Classification of Microflora
Based on the duration of stay in the macroorganism, microflora is divided into two main groups:
- Permanent (resident, indigenous). Continuously present on mucous membranes and skin. Subdivided into:
- Obligate: the basis of the microbiocenosis (bifidobacteria, lactobacilli, bacteroides).
- Facultative: a smaller part of the community (klebsiella, staphylococci, streptococci, certain fungi).
- Transient (allochthonous). Random microorganisms that cannot establish themselves long-term in the body due to immune defense and competition.
The foundation of the entire microbial community consists of obligate anaerobes.
Topography and Composition of the Microbiota
The body contains both densely populated areas and normally sterile zones (lungs, uterus, internal organs, kidneys, urinary bladder, trachea, and alveoli).
- Skin. Anaerobes here outnumber aerobes by 2–10 times (residing in deep layers: hair follicles, sebaceous glands). Typical residents: Staphylococcus epidermidis, coryneform bacteria, lipophilic Malassezia fungi. Escherichia coli on the skin is transient flora.
- Oral Cavity. The environment is alkaline, and population density is enormous (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 acts as a "sterilization chamber" — hydrochloric acid destroys most bacteria (concentration < $10^3$ cells/mL). The colon 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 layer of mucin: its inner layer is sterile and protects epithelial cells from contact with bacteria.
- Vagina. Anaerobes heavily predominate (10:1). In reproductive age, Gram-positive lactobacilli form the core. They break down glycogen (accumulated under the influence of estrogens) into lactic acid, lowering the pH to 4.0–4.6, which suppresses foreign microflora.
Main Functions of Normal Microflora
Normal flora acts as an independent extracorporeal organ that actively interacts with immunity and metabolism.
- Colonization Resistance (CR). The combined properties of the flora and the organism that prevent pathogens from colonizing mucous membranes. The primary role belongs to anaerobes (lacto- and bifidobacteria), which compete for nutrients 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 T-helper differentiation and suppresses IgE production (anti-allergic effect).
- Metabolic Role. Colonic microbes synthesize vitamins (K, B groups), break down cellulose, and participate in bile salt and cholesterol metabolism. Metabolic detoxification in the gut is comparable to the barrier function of the liver.
- Antimutagenic Activity. Bacteria can destroy carcinogens in the intestinal lumen, although during dysbiosis, certain bacterial enzymes may conversely convert safe substances into toxic ones.
Dynamics and Pathogenic Potential
The newborn's gastrointestinal 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 carries pathogenic potential. With decreased immunity or antibiotic use, the following may occur:
- Translocation: penetration of bacteria through mucous membranes into internal sterile environments.
- Breakdown of Tolerance: the immune system begins attacking its own bacteria, provoking bowel diseases (ulcerative colitis, Crohn's disease).
- Allergization: microbial decarboxylases promote excessive histamine release.
To correct disorders (restore CR), selective decontamination is used, along with preparations such as probiotics (live bacteria), prebiotics (flora growth stimulators), and synbiotics.