Microbiology Culture Media Overview

Culture Media

• Culture medium types:
  • Liquid: Diffused growth, no distinct characteristics for identification, difficult to isolate. Earliest liquid media included urine or meat broth used by Louis Pasteur.
  • Solid: Distinct colony morphology, characteristics are easily identified.

Solid Media

• Colony: Macroscopically visible collection of bacteria originating from a single cell.
• Earliest solid medium: Cooked potato (Robert Koch), but not satisfactory. Liquefied at 24°C.
• Gelatin: Not satisfactory, liquefied at 24°C.
• Agar: Universally used for solid media, derived from seaweed (Gelidium). Offers no nutritive value, unaffected by bacterial growth, melts at 98°C and solidifies at 42°C. 2% agar is commonly used.

Types of Culture Media

• Based on consistency:

  • Solid
  • Liquid
  • Semi-solid

• Based on constituents/ingredients:

  • Simple
  • Complex
  • Synthetic/defined
  • Special

Special Media

• Enriched media: Supports growth of a wide variety of organisms, including fastidious ones (difficult to grow). Often used for pathogen isolation. Include blood agar (whole blood is added) and chocolate agar (heat-treated blood is added).
• Selective media: Contains inhibitory substances that prevent the growth of certain microorganisms, while allowing others to grow. Increase the number of desired bacterial colonies. Examples include desoxycholate citrate, Mac Conkey's medium, TCBS (Thiosulfate Citrate Bile Salt Sucrose), and LJ medium.
• Indicator media: Contain an indicator that changes color when a microorganism grows in it. Examples include Wilson-Blair agar (S. typhi black colonies) and urease medium (yellow oranged) where urease-producing bacteria turn the medium pink.
• Differential media: Incorporate substances that enable differentiation between bacteria. MacConkey's agar, for example, differentiates based on lactose fermentation (pink colonies for lactose fermenters and colourless for non-lactose fermenters).

Transport Media

• Used for transporting samples, preserving delicate organisms during transport. Stuart's medium is one example.

Anaerobic Media

• Used to cultivate anaerobic organisms, such as those in Robertson's cooked meat medium or Thioglycolate medium.

Culture Methods

• Methods depend on the purpose—isolating pure cultures, demonstrating properties, testing antigens, susceptibility to antibiotics, and viable count estimates. Includes streak culture, lawn culture, stroke culture, and stab culture.

This quiz covers the essentials of culture media used in microbiology, focusing on the types, characteristics, and historical developments. Learn about liquid, solid, and semi-solid media, as well as different constituents used in their formation. Test your knowledge on the importance of agar and other media in bacterial growth and isolation.