12 September, 2025

Introduction

Transport media are specially designed media used to preserve the viability of microorganisms during transport from the site of collection to the laboratory without allowing their multiplication. They prevent the death of pathogens due to drying, pH changes, or exposure to oxygen.They are especially useful when immediate inoculation is not possible.

Characteristics of an Ideal Transport Medium

  • Prevents drying (desiccation) of the specimen

  • Maintains viability of organisms without multiplication

  • Inhibits overgrowth of commensals

  • Maintains specimen pH

  • Easy to prepare and sterilize

  • Non-nutritive

Types of Transport Media

  • General Purpose Transport Media

    • Used for a wide range of bacteria

    • Example: Stuart’s medium, Amies medium

  • Specialized Transport Media

    • Designed for specific organisms

    • Examples:

      • Cary-Blair medium → Enteric pathogens (Salmonella, Shigella, Vibrio, Campylobacter)

      • Venkatraman–Ramkrishnan medium (VR medium) → Vibrio cholerae

      • Sach’s buffered glycerol saline → Dysentery bacilli (Shigella), but not for S. dysenteriae type 1

      • Throat swab transport media → For Neisseria spp. and Corynebacterium diphtheriae

Composition & pH

  • Sodium glycerophosphate → Buffer

  • Sodium thioglycollate → Reducing agent, prevents oxidation

  • Agar → Provides semi-solid consistency

  • Methylene blue → Indicator of oxidation

  • Distilled water

  • pH: ~7.4

Principle of Transport Media

  • Non-nutritive → Prevents bacterial multiplication

  • Buffering system → Maintains stable pH

  • Reducing substances (thioglycollate, charcoal) → Protect organisms from oxygen/free radicals

  • Semisolid consistency → Prevents drying and maintains viability

Sterilization

  • Prepared medium is sterilized by autoclaving at 121°C for 15 minutes (where applicable).

  • Some specialized media are sterilized by tyndallization or filtration if heat-sensitive components are present.

Preparation

  • Suspend dehydrated medium in distilled water.

  • Heat gently until dissolved.

  • Dispense into transport vials.

  • Sterilize by autoclaving.

  • Store at 2–8°C.

Uses of Transport Media

  • Stuart’s Medium → Neisseria gonorrhoeae, Haemophilus influenzae, Streptococcus pneumoniae

  • Amies Medium (with charcoal) → Neisseria spp. (charcoal absorbs fatty acids that are toxic)

  • Cary-Blair Medium → Enteric pathogens (Salmonella, Shigella, Vibrio, Campylobacter)

  • Venkatraman–Ramkrishnan (VR) Medium → Vibrio cholerae

  • Buffered Glycerol Saline → Dysentery bacilli (Shigella)

  • Special blood culture bottles → Anaerobes and fastidious organisms

Limitations

  • Does not allow long-term storage (only short transport).

  • Improper temperature may kill sensitive organisms (Neisseria gonorrhoeae).

  • Certain pathogens require enriched media immediately (e.g., Mycobacterium tuberculosis).

Conclusion

Transport media play a critical role in diagnostic microbiology by ensuring the survival of microorganisms during transport. Different media are designed for different pathogens—Stuart’s and Amies for general use, Cary-Blair for enteric pathogens, and specialized ones like VR medium for Vibrio cholerae.

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