Topography and Distribution of Bacteria Along the GI Tract
In newborns, the digestive tube is entirely sterile. Its colonization begins in the first weeks of life when microorganisms (predominantly anaerobes) enter the body through the mouth. Subsequent distribution of bacterial populations is uneven, as each segment of the gastrointestinal tract features a specific environment.
- Oral cavity: Characterized by massive bacterial contamination, despite the pronounced bactericidal properties of saliva. One milliliter of oral fluid contains 10 to 100 million microbial cells.
- Stomach: The aggressively low pH strongly suppresses microflora growth. Gastric contents often remain completely sterile or contain no more than 1,000 microorganisms per 1 mL.
- Small intestine: In the duodenum and proximal 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: Serves as the primary reservoir with the maximum concentration of microorganisms. One gram of feces from a healthy human harbors 10 billion or more bacterial cells.
Eubiosis and the Influence of Diet
The balance of interactions among various species of microorganisms within the GI tract is known as eubiosis. This equilibrium establishes a specific saprophytic flora and reliably protects the host organism from invasion by pathogenic bacteria. Historically, the concept of improving health through microflora was studied by É. Metchnikoff, who proposed the hypothesis of replacing pathogenic flora with beneficial acidophilic bacteria using fermented dairy products.
To maintain eubiosis, a continuous, adequate, and balanced diet is critically important. Any pronounced changes in diet (an exogenous factor) transform the bacterial landscape:
- Vegetarian diet stimulates the growth of enterococci and eubacteria populations.
- Excess animal proteins and fats lead to a pathologic increase in the number of clostridia and bacteroides.
- Excess animal fats alone selectively increase the bacteroides population while concurrently decreasing the number of enterococci and bifidobacteria (fat deficiency causes the exact opposite changes).
- Milk diet (rich in lactose) promotes the active proliferation of bifidobacteria.
- Non-physiological components, such as freeze-dried foods or single-cell proteins, exert adverse effects on microbial homeostasis.
Physiological Role of Microorganisms
The normal intestinal microflora performs a spectrum of tasks essential for maintaining human vitality. A key function is the development of immunobiological reactivity. Bacteria provide colonization resistance, protecting the organism from the implantation and proliferation of pathogenic agents.
In addition to defense, the microflora fulfills 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, influencing 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) occurs alongside the decomposition of organic substances, producing amines, ammonium salts, and organic acids.
Disruption of this complex microbial biocenosis is termed dysbiosis. It arises against the background of various diseases or prolonged antibiotic therapy. This condition is dangerous due to the unchecked proliferation of opportunistic flora (Proteus, staphylococci, yeasts), which inevitably leads to complications.