Normal human microflora

Classification of microflora

According to the duration of stay in the macroorganism, microflora is divided into two main groups:

1. Permanent (resident, indigenous). Continuously present on the mucous membranes and skin. Subdivided into:
2. Obligate: the basis of microbiocenosis (bifidobacteria, lactobacilli, bacteroides).
3. Facultative: a minor part of the community (klebsiella, staphylococci, streptococci, certain fungi).
4. Transient (allochthonous). Accidental microorganisms that cannot establish themselves in the body for long due to immune defense and competition.

The basis of the entire microbial community is formed by obligate anaerobes.

Topography and composition of the microbiota

The body contains both densely populated areas and normally completely sterile zones (lungs, uterus, internal organs, kidneys, urinary bladder, trachea, and alveoli).

• Skin. Anaerobes here are 2–10 times more abundant than aerobes (residing in deep layers: hair follicles, sebaceous glands). Typical residents: Staphylococcus epidermidis, coryneform bacteria, lipophilic fungi Malassezia. Escherichia coli on the skin is transient flora.
• Oral cavity. Alkaline environment, huge population density (up to $10^8$ bacteria per 1 mL of saliva). Bacteroides, streptococci, and actinomycetes dominate. Dental plaque formation occurs through bacterial synthesis of polysaccharides (from sucrose) and binding to salivary receptors.
• Gastrointestinal tract. The stomach serves as a "sterilization chamber" — hydrochloric acid destroys most bacteria (concentration < $10^3$ cells/mL). The large intestine is the main reservoir of microflora ($10^{10}–10^{12}$ per 1 g of feces). Here, 95% of the flora consists of anaerobes (bifidobacteria, bacteroides, clostridia). The gastrointestinal barrier is reinforced by a mucin layer: its inner layer is sterile and protects epithelial cells from contact with bacteria.
• Vagina. Anaerobes sharply predominate (10:1). In reproductive age, the basis consists of gram-positive lactobacilli. They split glycogen (accumulated under the action of estrogens) into lactic acid, lowering the pH to 4.0–4.6, which suppresses foreign microflora.

Main functions of normal microflora

Normal flora is an independent extracorporeal organ that actively interacts with immunity and metabolism.

• Colonization resistance (CR). A set of properties of the flora and the organism that prevents pathogens from colonizing mucous membranes. The primary role belongs to anaerobes (lacto- and bifidobacteria), which compete for nutrition and secrete bacteriocins, organic acids, and hydrogen peroxide.
• Immunomodulatory function. Microbial antigens "train" innate immunity. For example, muramyl dipeptide stimulates macrophages to produce IL-12, which triggers the differentiation of T-helper cells and suppresses IgE production (anti-allergic effect).
• Metabolic role. Large intestine microbes synthesize vitamins (groups K, B), break down cellulose, and participate in the metabolism of bile salts and cholesterol. Metabolic detoxification in the intestine is comparable to the barrier function of the liver.
• Antimutagenic activity. Bacteria are capable of destroying carcinogens in the intestinal lumen, although during dysfunctions, some of their enzymes can, conversely, turn safe substances into toxic ones.

Dynamics and pathological potential

The newborn GI tract is sterile, but colonization begins within the first hours. By days 9–10, normal eubiosis forms with a dominance of bifidobacteria.

However, microflora also has pathological potential. With decreased immunity or antibiotic intake, the following may occur:

• Translocation: penetration of bacteria through mucous membranes into internal sterile environments.
• Breakdown of tolerance: the immune system begins to attack its own bacteria, which triggers bowel diseases (ulcerative colitis, Crohn's disease).
• Allergization: microbial decarboxylases contribute to the excessive release of histamine.

To correct disorders (restore CR), selective decontamination is used, as well as medications: probiotics (live bacteria), prebiotics (flora growth stimulants), and synbiotics.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Quorum Sensing mechanism (bacterial communication in populations).
• Structure and functions of the two-layer intestinal mucin barrier.
• Features of selective decontamination in microbiocenosis correction.
• Stages and mechanisms of biofilm formation on dental enamel.
• Development of the GI microbiota in a child's first year of life.

Other topics in the section "Microbial Ecology — Microecology"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.