Microbiology Media Types & Testing Methods

Microbiology Media Types

Selective Media

  • Definition: Selective media contain substances that inhibit the growth of some organisms while allowing others to grow abundantly.
    • Example: MacConkey Agar
    • Purpose: Selective for gram-negative organisms (inhibits gram-positive growth).
    • Differential for Lactose Fermentation: Contains lactose, allowing identification of lactose fermenters (coliforms).
      • Color Results:
      • Fermentation of lactose by coliforms leads to colonies with dark purple centers.
      • E. coli produces an emerald green sheen due to acid produced in lactose fermentation.
      • Non-fermenters appear colorless or have pink colonies.

Differential Media

  • Definition: Differential media allow certain organisms to change color differently than others growing on the same medium.
  • Purpose: Helps distinguish between different organisms based on their metabolic characteristics.

Complex Media

  • Definition: Complex media incorporate special nutrients and growth factors that may be required by certain organisms.
  • Example: Media containing yeast extract or pancreatic digest of casein enhances growth for fastidious organisms.
  • Utility: Helps in identifying bacteria by selectively promoting growth of some while inhibiting others.

Fermentation Tubes

  • Contents: These tubes contain carbohydrates that may include various sugars like Lactose, Dextrose, and Maltose, in a 1% concentration.
  • Phenol Red Indicator:
    • When bacteria ferment sugar, it produces acid leading to a color change in phenol red:
    • pH Change: When acid is produced, the pH drops, causing phenol red to change from red to yellow.
    • Gas Production: Gas (e.g., carbon dioxide) can be detected in a Durham tube, which is inverted inside the main broth tube before autoclaving.

SIM Medium (Sulfide-Indole-Motility)

  • Contents: This is a semisolid medium designed to test for hydrogen sulfide production, indole production, and motility.
    • Detection of Hydrogen Sulfide: Contains iron salts that turn black in the presence of hydrogen sulfide.
    • Bacteria that turn SIM black are classified as sulfide positive.
    • Indole Production: Bacteria that hydrolyze tryptophan produce an intermediate called indole, detectable by Kovac's reagent (adding drops of Kovac's reagent can yield a red color if indole is present).
    • Motility Test:
    • Motile bacteria will spread away from the stab line, producing turbid growth.
    • Non-motile bacteria will only grow along the stab line.

Thioglycollate Medium

  • Definition: An enriched complex medium typically containing yeast extract and pancreatic digest of casein, suitable for cultivating organisms that cannot grow in atmospheric levels of oxygen.
  • Preparation: Autoclaved to sterilize and reduce free oxygen gas.
  • Oxygen Gradient:
    • After autoclaving:
      • Top layer has approximately 10-15% O₂.
      • Middle layer has about 2-10% O₂.
      • Bottom has no O₂.
  • Oxygen Indicator: Resazurin is used as an oxygen indicator:
    • Color Change: Pink indicates oxidized conditions (O₂ present), and colorless indicates reduced conditions (O₂ absent).

Oxygen Requirements of Bacteria

  • Classification of Bacteria Based on Oxygen Requirements:
    • Obligate Aerobes: Require O₂ and thrive at the top of the medium.
    • Facultative Anaerobes: Can grow throughout the medium, better in O₂ presence.
    • Aerotolerant Anaerobes: Indifferent to O₂ levels and can grow in both conditions.
    • Microaerophiles: Grow in the middle region where O₂ concentration is lower.
    • Obligate Anaerobes: Grow towards the bottom of the tube where O₂ levels are minimal.

Viscosity of Media

  • Purpose of Agar: Added to make media more viscous, limiting the diffusion of oxygen.