Learning Objective

Differentiate Gram-positive vs Gram-negative bacterial envelopes by recognizing:

• Gram+ → thick peptidoglycan, lipoteichoic acids, no outer membrane.
• Gram– → thin peptidoglycan, outer membrane with LPS, wide periplasm with β-lactamases.

Common to Both (Gram+ and Gram–)

• Cytoplasmic membrane
• Peptidoglycan cell wall (thickness varies)
• Capsule (when present → anti-phagocytic)
• Pili/Fimbriae (adherence, conjugation)
• Flagella (motility, not universal)

Gram-Positive (Gram +)

• Thick peptidoglycan layer → provides strong structural support
• Lipoteichoic acids (unique to Gram +) → induce TNF-α, IL-1
• No outer membrane
• No endotoxin (LPS)
• Narrow periplasmic space

Gram-Negative (Gram –)

• Thin peptidoglycan layer (sandwiched between membranes)
• Outer membrane (unique to Gram –)
  • Outer leaflet: LPS (endotoxin) → lipid A triggers TNF, IL-1
  • Porins: transport across the membrane
  • Outer membrane proteins (OMPs): antigenic
• Wide periplasmic space (contains β-lactamases)
• More resistant to antibiotics due to an extra membrane barrier