Where is the gut microbiota located? A complete guide
Where is the gut microbiota located ? It is mainly located in the colon (large intestine), where it concentrates more than 95% of the microorganisms in the entire human digestive tract.
According to Inserm( [1], the human gut microbiota comprises approximately 100 trillion bacteria belonging to more than 1,000 different species. It is one of the densest and most complex microbial ecosystems in the human body.
- The gut microbiota is concentrated mainly in the colon (large intestine).
- It also colonizes the walls of thesmall intestine, albeit in lower density.
- The stomach and esophagus harbor very few microorganisms due to their acidity.
- The bacteria remain attached to the walls thanks to a protective biofilm
- The composition of the microbiota varies according to the digestive segments and from one individual to another.
What is the gut microbiota?
The gut microbiota, long known as gut flora, refers to all the microorganisms living in the digestive tract. It includes bacteria, viruses, fungi, and archaea.
These microorganisms are not mere passengers. They actively participate in food digestion, vitamin synthesis, and the regulation of the immune system.
Understanding where the gut microbiota is located helps to better understand why certain foods or supplements act directly on your digestive health.
Where is the gut microbiota located in the body?
Where exactly is the gut microbiota located ? It colonizes the entire digestive tract, from the mouth to the anus, but its density varies considerably depending on the segment.
Here is the breakdown by digestive zone:
| Digestive segment | Microbial density | Dominant microorganisms |
|---|---|---|
| Mouth / esophagus | Low (10² – 10³ CFU/mL) | Streptococci, Veillonella |
| Stomach | Very low (10¹ – 10³ CFU/mL) | Helicobacter pylori, Lactobacillus |
| Small intestine (duodenum) | Moderate (10³ – 10⁴ CFU/mL) | Lactobacillus, Streptococcus |
| Small intestine (ileum) | High (10⁷ – 10⁸ CFU/mL) | Enterococcus, Bacteroides |
| Colon (large intestine) | Very high (10⁹⁰ – 10¹² CFU/mL) | Firmicutes, Bacteroidetes, Bifidobacterium |
CFU/mL = colony-forming units per millilitre. The higher the number, the greater the bacterial density.
The colon: the main site of the gut microbiota
The colon is where the gut microbiota is found in the highest concentration. It represents less than 20% of the length of the digestive tract but is home to more than 95% of the total bacteria.
Why such a concentration? The colon offers ideal conditions: a less acidic pH, a slower transit speed and availability of fermentable nutrients (dietary fiber).
The bacteria in the colon ferment undigested fiber and produce SCFAs (short-chain fatty acids), which are essential for nourishing the cells of the intestinal wall. Learn more about the link between probiotics and the gut microbiota.
The small intestine: a less dense but active microbiota
Where is the gut microbiota located in the small intestine? It is present along its entire length (approximately 6 to 7 meters), but at a much lower density than in the colon.
The density gradually increases from the duodenum to the terminal ileum. The ileum, the last part of the small intestine, harbors significant bacterial populations that prepare the transition to the colon.
- The duodenum is very acidic: few bacteria survive there.
- The jejunum primarily hosts Lactobacillus and Streptococcus bacteria.
- Theileum contains Bacteroides and Enterococcus in increasing quantities.
The stomach and esophagus: almost sterile areas
The stomach is one of the places in the digestive tract where the gut microbiota is least represented. Its highly acidic pH (between 1.5 and 3.5) destroys the majority of ingested microorganisms.
Only a few resistant species survive there, notably Helicobacter pylori, which is associated with certain gastric ulcers. The esophagus, on the other hand, is rapidly traversed by food and does not constitute a stable habitat for bacteria.
This gastric acidity plays a protective barrier role against foodborne pathogens.
How do bacteria remain attached to the intestinal walls?
Gut bacteria do not float freely in the digestive contents. They attach to the intestinal walls thanks to a biofilm, a protective layer of polysaccharides that they secrete themselves.
This biofilm allows them to resist the digestive flow and pH variations. It also constitutes an exchange interface between the bacteria and the intestinal epithelial cells.
- The biofilm protects bacteria from chemical attacks (bile, acid).
- It facilitates bacterial communication (quorum sensing).
- It modulates the permeability of the intestinal mucosa.
- An unbalanced biofilm can promote dysbiosis.
What bacteria make up the gut microbiota?
The intestinal microbiota is dominated by two major bacterial phyla: Firmicutes and Bacteroidetes, which together represent more than 90% of the bacteria in the colon.
A study published in Nature [2] showed that the Firmicutes/Bacteroidetes ratio varies according to health status and body weight.
- Firmicutes : Lactobacillus, Clostridium, Ruminococcus
- Bacteroidetes : Bacteroides, Prevotella
- Actinobacteria : Bifidobacterium (important in infants)
- Proteobacteria : Escherichia coli (in small proportions in healthy individuals)
- Verrucomicrobia : Akkermansia muciniphila (protective of the mucous membrane)
Many readers tell us that the discovery of intestinal bacterial diversity has helped them to better understand the importance of a varied diet to maintain their microbiota.
