Expansion
Fermentation, vitamin synthesis, colonisation resistance and immune education
Expansion
The colon contains around 10 to 100 trillion organisms, mostly anaerobes, with a combined genome far larger than our own.
Functions:
- Fermentation of undigested carbohydrate to short-chain fatty acids, of which butyrate is the main fuel of the colonocyte. This salvages energy that would otherwise be lost
- Vitamin synthesis: vitamin K and several B vitamins
- Colonisation resistance: occupying niches and competing for nutrients, preventing overgrowth by pathogens
- Immune education: driving development of gut-associated lymphoid tissue, regulatory T cells and IgA
- Metabolism of bile acids and of some drugs, such as sulfasalazine and digoxin
Disruption (dysbiosis) after antibiotics allows Clostridioides difficile to proliferate, and faecal microbiota transplantation restores resistance with high success rates, which is perhaps the clearest demonstration of the microbiota’s functional role.
The vitamin K contribution also explains why prolonged broad-spectrum antibiotics can prolong the prothrombin time.