Exer 4 Culture Media Preparation for Microorganisms

Microorganisms and Culture Media

Introduction

  • Microorganisms require nutrients for biological functions and survival.
  • In laboratory settings, microorganisms are cultured using artificial nutrient sources called culture media.

Expected Learning Outcomes

  • Describe and perform major steps in culture media preparation.
  • Determine the effect of pH and sterilization on gelling power and microorganism growth.

Culture Medium

  • Definition: Any nutrient material prepared for cultivating microorganisms.
  • Must provide essential elements for growth:
    • Key Elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Sulfur (S).
  • Ingredients vary according to the type of media.

pH and Microorganisms

  • Importance of pH: Different microorganisms grow best at specific pH levels:
    • Acidophiles: pH < 5.5
    • Neutrophiles: pH 5.5 - 8
    • Alkaliphiles: pH > 8
  • Adjust pH of culture media according to the microorganism being cultivated.

Types of Culture Media

Based on Composition
  1. Chemically Defined (Synthetic) Medium
    • Exact known chemical composition.
  2. Complex Medium
    • Composed of extracts/digests from yeast, meat, or plants; precise composition unknown.
    • Examples: Nutrient Agar, Eosin Methylene Blue Agar (EMBA).
Based on Consistency
  • Solid Medium: 1.5 - 2.0% agar; used for enumeration, isolation, and characterization.
  • Liquid Medium: 0% agar; used for growing large quantities of microorganisms.
  • Semi-Solid Medium: 0.5 - 0.7% agar; used for bacterial motility determination.
Based on Purpose/Use
  1. General Purpose Medium
    • Supports growth of most microorganisms.
    • Examples: Nutrient Agar, Nutrient Broth, Plate Count Agar.
  2. Selective Medium
    • Promotes specific organism growth while inhibiting others.
    • Example: Mannitol Salt Agar (selects for Staphylococcus).
  3. Differential Medium
    • Differentiates microorganisms based on metabolic reactions.
    • Examples: MacConkey Agar, Eosin Methylene Blue Agar.
  4. Enrichment Medium
    • Increases the number of desired microorganisms to detectable levels.
    • Examples: Blood Agar, Lactose Broth.
  5. Characterization Medium
    • Tests metabolic activities of specific microorganisms.
    • Example: Citrate Agar (tests for citrate utilization).

Media Preparation

Initial Steps
  • Determine total volume of medium to prepare.
  • Common glassware volumes include:
    • Petri dish: 15-20 ml
    • Big test tubes (stabs/broth): 10 ml; (slants): 6-8 ml
    • Small tubes (stabs/broth): 5 ml; (slants): 3 ml
    • Erlenmeyer flasks: Not more than 60% total capacity.
Calculation Example
  • Prepare 10 plates of Nutrient Agar (NA) and calculate:
    • 10 x 20 ml = 200 ml (plates)
    • 10 x 10 ml = 100 ml (big stabs)
    • 10 x 3 ml = 30 ml (small slants)
  • Total volume: 200 ml + 100 ml + 30 ml = 330 ml.
Component Computation
  • When using dehydrated medium, calculate amounts of each component needed:
    • Nutrient Agar Components per Liter:
    • Peptone: 5 g
    • Meat extract: 3 g
    • Agar: 15 g
  • For 330 ml of Nutrient Agar:
    • Peptone: (5 g / 1000 ml) * 330 ml = 1.65 g
    • Meat Extract: (3 g / 1000 ml) * 330 ml = 0.99 g
    • Agar: (15 g / 1000 ml) * 330 ml = 4.95 g

pH Adjustment

  • Adjust pH via titration if necessary:
    • Use specific indicator dyes for different pH levels:
    •  - bromthymol blue
    •  - bromcresol green
    •  - phenolphthalein
  • For titration example, prepare pH 7 solution, calculating volumes of titrant needed based on sample size.

Sterilization

  • Cover media with cotton plugs prior to sterilization.
  • Purpose: Allow gas exchange while preventing microbial contamination.

Conclusion

  • Understanding the preparation, properties, and applications of culture media is essential in microbiology for the growth and study of microorganisms.