The bacterial populations within the human body vary drastically by location. Perhaps the most fascinating in this variation is the GI tract – starting with your mouth, ending with your large intestine. To visualize this proportion, we can use a conceptual scale, where the bacterial concentration of the (relatively-sterile) eyeball is set as our baseline, represented by the size of a House.
| Body Area | Relative Size Comparison |
| Eyeball, Kidneys, Liver | A House |
| Stomach (note: stomach acid and enzymes kill most of the bacteria) | A large office building |
| Mammary tissue (while breastfeeding) | A large shopping mall |
| Duodenum (first part of your small intestine) | A city block |
| Jejunum (second part of your small intestine) | A large city |
| Ileum (final part of your small intestine) | A large country |
| Mouth (obviously: pre-stomach acid) | The continental United States |
| Terminal Ileum, Skin, Hair, Ear Canal | The surface area of the Earth |
| Armpit Skin, Toes | The planet Jupiter |
| Large Intestine | The Sun |
The digestive tract tells a fascinating story of how gut bacteria colonizes the human GI tract. The mouth’s abundant bacteria can vary wildly, within hours – both in concentration and type. The stomach’s acidic environment kills most bacteria populations, but as food moves to the duodenum, the pH becomes more neutral and bacteria again begin to thrive. This population grows exponentially through the small intestine, reaching its peak in the large intestine, where the environment is ideal for microbial communities.
The small intestine bacteria play a crucial role in food digestion and absorption, whereas the bacteria in the large intestine perform a “cleanup” role. This is why drinking kefir helps get your dairy back – it’s a helpful cycle of bacterial colonization, who then changes the gut terrain to accommodate their needs, which leads to more established cocolonization, etc. – ultimately leading to better dairy digestion.
Note that too much bacteria in the small intestine is called “SIBO” – Small intestine bacterial overgrowth. If you’re sensitive to fermented foods, chances are that (a) you have low stomach acid, and (b) you have SIBO, at least temporarily after you eat high-bacterial foods. Also note that most healthy internal organs like the kidneys and liver are sterile, a state that is essential for their function. An exception is mammary tissue during breastfeeding, which actively harbors a bacterial community that is transferred to the infant, playing a key role in developing the baby’s microbiome.
