Overview
This lecture covers the structure, components, and functions of bacterial cell walls, the differences between gram-positive and gram-negative bacteria, and the Gram staining procedure.
Bacterial Cell Wall Structure and Function
- The bacterial cell wall is located outside the cell membrane and provides structural strength.
- It maintains cell shape and protects against osmotic lysis.
- Its strength comes from peptidoglycan, a mesh-like polymer of sugars and amino acids.
Gram-Positive vs. Gram-Negative Cell Walls
- Gram-positive bacteria have thick, multilayered peptidoglycan cell walls.
- Teichoic acids are embedded in gram-positive cell walls, enhancing rigidity and shape maintenance.
- Gram-negative bacteria have a thin peptidoglycan layer and an additional outer membrane.
- The outer membrane of gram-negative bacteria contains lipopolysaccharide (LPS).
Lipopolysaccharide (LPS) Structure and Function
- LPS has three parts: O antigen (outermost), core polysaccharide, and lipid A (anchors LPS).
- LPS stabilizes the outer membrane and blocks certain chemicals.
- O antigen triggers host immune responses; lipid A acts as an endotoxin causing illness symptoms.
Gram Staining Procedure and Results
- Crystal violet dye stains both gram-positive and gram-negative cells purple.
- Iodine forms an insoluble complex with crystal violet.
- Alcohol dehydrates peptidoglycan, trapping dye in gram-positive but not gram-negative cells.
- Gram-positive cells retain purple color; gram-negative cells lose color after alcohol wash.
- Safranin counterstain colors gram-negative cells pink/red while gram-positive cells remain purple.
Key Terms & Definitions
- Peptidoglycan — A polymer of sugars and amino acids that forms the bacterial cell wall mesh.
- Teichoic Acid — Molecule in gram-positive cell walls that adds rigidity and shape stability.
- Lipopolysaccharide (LPS) — Large molecule in gram-negative outer membranes, with O antigen, core polysaccharide, and lipid A components.
- Endotoxin — Toxin (e.g., lipid A) inside bacterial cells, released when cells disintegrate.
- Gram Stain — Laboratory method to distinguish between gram-positive and gram-negative bacteria by cell wall structure.
Action Items / Next Steps
- Review diagrams of gram-positive and gram-negative cell walls.
- Practice explaining the steps and outcomes of the Gram stain procedure.