Differences Between Gram Positive and Gram Negative Bacteria

Bacterial Cell Structure

• Plasma Membrane (Green)
  • Innermost layer
  • Phospholipid bilayer with proteins
  • Regulates entry and exit of substances
• Cell Wall (Gold)
  • Middle layer
  • Made of peptidoglycan
  • Provides structural support
  • Critical for distinguishing gram positive and gram negative bacteria
• Capsule (Red)
  • Outer layer
  • Sticky, used for attachment and protection

Gram Positive vs. Gram Negative Bacteria

Gram Positive Bacteria

• Plasma Membrane
  • Present
• Cell Wall
  • Thick layer of peptidoglycan (approximately 5 layers)
• Capsule
  • Present
• Treatment
  • More easily treated with antibiotics
  • Lack an additional outer phospholipid bilayer
• Gram Stain Result
  • Appears violet/purple due to retention of crystal violet dye

Gram Negative Bacteria

• Plasma Membrane
  • Present
• Cell Wall
  • Thinner layer of peptidoglycan (approximately 2 layers)
• Additional Outer Membrane
  • Additional phospholipid bilayer
• Capsule
  • Present
• Treatment
  • More resistant to antibiotics
  • Harder for antibiotics to penetrate the additional membrane
• Gram Stain Result
  • Appears reddish/pink due to loss of crystal violet and retention of saffrin

The Gram Stain Process

Purpose

• Identifies infectious bacteria
• Determines appropriate treatment

Steps

1. Obtain Bacteria
   • Use an inoculation loop to transfer bacteria to a glass slide
2. Fix Bacteria to Slide
   • Pass slide over flame to fix bacteria
3. Staining
   • Add Crystal Violet (purple dye)
   • Add Iodine (bonds with crystal violet)
4. Alcohol Wash
   • Shrinks cell walls
   • Dissolves capsules
   • Gram positive retains color, gram negative loses color
5. Apply Saffrin
   • Red/pink dye
   • Binds with lipids in the cell wall

Results

• Gram Positive
  • Retains purple color from crystal violet
• Gram Negative
  • Appears pink due to saffrin

Conclusion

• Identifying whether bacteria are gram positive or negative is crucial for treatment
• Gram positives are more susceptible to antibiotics due to a single phospholipid bilayer
• Gram negatives are more resistant due to an additional outer membrane

Note: Refer to actual photographs to see differences in color between gram positive (purple) and gram negative (pink) bacteria.