Etiology and classification of disorders
Disruptions in the body's microecology rarely occur spontaneously. The main risk factors include iatrogenic interventions (uncontrolled antibiotic therapy, consequences of chemo- and radiation therapy, extensive surgical procedures), irrational nutrition (dietary factor), as well as environmental influences and severe stress.
The classification of dysbiotic conditions is based on several principles:
- By etiology: proteus, staphylococcal, fungal, and other types of disorders are distinguished depending on the dominant causative agent.
- By localization: lesions can affect the oral cavity, vagina, or intestines.
Intestinal lesions have their own specific features. Isolated colonic dysbiosis is distinguished, along with a specific pathology of the small intestine known as SIBO (small intestinal bacterial overgrowth syndrome, or Bacterial overgrowth syndrome).
Such flora disorders are associated with a huge spectrum of diseases. Gastrointestinal manifestations include diarrhea or constipation, gastritis, colitis, peptic ulcer disease, and malabsorption syndrome. Systemic disorders include blood pressure fluctuations (hypo- and hypertension) and cholesterol imbalances, the development of urolithiasis, and allergies. In addition, flora imbalance increases the risk of dental caries, liver damage, arthritis, and even malignancies.
Methods for diagnosing microbiocenosis
To accurately assess the state of the flora, a comprehensive approach is used. The clinician's and microbiologist's arsenal includes three basic methods:
- Bacteriological (classic culture of the material).
- Molecular genetic (PCR diagnostics).
- Instrumental and analytical (chromatography-mass spectrometry).
A special place is occupied by the study of metabolites. Using gas-liquid chromatography (GLC), specialists determine the level of short-chain fatty acids, which serve as markers of anaerobic bacterial activity.
Biochemical diagnostics is based on detecting specific metabolic products (aldehydes, enzymes, hydroxy fatty acids) that indicate an imbalance. A classic example is the detection of dipeptides ($\beta$-aspartyl-glycine and $\beta$-aspartyl-lysine) in a patient's stool. Normally, these substances are completely metabolized by healthy anaerobic flora; therefore, their presence in the sample reliably indicates a disruption of the intestinal microbiocenosis.
Bacteriological method and selective media
Classical microbiological diagnostics aims to determine the quantitative and species composition of the biotope (throat, skin, intestines). For this, the imprint method or seeding of dilutions of the test material onto nutrient media is used.
It is extremely important for a student to memorize the correspondence of specific groups of microorganisms and selective media for their cultivation:
- Bifidobacteria: Blaurock's medium.
- Lactobacilli: MRS-2 medium.
- Bacteroides: anaerobic blood agar.
- Enterobacteria: Levin's or Endo media.
- Enterococci: bile-blood agar.
- Streptococci and hemophils: blood agar.
- Pseudomonas aeruginosa: meat peptone agar (MPA) supplemented with furagin.
- Fungi: Sabouraud medium.
Principles of treatment and drug groups
The basis of dysbiosis correction is selective decontamination of pathogens and oral administration of specialized drugs to restore normoflora.
Several key drug groups are distinguished:
- Prebiotics. These are microbial components or non-microbial substances that stimulate the growth of beneficial resident flora. Chemically, they are low-molecular-weight carbohydrates (oligosaccharides, fructooligosaccharides). They are naturally found in food products and breast milk.
- Probiotics (eubiotics). Medications based on live bacteria. They are divided into two subgroups. The first contains representatives of the normal flora (bifidobacteria, lactobacilli, E. coli — for example, bifidumbacterin or colibacterin). The second group consists of self-eliminating antagonists. They do not normally inhabit the body, but they effectively suppress pathogens and are then completely eliminated (examples: Bacillus subtilis bacteria or Saccharomyces boulardii yeasts).
- Synbiotics. Combined agents that simultaneously include live bacteria (probiotic) and a nutrient medium for them (prebiotic).
- Enterosorbents. Used to bind and eliminate toxic metabolites and opportunistic microorganisms from the intestinal lumen (for example, Enterosgel or activated charcoal).
Briefly: answers to topical questions
Which microorganisms constitute the obligate and transient microflora of the large intestine?
The obligate microflora of the large intestine is represented predominantly by anaerobes. Key groups include bacteroides, bifidobacteria, clostridia, prevotella, ruminococci, and porphyromonas. Sources also indicate the participation of lactobacilli and Escherichia coli in the normal microflora and colonization resistance. The specific composition of the transient microflora and the distribution of the listed microorganisms into obligate and transient flora are not provided in the sources.
What specific protective and metabolic functions does the normal microflora perform in the human body?
The normal microflora performs a complex of protective and metabolic functions, ensuring colonization resistance and regulation of metabolism.
- Protective functions — suppression of pathogenic flora, stimulation of nonspecific immunity, intermicrobial antagonism (synthesis of organic acids, hydrogen peroxide, bacteriocins), and competition for nutrients.
- Metabolic functions — regulation of water-salt metabolism, intestinal gas composition, peristalsis, as well as protein, carbohydrate, and fatty acid metabolism.
- Synthetic functions — production of vitamins (B6, K, E, nicotinic and folic acids), amino acids, and enzymes.
- Detoxification functions — hydrolysis of metabolic products, transformation of drugs, and neutralization of carcinogens.
How to memorize media: Bifidobacteria — Blaurock (both start with B), Enterobacteria — Endo and Levin (EL = Enterobacteria Love).
Class and exam questions
What is the fundamental difference between dysbacteriosis and dysbiosis?
Dysbacteriosis implies qualitative and quantitative shifts exclusively in the composition of the bacterial flora. Dysbiosis is a broader concept that describes changes not only in bacteria, but also in other groups of microorganisms, such as viruses and fungi.
What is SIBO?
SIBO stands for small intestinal bacterial overgrowth syndrome (Bacterial overgrowth syndrome). It is a specific variant of dysbiosis localized in the small intestine.
What does the appearance of β-aspartyl-glycine in feces indicate?
This is a metabolic marker. In a healthy intestine, this dipeptide is cleaved by anaerobic flora. Its detection indicates a decrease in anaerobic activity and a disruption of the microbiocenosis.
How do second-group probiotics (antagonists) work?
Unlike the first group, they contain microorganisms that do not normally live in our bodies (e.g., Bacillus subtilis). They temporarily colonize the GI tract, suppress the growth of pathogens, and are then independently eliminated from the body.
What else to study on this topic
- Pathogenetic mechanisms of malabsorption syndrome formation in dysbiosis.
- The role of short-chain fatty acids in regulating intestinal functions.
- Features of selective decontamination in pronounced dysbiosis.
- Principles of interpreting chromatography-mass spectrometry results of the microbiota.
Study the full topic in the «Microbiology» course
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