Bacterial Morphology and Cell Wall

Bacterial Morphology

• Coccus: Spherical shape of bacteria.
• Bacillus: Rod-shaped bacteria.
• Coccobacillus: Oval shape, intermediate between cocci and bacilli.
• Diplococcus: Bacteria occurring in pairs of cocci.
• Streptococcus: Bacteria arranged in chains of cocci.
• Staphylococcus: Clusters of cocci bacteria.
• Tetrad: Clusters of four cocci.

Glycocalyx and Functions

• Glycocalyx: Comprised of polysaccharides and glycoproteins, formed into a capsule or slime layer.
• Functions:
  • Poorly antigenic.
  • Inhibits phagocytosis.
  • Promotes bacterial attachment.
  • Acts as a virulence factor in some bacterial species.

Cell Wall Structure

• Cell Wall: Semi-rigid structure made of cross-linked peptidoglycan, crucial for maintaining bacterial shape and preventing lysis.
• Gram Staining:
  • Gram-positive: Retains violet stain; thick peptidoglycan layer (~25 layers).
  • Gram-negative: Retains pink counter stain; thin peptidoglycan layer (~2 layers).

Gram-Negative Bacteria Structure

• Periplasmic Space: Area between cytoplasmic membrane and outer membrane in Gram-negative bacteria, facilitates transport of proteins and enzymes.
• Outer Membrane:
  • Bilayer with inner layer like cytoplasmic membrane and outer layer containing phospholipids and lipopolysaccharides (LPS), which serve as a protective barrier.
• LPS Structure:
  • O Antigen: Antigenic component, composed of long carbohydrate units, highly variable among species.
  • Core Polysaccharide: Branched structure connecting O antigen to lipid A.
  • Lipid A: Embedded in outer membrane, acts as an endotoxin and is a virulence factor.

Toxins

• Exotoxins: Proteins secreted by living bacteria, contributing to virulence.
• Endotoxins: Structural elements of lipopolysaccharide, specifically lipid A.

Bacterial Virulence Factors

• Teichoic and Lipoteichoic Acids: Found in Gram-positive bacteria, these act as virulence factors.
• Flagella: Filamentous protein structures providing motility; source of H antigen.
• Pili: Hair-like structures facilitating adherence and genetic material exchange (conjugation), also a virulence factor.
• Endospores: Dormant, resistant forms produced by mainly Gram-positive bacteria in stress; examples include Bacillus anthracis and Clostridium tetani.
• Virulence Factor for Glycocalyx: Functions in antiphagocytic properties.

Antibiotics and Their Targets

• Fosfomycin: Inhibits MurA, crucial in N-acetylglucosamine (NAG) synthesis.
• D-Cycloserine: Interferes with addition of D-Alanine.
• Daptomycin: Disrupts membrane polarization.
• Oxacillin/Penicillins: Target transpeptidase for crosslinking in peptidoglycan.

Flagella Arrangement

• Monotrichous: Single flagellum at one pole.
• Lophotrichous: Multiple flagella at one pole.
• Amphitricous: Multiple flagella at both poles.
• Peritrichous: Flagella distributed all around the bacterial surface.

Protein Secretion Systems

• Type 1, 3, and 4 Secretion Systems (T1SS, T3SS, T4SS): Mechanisms to secrete proteins across membranes in Gram-negative bacteria.
• Type 3 Secretion System (T3SS): Injects virulence factors directly into host cells using a needle-like structure.