MM - LU6

Learning Unit 6

Normal Microbiota of Body

  • Healthy internal tissues (e.g., brain, blood, cerebrospinal fluid, muscles) are free of microorganisms.

  • Surface tissues (skin and mucous membranes) contact microorganisms and can be colonized.

  • Normal microbiota: mixture of microorganisms (mainly bacteria, fewer fungi and protozoa) found at anatomical sites.

Importance of Understanding Normal Human Microbiota

  • Specific microorganism locations inform potential infection risks.

  • Provides insight into sources and significance of microorganisms isolated from infections.

  • Understanding growth causes and consequences of microorganisms absent at specific body sites.

  • Recognizing the role of normal microbiota in stimulating host immune responses against pathogens.

Distribution of Normal Microbiota

  • Symbiotic relationships:

    • Commensalism: symbiont benefits; host unharmed.

    • Mutualism: both benefit.

    • Parasitism: symbiont benefits; host harmed.

  • Types of associations:

    • Ectosymbiosis: one organism outside another.

    • Endosymbiosis: one organism inside another.

Skin Microbiota

  • Skin epidermis is not conducive for microbial colonization due to:

    • Drying and moisture lack causing dormancy of resident microbiota.

    • Moist areas (scalp, ears, anal regions, palms) support microbiota.

    • Slightly acidic nature and hyperosmotic surface (due to sweat) discourage microbial colonization.

Factors Controlling Skin Microbiota

  • Bactericidal inhibitory substances help control colonization and infections.

  • Skin bacteria typically found on dead cells or near oil/sweat glands which provide nutrients (water, amino acids, urea, electrolytes, fatty acids).

  • Yeasts (e.g., Pityrosporum ovale) and fungi (causing athlete's foot and ringworm) can colonize skin.

  • Common bacteria include Propionibacterium acnes, linked to acne.

Respiratory Tract Microbiota

  • Upper and lower respiratory tracts (trachea, bronchi, bronchioles, alveoli) lack normal microbiota due to:

    • Continuous mucus stream by ciliated epithelial cells.

    • Phagocytic action of alveolar macrophages.

    • Bactericidal effects of lysozyme in nasal mucus.