Key Points on Bacterial Structure and Antibiotic Resistance

Structural Overview of Bacteria

• Basic Structure:
  • Cell membrane surrounded by a cell wall.
  • Some bacteria have an additional outer layer.
  • Internal components include:
    • Cytoplasm
    • Ribosomes
    • Nuclear region
    • Granules/vesicles (in some species)
• External Structures:
  • Capsule
  • Flagella
  • Pili
• Gram-Negative Bacteria:
  • Periplasmic space between the cell membrane and the cell wall.
• Gram-Positive Bacteria:
  • Lacks periplasmic space; has periplasm for metabolic digestion.

Cell Wall Composition

• Peptidoglycan:
  • Key component of the bacterial cell wall.
  • Made of N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) cross-linked by amino acids.
• Function:
  • Maintains cell shape.
  • Prevents bursting from osmotic pressure.

Peptidoglycan Synthesis

1. Addition of five amino acids to N-acetylmuramic acid.
2. N-acetylglucosamine added to form a peptidoglycan precursor.
3. Precursor transported to cell wall acceptor in periplasm.
4. Cross-linking by enzymes:
   • Transpeptidase
   • D-alanilcarboxypeptidase

• Penicillin-Binding Proteins:
  • Enzymes involved in peptidoglycan cross-linking.
  • Targeted by beta-lactam antibiotics.

Antibiotic Mechanism

• Beta-Lactam Antibiotics:
  • Includes penicillins and cephalosporins.
  • Bind to transpeptidase and D-alanilcarboxypeptidase, blocking cross-linking.
• Effect on Bacteria:
  • Damages bacterial cells by preventing wall synthesis.
  • High internal osmotic pressure in gram-positive bacteria leads to cell lysis in low osmotic environments.
  • Autolysins are released to digest existing cell walls.

Bacterial Resistance Mechanisms

Transformation

• Transfer of chromosomal genes between bacteria.
• Naked DNA from dead bacteria can be taken up by nearby bacteria.
• Resistance genes integrated into host chromosome via homologous transformation.
• Result: Altered penicillin-binding proteins with reduced affinity for beta-lactams.

Conjugation

• Plasmids carrying resistance genes can be passed between bacteria.
• Channel formed during close contact allows plasmid transfer.
• Beta-Lactamases:
  • Enzymes that inactivate beta-lactam antibiotics.
  • Inducible in gram-positive bacteria; produced constitutively in gram-negative bacteria.
• Mechanism of Action:
  • Gram-positive: Enzymes released into the environment.
  • Gram-negative: Enzymes retained in periplasmic space, more effective.
• Outcome:
  • Destruction of the beta-lactam ring, leading to antibiotic resistance.