Microbiology · MU Sofia

103. Human Microbiota – Physiological Significance and Etiological Role

Explains what the human microbiota is, where its main communities sit on the body, and which microbes dominate each site, presented in a table. Covers the protective, metabolic and immune-maturing roles of these microbes, then how disruption of the flora, or dysbiosis, underlies opportunistic infections, chronic and autoimmune conditions and antibiotic-related disease. Ends with probiotics, prebiotics, faecal transplantation and a summary table of clinical implications.

Inside: 7 sections · 5 labelled pictures · 3 tables · 16 flashcards · 3 min

Topic 103 of 113 in the MU Sofia syllabus for Microbiology (Medicine, Year 3). The free AceMed note covers it in 7 sections, about 267 words (3 min of reading), with 16 flashcards.

In this note

  1. Human Microbiota – Physiological Significance and Etiological Role
  2. I. Definition
  3. II. Distribution of Human Microbiota
  4. III. Physiological Significance of Human Microbiota
  5. IV. Etiological Role in Disease (Dysbiosis)
  6. V. Microbiota Restoration and Therapeutic Use
  7. VI Clinical Relevance Summary

I. Definition

  • The human microbiota refers to the collection of commensal, symbiotic, and pathogenic microorganisms living on and inside the human body.
  • Includes bacteria, viruses, fungi, protozoa.
  • The microbiome refers to their collective genomes.

II. Distribution of Human Microbiota

Note: Fetus is sterile at birth; microbiota is acquired during and after delivery.

III. Physiological Significance of Human Microbiota

A. Protective Role

  • Colonization resistance:
  • Prevents pathogen overgrowth by:
  • Competing for nutrients and space
  • Producing bacteriocins
  • Altering pH (e.g., lactic acid from Lactobacillus)
  • Produces biofilms that stabilize mucosal barriers.

B. Metabolic Functions

  • Fermentation of undigested carbohydrates → short-chain fatty acids (SCFAs)
  • Synthesis of vitamins: especially vitamin K and B group (e.g., B12, folate, biotin)
  • Involved in bile acid metabolism, urea degradation

C. Immune System Maturation

  • Stimulates development of GALT (gut-associated lymphoid tissue)
  • Promotes tolerance to commensals and activation against pathogens
  • Triggers IgA production at mucosal surfaces

IV. Etiological Role in Disease (Dysbiosis)

Dysbiosis = Disruption or imbalance in the normal microbiota.

A. Opportunistic Infections

  • Commensals may cause disease when they translocate or grow excessively.
  • Examples:
  • E. coli → UTI when enters urinary tract
  • Candida albicans → thrush in immunosuppressed patients
  • Staph. epidermidis → catheter-related infections

B. Microbiota and Chronic Disease

  • Gut dysbiosis implicated in:
  • IBD (Crohn’s, Ulcerative colitis)
  • Obesity, Type 2 Diabetes
  • Colon cancer
  • Neuropsychiatric disorders (via gut-brain axis)

C. Antibiotic-Associated Complications

  • Broad-spectrum antibiotics destroy normal flora → overgrowth of:
  • Clostridioides difficile → pseudomembranous colitis
  • Yeast → candidiasis

D. Autoimmunity and Allergies

  • Lack of microbial exposure in early life → "hygiene hypothesis"
  • Linked to asthma, eczema, autoimmune diseases (e.g., T1DM, MS)

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