.
2Ferment the day's fibre
Hours after a bean or oat meal, gases and acids rise. That is ecology you can feel.
3Turnover
Many lineages double daily. A week of new food already redraws abundances.
4Disturb and recover
Infection, travel and antibiotics can dent diversity. Recovery is common; it is not guaranteed identical.
When a headline says the microbiome 'causes' a mood, read for association versus mechanism.
Keep exploring
More in Beneficial allies
Probiotics
Live microbes in food or capsules — strain-specific guests, not a whole microbiome and not medical advice.
Familiarity 90Fermentation
Microbes turning food into kimchi, doenjang, yoghurt and other living crafts — chemistry plus culture.
Familiarity 85Skin microbiome
A site-specific living film — oily, moist and dry neighbourhoods, not a sterile wrapper.
Familiarity 62Soil microbes
The hidden workforce of land — decomposers, mycorrhizae and a mostly uncultured majority.
Familiarity 48Nitrogen cycle
How microbes move nitrogen between air, soil, water and food — plus the industrial shortcut that feeds billions.
Familiarity 40