Experiment 12: Selective and Differential Media Study Notes

Experiment 12: Selective and Differential Media

Overview of Bacterial Growth in the Lab

  • Growth Requirements for Bacteria:
    • Bacteria need nutrients to survive and reproduce, provided via different types of media.
    • Types of Media:
    • Liquid Media: Example - Tryptic Soy Broth (TSB)
    • Solid Media: Example - Tryptic Soy Agar (TSA), which is TSB mixed with agar.
    • Semi-solid Media: Example - SIM agar (with less agar than TSA, used in later experiments).
  • Environmental Conditions Needed:
    • Temperature: Bacteria require specific temperatures for optimal growth.
    • Oxygen Requirement: Some bacteria require oxygen, while others may require anaerobic conditions or elevated levels of carbon dioxide.

Types of Growth Media

  • General Purpose Media:
    • Support growth for most bacterial organisms.
  • Selective Media:
    • Contains components that favor the growth of certain species while inhibiting others.
  • Differential Media:
    • Displays observable changes in bacterial appearance based on their metabolic properties, allowing for differentiation among species.
  • Enriched Media:
    • Contains extra nutrients such as vitamins or specific food sources to support fastidious species (those that require more growth factors).
  • Combination Media:
    • Media can be combined; for instance, MSA (Mannitol Salt Agar) and MAC (MacConkey Agar) serve both selective and differential purposes.

Media Utilized in the Experiment

  • Tryptic Soy Broth (TSB) and Tryptic Soy Agar (TSA):
    • Classified as General Purpose Media.
    • Support growth for non-fastidious bacteria and allow for colony morphology examination.
Examples of Bacteria on TSA
  • Serratia marcescens: Produces an orange/pink pigment at room temperature but not at higher temperatures.
  • Pseudomonas aeruginosa: Produces a green pigment.
  • Micrococcus luteus: Produces a yellow pigment.
  • Escherichia coli: Appears creamy/off-white and does not produce pigment.
  • Note: The pigmentations stem from bacterial activity, not the media itself.

Introduction of New Media Types

  • Mannitol Salt Agar (MSA):
    • Contains mannitol sugar and 7.5% sodium chloride (NaCl).
    • Properties:
    • Selective: Grows salt-loving bacteria (halophiles) due to high salt content.
      • Example: Gram-positive staphylococci thrive.
      • Non-halophilic bacteria experience osmotic shock and do not grow.
    • Differential: Bacteria fermenting mannitol produce acid, and the pH indicator phenyl red changes color.
      • Neutral pH: Agar remains red.
      • Acidic pH (from fermentation): Agar turns yellow.
      • Halophiles unable to ferment mannitol still grow; agar remains red/pink.
Examples of Bacteria on MSA

  • Staphylococcus aureus

  • Staphylococcus epidermidis

  • MacConkey Agar (MAC):

    • Contains lactose sugar and bile salts.
    • Properties:
    • Selective: Grows Gram-negative enteric bacteria; bile inhibits growth of non-enteric bacteria.
    • Differential: Fermentation of lactose results in acid production, leading to color change with neutral red pH indicator.
      • Neutral pH: Agar is yellow.
      • Acidic pH (from fermentation): Agar turns pink/red.
      • Enteric bacteria unable to ferment lactose remain colorless on the agar.
Examples of Bacteria on MAC

  • Escherichia coli: Lactose fermenting (appears pink).

  • Enterobacter aerogenes: Lactose fermenting (appears pink).

  • Non-lactose fermenting examples:

    • Proteus vulgaris (appears colorless)
    • Salmonella typhimurium (appears colorless)
    • Staphylococcus aureus (appears colorless)

  • Sheep’s Blood Agar (SBA):

    • Contains sheep's blood; categorized as enriched and differential.
    • Properties:
    • Enriched: Supports fastidious bacteria needing extra nutrients (iron, hemoglobin, B vitamins).
    • Differential: Visual indications of hemolysis:
      • Alpha (α) Hemolysis: Partial lysis of red blood cells results in greenish agar.
      • Beta (β) Hemolysis: Complete lysis results in clear agar.
      • Gamma (γ) Hemolysis: No lysis, agar remains unchanged.
    • Hemolysis is enhanced under low-oxygen conditions, typically used in a candle jar setup.
Candle Jar Setup for SBA
  • Procedure of using a Candle Jar:
    • Use a large jar (like a pickle jar).
    • Place inoculated blood plates in the jar, then place a lit candle on top.
    • Seal the jar; as the candle burns, it consumes oxygen and produces carbon dioxide, creating anaerobic conditions ideal for SBA.
Examples of Bacteria on SBA
  • Detailed documentation of bacteria on SBA not provided in the transcript.

Procedure Overview

  • Materials Required:
    • Obtain 8 pre-made bacterial cultures.
    • Acquire 6 plates (2 of each type: MSA, MAC, SBA).
    • Divide each plate into 4 quadrants and label accordingly (including group name, plate type, and bacteria species).
  • Inoculation Process:
    • Plate all bacterial cultures on all types of media plates.
    • Incubation Instructions:
    • Place SBA plates into the candle jar.
    • Other types of plates incubate on the culture cart in sleeve tins.

Evaluation of Results

  • Evaluation Criteria:
    • Confirm the plate type characteristics: selective, differential, enriched, etc.
    • If selective media is used:
    • Determine the significance of growth or lack of growth concerning the microbe.
    • If differential media is employed:
    • Evaluation is limited to microbes that grew – analyze appearances, metabolically relevant characteristics.
    • For enriched media:
    • Identify which microbes specifically need this enriched medium and the rationale behind it.