Lab Act 4

• Bacteria have almost the same refractive index as water, making them difficult to visualize under a microscope
• Staining


  1. One method for making microbial cells easier to visualize

  2. Simple stains use only one dye to stain the cell wall of bacteria

  3. Differential stains use two or more stains to categorize cells into groups
• Staining techniques

  • Allow the detection of cell morphology or shape
  • Differential stain provides additional information concerning the cell
• Components of the Gram stain

  • Crystal violet
  • Gram’s iodine
  • Decolorizer (acetone and alcohol)
  • Counterstain (safranin)
• Gram stain


  • Based on differences in the cell wall of bacteria
  • Used to categorize bacteria into Gram-positive and Gram-negative
• Peptidoglycan


  A carbohydrate found in the cell walls of some bacteria
• Lipopolysaccharides


  • Found in the cell walls of some bacteria along with peptidoglycan
  • Nonpolar and soluble in nonpolar organic solvents
• Malachite green staining procedure

  Differential staining used to distinguish between vegetative cells and endospores
• Types of bacteria based on Gram staining

  • Gram-positive
  • Gram-negative
  • Gram-variable
• Endospores


  • Highly resistant to harsh environmental conditions
  • Distinguished using the malachite green staining procedure
• Typical gram-positive bacteria

  • Staphylococci that produce boils
• Typical gram-negative bacteria

  • Bacilli that cause whooping cough
• Typical gram-variable bacteria

  • Bacilli that cause tuberculosis
• The lipopolysaccharide of the Gram-negative cell not only accounts for the staining reaction of the cell but also acts as an endotoxin. This endotoxin is released when the cell dies and is responsible for the fever and general feeling of malaise that accompanies a Gram-negative infection.