Identifying and Classifying Bacteria

Shapes of Bacteria

• Spherical (Coccus/Cocci):
  • Diplococci
  • Staphylococci
• Rod-shaped (Bacillus/Bacilli):
  • Single bacillus
  • Diplobacilli
  • Streptobacilli
• Spiral (Spirilla):
  • Vibrio
  • Spirillum
  • Spirochete

Identifying Bacteria by Color Change

• Gram Stain: Differentiates bacteria based on cell wall structure.
  • Gram-positive: Violet/Blue (due to peptidoglycan layer)
  • Gram-negative: Red/Pink (due to outer membrane)
  • Process:
    1. Crystal violet
    2. Iodine (mordant)
    3. Alcohol wash (decolorization)
    4. Safranin (counterstain)
• Acid-Fast Stain (Ziehl-Neelsen Stain): Identifies bacteria with a wax-like, impermeable cell wall containing mycolic acid.
  • Acid-fast bacteria retain the primary stain (Carbolfuchsin) and appear red.
  • Non-acid-fast bacteria lose the primary stain and take up the counterstain (Methylene Blue).
  • Process:
    1. Carbolfuchsin (primary stain)
    2. Heat (mordant)
    3. Acid Alcohol (decolorizer)
    4. Methylene Blue (counterstain)
  • Examples:
    • Mycobacterium tuberculosis
    • Mycobacterium leprae

Identifying Bacteria by Oxygen Requirements

• Aerobic Bacteria (Obligate Aerobes): Require oxygen to survive.
• Anaerobic Bacteria (Obligate Anaerobes): Require little or no oxygen to grow and reproduce.
• Facultative Bacteria: Can survive with or without oxygen.
• Oxygen Test: Bacteria are grown in a Thioglycolate broth to determine oxygen requirements.