What are the roles of the microbiota depending on its location?
The location of the gut microbiota directly determines its functions. Each digestive segment hosts populations adapted to specific tasks.
In the small intestine, bacteria participate in the digestion of carbohydrates and the absorption of nutrients. In the colon, they ferment fibers and produce essential metabolites.
- Digestion : breakdown of indigestible dietary fibers.
- Vitamin synthesis : vitamin K, B12, B9 (folate).
- Immunity : 70% of immune cells reside in the intestinal mucosa [3].
- Gut-brain axis : production of serotonin and neurotransmitters.
- Protection : competition with pathogens (barrier effect).
What is dysbiosis and how does it affect the localization of the microbiota?
Dysbiosis occurs when the balance of the microbiota is disrupted. It can alter the distribution of bacteria between the different segments of the digestive tract.
SIBO (small intestinal bacterial overgrowth) is a typical example: bacteria normally present in the colon colonize the small intestine in excess, causing bloating, pain, and diarrhea .
According to ANSES( [4], certain factors promote dysbiosis:
- Repeated or prolonged antibiotic therapy.
- A diet low in fiber and high in refined sugars.
- Chronic stress and lack of sleep.
- Excessive alcohol consumption.
- Certain medications (proton pump inhibitors, NSAIDs).
What factors influence the composition of the gut microbiota?
The composition of the gut microbiota is not fixed. It evolves throughout life under the influence of numerous environmental and behavioral factors.
A study published in Cell Host & Microbe [5] confirmed that diet is the main modulator of the microbiota in the short term, with measurable changes as early as 24 to 48 hours after dietary modification.
- Mode of delivery : babies born vaginally acquire a more diverse microbiota.
- Breastfeeding : promotes the development of Bifidobacterium.
- Nutrition : Prebiotic fibers nourish beneficial bacteria.
- Physical activity : associated with greater microbial diversity.
- ge : microbial diversity decreases with aging.
- Geography and culture : rural populations have a more diverse microbiota.
How to maintain the gut microbiota where it is found?
Maintaining a healthy gut microbiota, wherever it resides, begins primarily with diet. Prebiotics and probiotics are the two main levers.
Prebiotics (inulin, FOS, GOS) directly feed the bacteria in the colon. Probiotics provide live bacterial strains capable of temporarily boosting existing populations.
Consult our comprehensive guide on probiotics to choose the strains best suited to your needs.
Here are the most effective foods for supporting the gut microbiota:
- Fermented foods : yogurt, kefir, sauerkraut, kimchi, miso.
- Vegetables rich in fiber : artichoke, leek, garlic, onion, asparagus.
- Legumes : lentils, chickpeas, beans.
- Whole grains : oats, rye, barley.
- Fruits : banana (slightly green), apple, pear.
Learn exactly what a probiotic is and how it affects your gut flora.
Blond psyllium: an ally of the gut microbiota
Blond psyllium is one of the best-documented prebiotic fibers to support the gut microbiota where it resides, namely in the colon.
Its soluble fibers form a viscous gel in the colon, slowing transit and nourishing butyrate-producing bacteria. Butyrate is a short-chain fatty acid essential for the integrity of the colonic mucosa.
Discover all the benefits of psyllium on digestive health and the gut microbiota.
The gut microbiota and overall health: much more than just digestive flora
Understanding the location of the gut microbiota allows us to better grasp its influence on systems far removed from the digestive tract. The gut-brain axis, the gut-skin axis, and the gut-lung axis are currently active areas of research.
The NCCIH recognizes that the gut microbiota influences the systemic immune response, drug metabolism, and even some mood disorders.
- Mental health : 90% of serotonin is produced in the gut.
- Body weight : certain bacteria promote fat storage.
- Inflammation : an unbalanced microbiota increases intestinal permeability.
- Allergy : A diverse microbiota reduces the risk of allergies and asthma.
When should you consult a doctor about your gut microbiota?
Certain signs indicate that the gut microbiota, wherever it is located, is disrupted and requires medical evaluation. Don't delay in consulting a doctor if you experience several of these symptoms persistently.
- Chronic bloating and recurring abdominal pain.
- Unexplained alternating diarrhea/constipation.
- Intense fatigue with no identified cause.
- Repeated infections (candidiasis, urinary tract infections).
- Unexplained weight gain or loss.
- Persistent skin symptoms (eczema, acne, psoriasis).
A doctor may prescribe a microbiota analysis by metagenomic sequencing to identify specific imbalances.
- Where is the gut microbiota located ? Primarily in the colon, which is home to more than 95% of the 100 trillion bacteria in the human body.
- The small intestine also hosts an active microbiota, but in a much lower density, with an increasing concentration from the ileum towards the colon.
- The bacteria remain attached to the intestinal walls thanks to a protective biofilm which facilitates their exchange with the cells of the mucosa.
