intestinal flora
Meaning of intestinal flora: Resident microbes in their trillions, doing chemical work the human body never evolved the enzymes to do for itself.
Definition of intestinal flora
Intestinal flora is the community of bacteria and other microbes living in the gut, most of them in the large intestine. They ferment the fiber that human enzymes cannot break down, make certain vitamins, crowd out harmful organisms and help train the immune system. The mix differs between people and shifts with diet, age, illness and antibiotics.
What do these microbes do for the body?
| Job | How it works |
|---|---|
| Fermenting fiber | Breaks down plant material human enzymes cannot, releasing short chain fatty acids that feed the cells lining the colon |
| Making vitamins | Produces vitamin K and several B vitamins, biotin among them |
| Excluding invaders | Occupies space and consumes nutrients that harmful organisms would otherwise use |
| Training immunity | Constant low level contact teaches the immune system what to tolerate |
| Handling drugs and bile | Alters some medicines and recycles bile acids |
The biotin and vitamin K produced this way add to what comes from food rather than replacing it.
Where do they live and where do they come from?
Numbers climb steeply along the gut. The stomach and upper small intestine hold few, kept down by acid and rapid flow, while the colon holds the overwhelming majority. A mucus layer keeps most microbes at a distance from the lining cells and from the intestinal villi that absorb nutrients. Colonization begins at birth and is shaped by how a baby is delivered and fed, then settles into an adult pattern during early childhood. Diet is the strongest ongoing influence, particularly the amount and variety of plant fiber.
What disturbs the balance?
Antibiotics are the most abrupt disturbance, since they kill useful bacteria along with the intended target. Most of the community recovers over weeks, although some members do not return. The gap left behind lets resistant organisms take over, which is how Clostridioides difficile causes severe diarrhea after a course of antibiotics. Illness, surgery and major changes in diet also shift the mix. Transplanting stool from a healthy donor is now an established treatment for repeated infection of that kind, which shows how much the resident community matters.
What is still uncertain?
Differences in the gut community have been reported in obesity, inflammatory bowel disease, diabetes and several conditions affecting mood. Whether those differences cause the condition or follow from it is unresolved in most cases, and the marketing of probiotics has run well ahead of the evidence. What is established is that this community is metabolically active on a large scale and interacts continuously with the immune system.
Also known as
Used in a sentence
The intestinal flora was disrupted by broad spectrum antibiotics, allowing Clostridioides difficile to overgrow.
Good to know
Differences in intestinal flora have been reported in many conditions, but whether the difference causes the condition or follows from it is unresolved in most cases, and antibiotics disturb the community in ways that can take weeks to settle.
Word origin
Where “intestinal flora” comes from
Flora comes from the Latin flos, meaning flower, a word applied to gut microbes when bacteria were still classed as plants.