Harvey's Garden

Bacterial structures

2 min read

Cell Wall & Membranes

  • Gram-Positive (+)
    • Structure: Thick peptidoglycan layer, single cell membrane. Contains teichoic acid and lipoteichoic acid (LTA). LTA induces TNF-α and IL-1.
    • Analogy: A thick brick wall (peptidoglycan) with reinforcing bars (teichoic acids).
    • Gram Stain: Retains crystal violet, appears purple/blue.
    • Key Features: More susceptible to penicillin.
  • Gram-Negative (-)
    • Structure: Thin peptidoglycan layer in the periplasmic space between inner and outer membranes.
    • Analogy: A thin brick wall surrounded by a chain-link security fence (outer membrane).
    • Outer Membrane: Contains endotoxins.
      • Lipopolysaccharide (LPS): The major endotoxin.
        • Analogy: The alarm system on the security fence.
        • Lipid A: The siren of the alarm system; triggers a massive immune response (shock, fever, DIC).
        • O-Antigen: The specific model number of the alarm system, used for identification (e.g., E. coli O157).
      • Lipooligosaccharide (LOS): Found in some species (e.g., Neisseria). Has endotoxic Lipid A but lacks the O-antigen.
        • Analogy: A generic, but equally loud, motion-detecting siren that lacks a specific model number. Its structural variability helps evade the immune system.
  • Atypical Cell Walls
    • Mycobacteria: Contain mycolic acid. Do not Gram stain; require acid-fast stain.
      • Analogy: A waxy, waterproofed wall that repels the purple/pink “paint” of Gram stain.
    • Mycoplasma: No cell wall. Cell membrane contains sterols.
      • Analogy: A tent with flexible poles (sterols) instead of rigid walls.

External Structures

  • Capsule
    • Patho/Etiology: A well-organized polysaccharide layer.
    • Function: Antiphagocytic virulence factor.
    • Analogy: A slippery, invisibility cloak that helps the bacterium evade capture by immune cells.
  • Glycocalyx (Slime Layer)
    • Patho/Etiology: A loose, disorganized polysaccharide layer.
    • Function: Mediates adherence and forms biofilms.
    • High-Yield Examples:
      • S. epidermidis: Adheres to IV catheters and prosthetic devices.
      • P. aeruginosa: Biofilm formation in the lungs of cystic fibrosis patients and on contact lenses.
      • Non-typeable H. influenzae: Biofilms in otitis media.
    • Analogy: Sticky glue that helps bacteria clump together and stick to surfaces (like catheters or teeth).
  • Pili (Fimbriae)
    • Patho/Etiology: Hair-like proteinaceous appendages.
    • Function: Adhesion to host cells. Sex pilus is used for conjugation.
    • Analogy: Velcro hooks for grabbing onto host cells. The sex pilus is a “tunnel” for passing genetic information.
  • Flagella
    • Patho/Etiology: Long, helical filament for motility.
    • Function: Provides movement (chemotaxis). H-antigen.
    • Analogy: A motorboat propeller that drives the bacterium through its environment.

Intracellular Structures

  • Spores (Endospores)
    • Patho/Etiology: Dormant, highly resilient structures formed by some gram-positive rods (Bacillus, Clostridium).
    • Key Components: Core contains dipicolinic acid.
    • Function: Survival in harsh conditions.
    • Analogy: A survival bunker or panic room that protects the bacterium’s DNA during a disaster, allowing it to reactivate when conditions improve.

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