Microbial Culture Media Overview

Microbial Culture Media

• Microbial culture media provides nutrients for microbial growth, akin to any other organism.
• Classifications of culture media:
  • Chemical Composition: Simple, defined, complex
  • Physical nature: Solid, liquid
  • Function: Supportive, enriched, selective, differential

Types of Media

• Simple or Basal Media:
  • Routinely used to grow non-fastidious bacteria (e.g., Escherichia coli, Enterobacter, Pseudomonas aeruginosa).
  • Contains a carbon source, nitrogen source (e.g., beef extract, yeast extract, peptone), salts (e.g., sodium chloride), and agar (for solid media).
  • Examples: Nutrient broth, nutrient agar, peptone water.
  • Used to isolate microbes from patient samples or sources.
• Defined Media or Synthetic Media:
  • Carbon and nitrogen sources are precisely defined (exact proportions).
  • Contrasts with simple media, where nitrogen sources are undefined (e.g., beef extract or yeast extract).
• Enriched Media:
  • Contains extra nutrients for nutritionally demanding bacteria (fastidious organisms).
  • Examples:
    • Blood agar (5-10% sheep blood added to minimal media) suitable for growing Staphylococcus pneumonia and other organisms.
    • Chocolate agar (prepared by heating blood agar, lysing blood cells) suitable for growing Haemophilus influenzae.
• Selective Media:
  • Suppresses the growth of one group of microorganisms while allowing another to grow.
  • Examples:
    • Mannitol Salt Agar (MSA): Isolates Staphylococcus aureus; S. aureus colonies ferment mannitol, producing yellow colonies.
    • McConkey Agar: Selective and differential; isolates gram-negative and enteric bacteria; differentiates lactose fermenters (pink colonies, e.g., Klebsiella, Enterobacter) from lactose non-fermenters (yellow colonies, e.g., Salmonella, Shigella).
    • Campylobacter agar: Designed for isolating Campylobacter jejuni from rectal or fecal swabs; contains antibiotics (vancomycin, polymyxin B) to inhibit other bacterial growth.
• Differential Media:
  • Also known as Differential Indicator Media or Chromogenic Media.
  • Differentiates bacterial species based on colony color resulting from the metabolism of chromogenic substrates.
  • Examples:
    • Mannitol Salt Agar: Yellow colonies indicate Staphylococcus aureus.
    • McConkey Agar: Pink colonies indicate lactose fermenters; yellow colonies indicate lactose non-fermenters.
    • Sabouraud's agar: Selective for fungi isolation, commonly used to isolate Candida albicans.

Physical Nature of Media

• Solid Media:
  • Typically used in plates.
  • Contains agar.
• Liquid Media:
  • Does not contain agar, therefore does not solidify.
  • Example: Robertson's Cooked Meat Media.
  • Used for cultivating aerobic or anaerobic microorganisms.
  • Useful for small samples, as they can be inoculated, incubated, then transferred to solid media for isolation.
  • Industrially important: Growing large volumes of bacterial cultures for antibiotic, insulin, and other recombinant protein production.