Gastrointestinal microflora

Topography and distribution of bacteria by GI tract segments

In newborns, the digestive tract is completely sterile. Its colonization begins in the first weeks of life, when microorganisms (predominantly anaerobic) enter the body through the mouth. Subsequent distribution of bacterial populations is uneven, as each section of the gastrointestinal tract has a specific environment.

• Oral cavity: characterized by a huge bacterial contamination, even despite the pronounced bactericidal properties of saliva. There are from 10 to 100 million microbial cells in one milliliter of oral fluid.
• Stomach: aggressively low pH strongly inhibits the growth of microflora. Stomach contents often remain completely sterile or contain no more than a thousand microorganisms per 1 ml.
• Small intestine: in the duodenum and initial sections of the jejunum, bacteria are virtually absent (similar to the stomach). However, as chyme moves toward the ileum, the concentration of microorganisms gradually increases, reaching 1 million cells per 1 ml.
• Large intestine: is the main reservoir with the maximum concentration of microorganisms. One gram of feces in a healthy person contains 10 billion or more bacterial cells.

Eubiosis and the influence of diet

The balance of relationships between various species of microorganisms within the GI tract is called eubiosis. This balance forms a specific saprophytic flora and reliably protects the host organism from the invasion of pathogenic bacteria. Historically, the concept of improving health through microflora was studied by I.I. Mechnikov, who proposed the hypothesis of replacing pathogenic flora with beneficial acidophilic bacteria using fermented milk mixtures.

To maintain eubiosis, a constant, adequate, and balanced diet is critical. Any pronounced changes in diet (an exogenous factor) transform the bacterial landscape:

1. Vegetarian diet stimulates the growth of enterococcal and eubacterial populations.
2. Excess of animal proteins and fats leads to a pathological increase in the number of clostridia and bacteroides.
3. Excess of animal fats alone selectively increases the bacteroides population while decreasing the number of enterococci and bifidobacteria (fat deficiency causes the exact opposite changes).
4. Dairy diet (rich in lactose) promotes the active reproduction of bifidobacteria.
5. Non-physiological components, such as freeze-dried foods or single-cell proteins, have an adverse effect on microbial homeostasis.

Physiological role of microorganisms

The normal intestinal microflora performs a whole range of tasks necessary to maintain human life. The key one is the formation of immunobiological reactivity. Bacteria provide colonization resistance, protecting the body from the introduction and reproduction of pathogenic agents.

In addition to defense, the microflora carries out the following functions:

• Synthetic: microbial synthesis produces vitamin K and B vitamins, partially covering the human physiological requirement for these nutrients.
• Digestive: bacterial enzymes take over the breakdown of complex substances that were not digested in the small intestine. These include pectins, cellulose, and hemicellulose.
• Metabolic: microorganisms process undigested residues, transforming them into absorbable substances. The flora also regulates the enterohepatic circulation of bile components, affecting the liver, and actively participates in the metabolism of proteins, phospholipids, bile and fatty acids, cholesterol, and bilirubin.
• Destructive: inactivation of digestive juice enzymes (trypsin, amylase, alkaline phosphatase, enterokinase) and decomposition of organic substances with the formation of amines, ammonium salts, and organic acids occurs.

Disruption of this complex microbial biocenosis is called dysbiosis. It occurs against the background of various diseases or with prolonged use of antibiotics. This condition is dangerous due to the rapid proliferation of opportunistic flora (Proteus, staphylococci, yeast), which inevitably leads to complications.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of formation of intestinal colonization resistance.
• Enterohepatic circulation of bile components and the role of microflora in it.
• Historical contribution of I.I. Mechnikov to the study of acidophilic bacteria and eubiosis.
• Specific enzymatic activity of bacteria in the breakdown of pectins and cellulose.
• The danger of opportunistic flora in advanced stages of dysbiosis.

Other topics in the "Digestion" section

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