Bacteria in the Gut: From the Mouth to the Colon

The bacterial community changes as food travels from the mouth to the colon. Gut and Physiology Syndrome uses those different locations to explain why the name of a bacterium is only part of the story. The environment around it, the sample being tested and the job of that part of the digestive tract also matter.^[1]

The Mouth and Esophagus

The book reports hundreds of bacterial species in the mouth and describes different communities on the teeth, gums, tongue and other oral surfaces. It also notes overlap between bacterial groups found in saliva and stool. The important idea is that the digestive tract contains connected but distinct habitats.^[1]

A result from one site should therefore stay attached to that site. A saliva sample and a stool sample are not interchangeable reports about every surface in the body.

The Stomach and Its Acid Environment

Campbell-McBride describes the stomach as comparatively sparsely populated because of its acidic environment. She names acid-tolerant organisms, including some yeasts, lactobacilli and H. pylori, while discussing the stomach lining and acid production. Her account connects changes in acidity and mucus with changes in microbial growth.^[1]

The book also criticizes the effects of some medicines on that environment. An ecological explanation of stomach symptoms is not enough to decide that a prescribed medicine should be stopped. Keep the reason it was prescribed, the actual diagnosis and the GAPS food plan as separate questions.

H. Pylori Needs Its Own Clinical Context

The chapter discusses H. pylori as a longstanding human associate and describes possible relationships with the stomach environment and appetite hormones. It also gives the author’s view that treatment should be reserved for serious stomach problems. That opinion should not become advice to wait for an ulcer or cancer before getting care for confirmed infection.^[1]

The American College of Gastroenterology’s 2024 guidance recommends treatment for people found to have H. pylori infection and confirmation of cure afterward. A positive result needs interpretation and a care plan; a discussion of microbial ecology does not replace either.^[2]

The Small Intestine

The book describes a population gradient through the small intestine, with more microbes toward the ileum than near the stomach. It connects low stomach acidity with a greater microbial population and discusses small intestinal bacterial overgrowth, or SIBO. This is also a major site of nutrient absorption, so the condition of its lining matters for nourishment.^[1]

SIBO names an overgrowth in a location. IBS describes a symptom-based disorder and should not be treated as an identical label. For the fuller GAPS discussion, use the SIBO guide and its food, ferment and stomach-acid companion.

The Colon and Fermentation

The colon contains a dense and diverse microbial community. Campbell-McBride compares its fermentation role with the rumen of plant-eating animals, while noting that the two digestive systems are arranged differently. Microbial fermentation of material reaching the colon can produce short-chain fatty acids that the body uses.^[1]

She lists groups including Bacteroides, Bifidobacteria, Eubacteria and Clostridia in the discussion of healthy stool samples. A familiar group name does not make every member harmless, or tell you what its presence means at a different site.

What a Stool Sample Can Show

The book emphasizes that stool does not directly sample every living community along the gut wall. It also describes how food can shift the bacterial pattern seen in stool. Those are reasons to keep the sample and the question in view, not to assume that all stool tests are useless or that everyone needs a gut biopsy.^[1]

When reading a report, keep the sample type, the method and the organism names together. Then ask what the result can actually answer. The protozoa and stool-test guide works through that distinction for parasite names.

Prebiotics, Fiber and the GAPS Stage

Campbell-McBride explains that prebiotic carbohydrates can feed different members of a microbial community. She uses this to explain why foods or supplements rich in certain fibers may increase gas or discomfort when digestion is struggling. The passage does not create a single permanent no-vegetable rule for everyone following GAPS.^[1]

Use the actual food form and introduction sequence for Intro or Full GAPS. Record which food, amount and preparation changed rather than putting all fiber into one category.

For a companion to this topic, look for the 90-Day Family Food Diary in my printable catalog.

Return to the GAPS gut-flora guide for the wider picture and the links to each part of this series.

References

1. Campbell-McBride, N. Gut and Physiology Syndrome (2020), “Bacteria,” digestive-tract locations, stool samples, acidity, colon fermentation and prebiotics, printed pages 18–22.
2. American College of Gastroenterology. Treatment of Helicobacter pylori Infection: 2024 Guideline Highlights, treatment and test-of-cure guidance.