Gram Positive and Gram Negative Bacteria

Gram Staining

  • Gram staining is a differential technique to distinguish bacteria based on color after applying dyes.
  • Gram-positive bacteria stain bluey-purple, while gram-negative stain fuchsia pink.
  • Christian Gram discovered that some bacteria retain an initial stain after alcohol wash (gram-positive), while others lose it and take up a counterstain (gram-negative).
  • Gram positive bacteria keep that initial stain, which is a purpley color.
  • Gram negative bacteria don't take up the gram stain, and take up a fuchsia pink color.

Gram Staining Process

  1. Apply crystal violet (purple dye) to bacteria.
  2. Add iodine (mordant) to fix the stain.
  3. Wash with alcohol; gram-negative bacteria lose the purple dye.
  4. Apply safranin (counterstain); gram-negative bacteria turn fuchsia pink.

Bacterial Cell Wall

  • Gram-positive bacteria have a thick peptidoglycan cell wall with teichoic acids.
  • Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide.

Penicillin Susceptibility

  • Gram + bacteria are more susceptible to penicillin.
  • Penicillin inhibits amino acid cross-linking in the peptidoglycan layer during bacterial growth.
  • The outer membrane in gram-negative bacteria prevents penicillin from reaching the peptidoglycan layer.

Cell Membrane

  • The bacterial cell membrane is a phospholipid bilayer similar to eukaryotic cells.
  • Functions include synthesis, respiration (ATP production), and secretion of extracellular enzymes or toxins.
  • It's flexible and ruptures easily if the cell wall is disrupted.

Staphylococcus Aureus Example

  • Shape: Coccus (spherical).
  • Arrangement: Staphylococcus (clusters like grapes).
  • Color: Aureus (yellow or gold).
  • Gram Stain: Gram-positive (dark bluish violet).
  • Cell Wall: No outer cell membrane with lipopolysaccharide.
  • Penicillin Susceptibility: More susceptible due to reliance on peptidoglycan.