Learning Objective

Understand the structural differences between Gram-positive and Gram-negative bacteria, the composition and synthesis of peptidoglycan, and how antibiotics target bacterial cell wall formation for clinical effect.

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Bacterial Cell Structure (High-Yield for Medical Students)

Bacteria are surrounded by a cell envelope that protects them from osmotic lysis and maintains structural integrity. This envelope includes:

• Cytoplasmic membrane
• Peptidoglycan cell wall
• (In Gram-negative bacteria) an outer membrane

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Cytoplasmic Membrane

The cytoplasmic membrane contains enzymes essential for building the bacterial cell wall:

• Transpeptidases
• Carboxypeptidases

These enzymes are collectively called Penicillin-Binding Proteins (PBPs) because they are the primary targets of β-lactam antibiotics (e.g., penicillins, cephalosporins).

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Peptidoglycan Cell Wall

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Peptidoglycan provides rigidity and prevents osmotic rupture of bacterial cells.

Gram-Positive Bacteria

• Thick peptidoglycan layer
• No outer membrane
• Contains:
  • Teichoic acids
  • Lipoteichoic acids → aid in bacterial adhesion
• May contain virulence factors such as:
  • M protein (Streptococcus) → inhibits phagocytosis

Gram-Negative Bacteria

• Thin peptidoglycan layer
• Surrounded by an outer membrane
• The outer membrane contains:
  • Lipopolysaccharide (LPS) → endotoxin

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Activity

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Lipopolysaccharide (LPS) — Gram-Negative Only

LPS consists of:

Clinical Importance:

• Lipid A binds TLR-4 receptors on macrophages
• Triggers cytokine release (e.g., TNF-α, IL-1)
• Can lead to:
  • Fever
  • Hypotension
  • Septic shock

LPS may also activate B cells directly without T-cell assistance.

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Peptidoglycan Composition

Peptidoglycan is made of repeating sugar chains:

• N-acetylglucosamine (NAG)
• N-acetylmuramic acid (NAM)

These chains are cross-linked by short peptide bridges:

• Synthesized initially as pentapeptides
• Terminal D-alanine is removed during cross-linking → forms tetrapeptide
• Requires an amino acid with an amine group for cross-linking:
  • L-lysine
  • Diaminopimelic acid

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Activity

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Steps in Peptidoglycan Synthesis

1. NAG and NAM precursors are synthesized in the cytoplasm.
2. Pentapeptide side chains are added to NAM.
3. Units are transported across the membrane by bactoprenol.
4. Transglycosylases link NAG-NAM units to the growing cell wall.
5. Transpeptidases (PBPs) cross-link peptide chains for strength.

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Antibiotic Targets (Step 1 Favorite)

Peptidoglycan synthesis occurs only in bacteria, making it an excellent drug target.