Gram Positive and Gram Negative Bacteria

Gram Positive vs. Gram Negative Bacteria

Staining Technique

  • Gram and gram-negative bacteria are differentiated using a staining technique.
  • This involves applying colored dyes to bacteria for observation under a microscope.
  • Two main groups: gram-positive & gram-negative (also acid-fast).
  • Gram-positive: blue-purple color.
  • Gram-negative: fuchsia-pink color.
  • Differential staining technique distinguishes these groups.

Christian Gram's Discovery

  • Christian Gram observed that some bacteria could be decolorized with alcohol after staining.
  • Gram-positive bacteria retain the initial purple color after alcohol treatment.
  • Gram-negative bacteria lose the initial stain.

Gram Staining Process

  1. Apply purple dye (crystal violet) to bacteria.
  2. Add iodine (a mordant) to fix the stain.
  3. Wash with alcohol: gram-positive remain purple; gram-negative lose color.
  4. Apply counterstain (safranin): gram-negative turn fuchsia pink.
  • Primary stain: crystal violet (violet color).
  • Gram staining is based on bacterial cell wall structure.

Bacterial Cell Wall

  • Gram-positive: thick peptidoglycan layer with teichoic acids.
  • Gram-negative: thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS).

Penicillin Sensitivity

  • Gram-positive bacteria are more sensitive to penicillin.
  • Penicillin inhibits peptidoglycan cross-linking during bacterial growth.
  • Gram-negative bacteria's outer membrane prevents penicillin from reaching the peptidoglycan layer.

Cell Membrane

  • Located below the cell wall; a phospholipid bilayer (similar to eukaryotic cells).
  • Functions: synthesis of cell components, respiration, ATP synthesis, secretion of extracellular enzymes or toxins.
  • Water can easily pass through the membrane.
  • The membrane is flexible and ruptures easily when the cell wall is compromised.

Staphylococcus Example

  • Staphylococcus aureus: a gram-positive bacteria.
  • Coccus: spherical shape. (Cocci is plural )
  • Staphylo: bacteria cluster together (like grapes).
  • Aureus: yellow or gold color.
  • Gram-positive: retains the violet color.
  • Cell wall: lacks an outer membrane with lipopolysaccharide.
  • Susceptibility to penicillin: more susceptible due to reliance on peptidoglycan.