In-Depth Notes on Pathogenic Gram Positive Bacteria and Mycobacterium

Overview of Pathogenic Gram Positive Bacteria

  • Focus on Gram Positive Rods, specifically Acid-Fast Bacteria (AFB) like Mycobacterium.

Acid-Fast Bacteria Characteristics

  • Defining Acid-Fastness:
    • Special staining technique used for bacteria with a thick, waxy material in their cell walls, known as mycolic acid.
    • The term relates to the ability of these bacteria to resist decolorization by acid alcohol, which is a stronger solution than that used in the Gram staining process.
    • Acid-fast bacteria retain the primary stain after being washed with acid, differentiating them from other bacteria.
  • Key Genera:
    • Includes Mycobacteria, Nocardia, and Actinomycetes species.

Structure of Acid-Fast Bacterial Cell Walls

  • Components of Cell Wall:
    • Mycolic Acid:
    • Waxy lipids (with 60-90 carbon atoms) that form a thick layer on top of the peptidoglycan layer, enhancing resistance to drying and antibiotics.
    • Peptidoglycan Layer:
    • Underlies the mycolic acid and contributes to structural integrity.
    • Arabinogalactan:
    • A polysaccharide that links mycolic acids and peptidoglycan, providing additional strength.
  • Cell Wall Structure:
    • Mycobacterial cell envelope consists of a 7 nm thick cytoplasmic membrane (CM) and a 10 nm thick outer membrane (OM).

Characteristics of Mycobacterium Species

  • True Acid-Fast Bacteria:
    • Key pathogens include Mycobacterium tuberculosis (causes tuberculosis), Mycobacterium leprae (causes leprosy), and Mycobacterium avium-intracellulare.
  • Unique Properties:
    • Mycolic acids make them water-resistant and waxy.
    • Slows down growth due to limited nutrient uptake; however, it also protects against lysis after phagocytosis.
    • Colonies can take weeks to become visible in the laboratory and do not retain the Gram stain because the stain cannot penetrate their cell walls.

Specific Mycobacterial Diseases

Mycobacterium tuberculosis
  • A significant human pathogen known for causing tuberculosis (TB):
    • Historical significance: Found in Egyptian and Native American mummies dated between 140 and 1000 AD.
    • When the immune system is compromised, TB can spread, causing granulomas in the lungs.
    • Factors contributing to the disease include malnutrition, overcrowding, stress, and other illnesses.
    • Known historically as the "White Plague" due to its severe impacts on health.
Insidious Nature of Tuberculosis
  • Many individuals remain asymptomatic for years; 90-95% of those infected do not develop active TB.
  • Formerly called 'consumption' because the disease leads to significant weight loss and deterioration.
  • Treatment historically involved rest and fresh air, which supported the immune system's ability to combat the infection.