Overview
This lecture explains the structure and functions of bacterial cell walls, focusing on the differences between gram-positive and gram-negative types and their implications for infection and treatment.
Cell Wall Basics
- Bacterial cell walls provide structural support, preventing the cell from bursting.
- Most bacterial antibiotics target and weaken the cell wall, causing cells to lyse.
- All bacteria (except those with atypical walls) have either gram-positive or gram-negative cell walls.
Peptidoglycan (Murine) Structure
- Peptidoglycan consists of carbohydrate (glycan) chains cross-linked by protein (peptide) bonds.
- Carbohydrate chains act as a backbone; protein cross-links provide strength.
- Peptidoglycan is also known as murine.
Gram-Positive Cell Walls
- Gram-positive bacteria have thick, multilayered peptidoglycan cell walls.
- These walls give strong mechanical protection.
- Gram-positive bacteria lack an outer membrane.
Gram-Negative Cell Walls
- Gram-negative bacteria have thin peptidoglycan walls (1–4 layers).
- A periplasmic space exists between the plasma membrane and cell wall, containing enzymes and transport proteins.
- An outer lipid membrane (LPS or lipopolysaccharide membrane) surrounds the cell wall.
- The LPS membrane has a strong negative charge, helping bacteria evade phagocytosis and complement system.
- The outer membrane blocks entry of detergents, heavy metals, bile salts, some antibiotics, and lysozyme.
- Lipid A, a toxic component of LPS, is released when the bacteria die, causing fever, clotting, vessel dilation, and shock.
Comparison & Clinical Implications
- Gram-positive: strong against physical/mechanical attacks but less resistant to chemical agents.
- Gram-negative: less durable mechanically, but highly resistant to immune system and antibiotics due to the outer membrane and periplasmic space.
- Gram-negative bacteria pose a greater clinical challenge and are the focus of most drug-resistance research.
Key Terms & Definitions
- Peptidoglycan (Murine) — a mesh-like polymer of sugars and amino acids forming the bacterial cell wall.
- Gram-Positive — bacteria with thick peptidoglycan walls and no outer membrane.
- Gram-Negative — bacteria with thin peptidoglycan walls, periplasmic space, and an outer LPS membrane.
- LPS (Lipopolysaccharide) — a molecule forming the outer membrane of gram-negative bacteria.
- Lipid A — the toxic component of LPS, also called endotoxin.
- Periplasmic Space — the region between the plasma membrane and the peptidoglycan wall in gram-negative bacteria.
- Phagocytosis — process by which immune cells ingest and destroy microbes.
- Complement System — part of the immune system that helps clear microbes.
Action Items / Next Steps
- Watch the follow-up video on atypical bacterial cell walls.