Transport Media: Principle, Uses, Types, Examples

Culture media are solid, semisolid, or liquid preparations containing essential nutrients and minerals vital for microorganisms’ survival and growth. They are also called growth media. They provide nutritional requirements and a place to grow the microorganisms in the laboratory. Based on physical state, culture media are classified as solid, semi-solid, and liquid culture media. Similarly, based on their function (intended use), they are classified as general purpose (supportive) media, selective media, differential (indicator) media, enriched media, enrichment media, sugar media, and transport media.

Transport media are those culture media that are used to maintain the viability of pathogenic microorganisms present in a clinical sample and to prevent the potential pathogens to be overgrown by other commensals or contaminants in the specimen during the transportation of the clinical specimen to the clinical/diagnostic laboratory.

Transport media are typically buffer solutions with minimum nutrients like carbohydrates, peptones, and salts, excluding growth factors. Some also contain selective inhibitory components. Due to this minimum nutrient and lack of carbon, nitrogen, and organic growth factors, microorganisms present in the specimen do not multiply but remain viable. 

Principle of Transport Media

Transport media only contains enough energy sources like peptone and carbohydrates to maintain the viability of microorganisms potentially present in a specimen. They are well buffered and pH maintained so the present organisms do not suffer from chemical stress. 

No additional carbon, nitrogen, and other organic and inorganic (ions and minerals) growth factors are incorporated in the medium. Some may also contain inhibitory substances that selectively inhibit the growth and multiplication of microorganisms. Due to this, the microorganisms can’t multiply but remain viable for a certain duration under optimum physical conditions. Hence, the transport media tends to preserve the microbiome of the specimens in their original condition till they are processed in a lab. 

Uses of Transport Media

• To maintain the specimen and its microbiome in their original state after specimen collection to the processing period.
• To maintain the viability of specific pathogenic microorganisms potentially present in a clinical specimen.
• To prevent commensals and contaminants from overgrowing the pathogenic microorganisms potentially present in a clinical specimen.

Types of Transport Media

• Based on their physical state, transport media may be classified as semi-solid or liquid transport media. 
• Based on their utility, they can be classified as bacterial transport media, viral transport media, and parasite transport media. Specimens for fungal culture are generally transported without any transport media.

Some Common Transport Medium and Intended Use

Applications of Transport Media

• They are used for transferring clinical specimens from the collection center to diagnostic laboratories; especially from remote or resource-limited places to a laboratory. 
• They are used to preserve clinical specimens if there is some delay in processing/culturing clinical specimens. 
• They are used while sharing or transferring microbial cultures which are highly sensitive to environmental change. 
• They are used to suppress the growth of contaminants and other microbial species of non-interest in clinical samples. 
• They are used to maintain the viability of anaerobes and fastidious organisms and obligate pathogens in the specimen. 
• They are also used in transporting food samples, water samples, and other biological samples for analysis. 

Limitations of Transport Media

• As the transport mediums are limited in nutrients, they do not support the survival of organisms for a longer duration. 
• Though the transport medium inhibits and prevents the overgrowth of many contaminants, some contaminants having similar metabolic, physical, and chemical requirements also remain viable. 
• Physical conditions like temperature and pressure must be maintained. 
• Specimens must be cultured in another culture medium for isolation and diagnosis.

References

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