Gut Microbiome Physiology
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Direct answer
The human gut harbours roughly 38 trillion bacteria — about one bacterial cell per human cell by the revised 2016 estimate (Sender and colleagues), weighing 1-2 kilograms, with the colon alone packing 10 raised to the power 11-12 organisms per gram of contents. Two phyla, Firmicutes and Bacteroidetes, dominate, and their combined gene repertoire exceeds the human genome roughly 150-fold, functioning as a metabolic organ we did not inherit but acquired at birth. Physiological services include fermentation of undigested fibre into short-chain fatty acids (butyrate supplies 60-70 per cent of colonocyte energy), synthesis of vitamin K and several B vitamins, conversion of primary to secondary bile acids, maturation of gut-associated lymphoid tissue, and colonisation resistance against Clostridioides difficile — the last demonstrated clinically by faecal microbiota transplant curing roughly 85-90 per cent of recurrent C. difficile infection.
What you must remember
- Density gradient by segment: stomach and duodenum sparse (10 raised to 1-3 per gram), jejunum 10 raised to 4-5, ileum 10 raised to 7-8, colon 10 raised to 11-12 — the reason small-intestinal overgrowth is defined by jejunal aspirate culture above 10 raised to 5 colony-forming units per millilitre.
- Short-chain fatty acids: acetate, propionate and butyrate from fibre fermentation; butyrate is the preferred fuel of colonocytes (deficiency implicated in ulcerative colitis), propionate is gluconeogenic substrate in liver, acetate reaches muscle and brain.
- Vitamin K: bacterial menaquinones supply about half the daily requirement — the basis of haemorrhagic disease of the newborn after sterile gut birth and of vitamin K deficiency with prolonged broad-spectrum antibiotics; vitamin K prophylaxis at birth is standard Indian practice.
- Bile acid metabolism: deconjugation and 7-alpha-dehydroxylation produce secondary bile acids (deoxycholic, lithocholic) — enterohepatic circulation partners, but excess secondary acids are colonic-carcinogen candidates.
- Colonisation resistance: the resident flora blocks pathogens by niche occupancy, bacteriocins and short-chain-acid pH — its destruction by antibiotics is the opening for C. difficile.
- Immune education: germ-free animals have hypoplastic Peyer's patches and deficient IgA; commensals drive regulatory T-cell induction and tolerogenic tone — the physiological backdrop of the hygiene hypothesis.
- Dysbiosis associations: inflammatory bowel disease, irritable bowel syndrome, Clostridioides difficile, and (contested) obesity via altered Firmicutes:Bacteroidetes ratio — quote this ratio as a hypothesis, not settled fact.
- Terminology: probiotics (live organisms), prebiotics (fermentable fibre such as inulin), synbiotics (both), postbiotics (bacterial metabolites) — a definitions question in university vivas.
A worked case from antibiotics to transplant
Follow an elderly patient through repeated courses of broad-spectrum antibiotics for urinary infection. The colonic flora is flattened; colonisation resistance collapses; C. difficile spores germinate, toxins A and B destroy the epithelium, and pseudomembranous colitis follows. Metronidazole or oral vancomycin treats the episode, but each recurrence — around 20-25 per cent after the first — reflects a still-devastated ecosystem. By the second or third relapse, guideline-endorsed faecal microbiota transplant restores a diverse donor community and clears over 85-90 per cent of cases, a cure rate no antibiotic matches precisely because the treatment is ecology, not a single drug.
The same physiology explains the newborn: a sterile gut at delivery is colonised within days — vaginal delivery seeds lactobacilli-dominated flora, caesarean section a more cutaneous, staphylococcal profile — and breast milk oligosaccharides then feed commensal bifidobacteria. That early community primes immune maturation, which is part of the reasoning behind exclusive breastfeeding promotion in Indian national programmes.
Where students slip
Two slips recur. First, quoting the old 10:1 bacteria-to-cell ratio — the 2016 Sender re-estimate brought it to roughly 1:1 (about 38 trillion bacteria per 30 trillion human cells), and correcting the number signals current reading. Second, treating the microbiome as digester only: its immune-educating role (germ-free animals with hypoplastic GALT, IgA induction, T-regulatory cells) is the arm examiners probe in physiology, while the fermentation arm is assumed. Also keep the bile-acid story straight: the flora performs deconjugation and secondary-bile-acid conversion in the colon, not the active ileal reabsorption of conjugated bile salts — conflating the two mixes up gut segments and loses the ileal-resection question.
Frequently asked questions
Which segment of the gut is most densely colonised?
The colon, with about 10 raised to the power 11-12 bacteria per gram of contents, compared with near-sterility in the stomach and upper small intestine.
Why are short-chain fatty acids physiologically important?
Fibre fermentation yields acetate, propionate and butyrate; butyrate supplies most of the colonocyte's energy and supports epithelial barrier integrity, while propionate serves hepatic gluconeogenesis.
Why does vitamin K deficiency occur in newborns and after prolonged antibiotics?
The neonatal gut is sterile at birth, and broad-spectrum antibiotics eradicate the commensal flora that would otherwise synthesise menaquinones — hence intramuscular vitamin K prophylaxis at birth.
What is colonisation resistance?
The occupying resident microbiota blocks pathogen establishment through niche competition, bacteriocin production and luminal acidification; its loss to antibiotics allows Clostridioides difficile to proliferate.
In which condition is faecal microbiota transplant established therapy?
Recurrent Clostridioides difficile infection, where restoring donor microbial diversity clears roughly 85-90 per cent of cases, far exceeding repeat antibiotic courses.