- The location of the gut microbiota determines its functions: fiber digestion, vitamin synthesis, immune regulation, and neurotransmitter production.
- A diet rich in prebiotic fibers and fermented foods are the most effective ways to maintain the microbiota where it is found.
Frequently asked questions about where the gut microbiota is located
Where exactly is the gut microbiota located in the body?
The gut microbiota is found primarily in the colon (large intestine), which contains more than 95% of the microorganisms in the digestive tract. It is also present in the small intestine, increasing in density from the ileum to the colon, and in very small quantities in the stomach and esophagus.
Why is the gut microbiota primarily located in the colon?
The colon offers ideal conditions for bacteria: a less acidic pH than elsewhere, slow transit that allows time for bacteria to multiply, and an abundance of fermentable fibers that serve as a nutrient substrate. These factors explain why the gut microbiota is concentrated there in such large quantities.
Is the gut microbiota present in the stomach?
The stomach harbors very few microorganisms because its highly acidic pH (between 1.5 and 3.5) destroys most ingested bacteria. Only a few resistant species survive there, such as Helicobacter pylori. The stomach is therefore not a significant habitat for the gut microbiota.
What is the difference between microbiota and gut flora?
The two terms refer to the same reality. The term "intestinal flora" is the old name, replaced since the 2000s by "intestinal microbiota", which is more precise because it includes all microorganisms (bacteria, viruses, fungi, archaea) and not just bacteria.
How many bacteria are found in the gut microbiota?
The gut microbiota contains approximately 100 trillion bacteria, roughly 10 times more than the number of human cells. These bacteria belong to over 1,000 different species and collectively weigh about 2 kg in a healthy adult.
Where is the gut microbiota located in infants?
In infants, the gut microbiota is found in the same segments as in adults, but its composition differs. It is dominated by Bifidobacterium, which are favored by breast milk. The infant's microbiota gradually diversifies until it reaches a composition close to that of adults around the age of 2 to 3 years.
Can the gut microbiota migrate to other organs?
Under normal conditions, intestinal bacteria remain confined within the digestive tract thanks to the epithelial barrier. In cases of increased intestinal permeability (leaky gut), some bacteria or their metabolites can cross the mucosa and reach other organs, contributing to systemic inflammation.
How can I tell if my gut microbiota is healthy?
A balanced gut microbiota is characterized by regular bowel movements, the absence of chronic bloating, good energy levels, and a robust immune system. Stool analysis using metagenomic sequencing allows for the assessment of the diversity and composition of the microbiota in its natural habitat, namely the colon.
Do probiotics reach the right place in the gut?
Probiotics must survive gastric acidity and bile salts to reach the colon, where the gut microbiota is most concentrated. Gastro-resistant formulations (entersoluble capsules) significantly improve the survival of the strains to their site of action.
Does stress affect the location of the gut microbiota?
Chronic stress alters the composition and relative location of the gut microbiota via the gut-brain axis. It increases intestinal permeability, accelerates or slows transit, and promotes the proliferation of pro-inflammatory bacteria at the expense of beneficial species.
Can the gut microbiota be improved where it is located through diet?
Yes. A diet rich in prebiotic fiber (vegetables, legumes, whole grains) directly nourishes the bacteria in the colon. Fermented foods (yogurt, kefir, sauerkraut) provide live bacterial strains. Measurable changes in the composition of the gut microbiota can be observed within 24 to 48 hours of dietary modification.
Where is the gut microbiota located and what is its link to immunity?
The gut microbiota is in direct contact with 70% of the body's immune cells, located in the intestinal lining. This anatomical proximity explains why a balanced microbiota strengthens the immune response, while dysbiosis can promote autoimmune diseases and recurrent infections.
Key takeaways
Where is the gut microbiota located ? Primarily in the colon, which concentrates almost all of the 100 trillion bacteria in the digestive tract. The small intestine also harbors some, but in much lower concentrations. The precise location of the gut microbiota determines its digestive, immune, and neurological functions.
Maintaining the microbiota where it is located involves a diet rich in varied fibers, regular consumption of fermented foods and limiting factors of dysbiosis (antibiotics, stress, ultra-processed food).
Concrete action: Add a portion of fermented food (plain yogurt, kefir or sauerkraut) and a source of prebiotic fiber (garlic, onion, leek or artichoke) every day to nourish your gut microbiota directly in the colon, where it needs it most.
Sources and studies
- Gut microbiota (intestinal flora) — Inserm,
- An obesity-associated gut microbiome with increased capacity for energy harvest — Nature, Turnbaugh et al.,
- The gut microbiota shapes intestinal immune responses during health and disease — Nature Reviews Immunology, Round & Mazmanian,
- The gut microbiota: state of knowledge and health implications — ANSES,
- Gut-microbiota-targeted diets modulate human immune status — Cell, Wastyk et al.,
This article is provided for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional before use.
