LECTURE 04

Bacterial Cell Surface Structures

Gram-Negative Cell Wall
  • Components: Lipopolysaccharide (LPS), Lipid A, Core O-antigen, Phospholipid, Braun's lipoprotein, Peptidoglycan, Outer membrane, Periplasm, Cytoplasmic membrane, Porin.
  • Lipid A:
    • Hydrophobic membrane anchor of LPS.
    • Recognized by innate immune system; responsible for causing endotoxic shock.
  • Core Oligosaccharide: Contains sugars like ketodeoxyoctonate (KDO), heptoses, glucose, galactose, and N-acetyl glucosamine.
  • O-Antigen:
    • Composed of repeating oligosaccharide units, highly variable in structure.
Importance of Lipid A
  • Acts as an endotoxin recognized by Toll Like Receptor-4 in the immune system.
  • Triggers an inflammatory response which may lead to endotoxic shock – causing multi-organ failure.
  • Estimated 200,000 deaths per year in the US due to endotoxins.
Archaeal Cell Envelope
  • Characteristics:
    • Single membrane composed of monolayers of tetra-ether lipids, not bilayers, making it impermeable to protons.
    • S-layer made of protein layers can resist extreme environmental conditions (e.g., in extremophiles like Sulfolobus solfataricus).
  • Pseudomurein:
    • Found in some Archaea (like methanogens) but lacks peptidoglycan.
    • Contains N-acetylglucosamine and N-acetyltalosaminuronic acid linked by β,1-3 glycosidic linkages (resistant to lysozyme).
Surface or S-Layers
  • Present in most Archaea and some bacteria, composed of a paracrystalline arrangement of glycoprotein that forms a protective and porous structure.
Flagella
  • Functionality:
    • Confers movement and allows bacteria to swim via rotational movement (CW and CCW) powered by an ion-driven rotary motor.
    • Flagella arrangements can vary: monotrichous, lophotrichous, amphitrichous, and peritrichous.
Flagella Structure
  • Components:
    • Basal body (the motor), filament (the propellor), hook (connecting basal body and filament).
    • Proton movement in the flagellar motor can involve up to 1000 protons per rotation for Gram-negative bacteria.
Chemotaxis
  • Bacteria can move towards or away from chemical gradients (e.g., nutrients or repellents).
  • In peritrichous flagella (e.g., E. coli):
    • Counterclockwise rotation results in runs, while clockwise rotation leads to tumbles.
    • Runs can be extended or shortened based on environmental signals, facilitating a biased random walk.
Fimbriae and Pili
  • Fimbriae:
    • Thin, filamentous structures that help bacteria adhere to surfaces via adhesins at their tips.
    • Commonly peritrichous, important for forming biofilms.
  • Pili:
    • Similar structure to fimbriae but typically less in number and longer. Important for genetic exchange (conjugation) and motility (type IV pili).
  • Clinical Significance: Fimbriae and pili mediate adherence in pathogens, crucial in infections (e.g., E. coli, Neisseria gonorrhoeae).
E. coli P-Type or Type 1 Pili
  • Structure:
    • Composed of structural subunits (Pap A) and adhesins (Pap G) that bind to host epithelial cells, significant in urinary tract infections.
Summary of Bacterial Surface Structures
  • Bacterial surface structures play crucial roles in pathogenicity, environmental resistance, and motility. The structural variability and functional diversity among Gram-negative bacteria and Archaea illustrate adaptations to diverse habitats and interactions with host organisms.