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.
  • 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.