# Gut Microbiome Physiology > Gut microbiome physiology for MBBS Physiology: bacterial load by segment, short-chain fatty acids, vitamin K, dysbiosis and FMT. - Canonical URL: https://prepelephant.com/topics/mbbs/physiology/gut-microbiome-physiology - Exam / course: MBBS · Subject: Physiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Gut Microbiome Physiology", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/gut-microbiome-physiology ## 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.