Gram reaction

• Gram stain


  Procedure that reveals the morphology, arrangement, and cell wall information of bacteria
• Gram stain


  • Classifies bacteria as gram-positive or gram-negative
  • Gram-positive bacteria retain the primary stain crystal violet and appear purple
  • Gram-negative bacteria lose the primary stain and retain the secondary stain of safranin and appear pink
• Gram staining procedure


  1. Prepare smear of unknown bacterial cultures

  2. Stain smears with crystal violet, gram's iodine, 95% ethanol, and safranin

  3. Examine slides under HPO
• Gram-positive cocci in chains

  • Streptococcus pyogenes
• Gram-negative pleomorphic


  • Haemophilus influenzae
• Chemical differences between cell walls of gram-positive and gram-negative bacteria

  Explain differences in rate of decolorization
• Age of a culture


  Can affect gram stain reaction
• Gram-positive bacteria have a thick cell wall that retains the crystal violet dye during decolorization
• Gram-negative bacteria have a thin layer and an outer membrane that lose the dye more easily during decolorization