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Transport Media in Microbiology

It is a type of microbial culture medium. It is used to transport microbiological samples from one location to another. Microbiologists use transport medium to keep microbes in the sample or specimen alive when inoculation in growth media is delayed. In a microbiology lab, we need to transfer microbiological samples or specimens from one location to another for testing purposes. Sometimes we need to send samples from one city to another or from one nation to another. Therefore, transport media in microbiology are very essential for transport of clinical samples.

Transport mediums serve as buffers. It includes carbohydrates, peptones, and other nutritional components. It is deficient in growth factor. We must maintain the appropriate temperature and oxygen needs, as well as proper nutritional content, during this transport. If microbes are anaerobic, then it require transport media containing reducing agents. If we retain these anaerobic bacteria in the presence of oxygen, their growth pattern will change. To keep such bacteria alive, we normally need to add a reducing agent, such as sodium thioglycollate, which can act as a reducing agent. This sodium thioglycollate will deplete the medium of oxygen, resulting in anaerobic conditions. As a result, anaerobic bacteria will survive.

Transport Media Definition:

Transport media are special media for maintaining the microbiological specimen during transport of microbiological samples from one place to another.  It preserves the viability of microbes present in the sample and doesn’t allow their multiplication. Therefore, there is no bacterial overgrowth till sample arrives at the laboratory for further processing. Transport medium varies as per the types of specimen being collected. However, transport media are often categorized based on their physical condition as semisolid or liquid, as well as their utility as bacterial or viral transport media. To avoid microbial multiplication transport medium includes just buffers and salt. It lacks nutritional elements such as carbon, nitrogen, or organic growth factors.

Important Criteria of transport media in microbiology:

Transport media should fulfil the following criteria:

Temporary storage of specimen: The purpose of transport media is to keep the microbes in the sample alive. For this, the media should provide survival conditions for the microorganisms in the sample till sample or specimen reaches to lab for processing.

Maintenance of Viability of specimen: The transport media should preserve the viability of microbes in the sample.

Prevention of Microbial overgrowth: Transport media should prevent microbial multiplication. Addition of extra nutrients in this medium is not necessary. Transport media should not provide the nutrients for growth and multiplication of microbes in the sample.

Composition with Buffer and Salt: Buffers and salts in the transport media helps in maintaining viability of microbes in the sample. It contains only buffers and salts which provide survival conditions for the microbes present in the sample.

Absence of nutritional components: Transport media should be without nutritional components such as carbon, nitrogen, and organic growth factors. Therefore, transport media doesn’t allow the multiplication of the microorganisms in the sample

Anerobic condition for anaerobic bacteria: For isolation of anaerobic microbes the transport media used should be free from the molecular oxygen.

Examples of Transport media in Microbiology

Stuart’s transport medium:

It is semisolid medium used for preservation of microbiological samples during  transportation. It is recommended mainly for the preservation of specimens containing Neisseria species and other fastidious organisms. Stuart’s medium is non-nutritional i.e. it doesn’t contain any nutrients. This medium The reducing agent Sodium thioglycollate produces reducing environment in the medium therefore Transport of anaerobic bacteria is possible with Stuart transport medium. Sodium thioglycollate in Stuart medium improves the recovery of anaerobes. This medium also contains the Methylene Blue. Methylene blue acts as oxidation-reduction indicator. The monitoring of an anaerobic environment in the medium is detected by color change of the medium. When the medium is in reduced state then medium is colorless. When the medium is in oxidised state then medium become Blue colored.

Cary and Blair transport medium

Cary-Blair Transport Medium is semi-solid, non-nutritive medium used to transport clinical specimens containing enteric pathogens such as Shigella, Salmonella, Vibrio cholerae, and E. coli O157:H7. All these bacteria belongs to Enterobacteriaceae family and found in human feces. Since Cary-Blair Medium is non-nutritive, its minimum nutrients doesn’t  allow organisms in the specimen to proliferate. Cary-Blair Transport Medium is modified Stuart’s medium that has an enhanced buffering mechanism by substituting sodium glycerophosphate with inorganic phosphates. This new formulation minimizes Enterobacteriaceae overgrowth and contributes to the successful long-term preservation of Salmonella and Shigella. This medium has alkaline pH (pH 8.4) which reduces bacterial killing caused by acid production. Cary-Blair is the preferred medium for transporting and preserving Vibrio cholerae.

Amies transport media

Amies media is the modified form of Stuart’s Transport Medium. It contains charcoal and an inorganic phosphate buffer in place of glycerophosphate. It is non-nutritive, semi-solid media used for transporting clinical swab specimens to laboratories. This transport medium preserves the anaerobic bacteria like Neisseria gonorrhea and other pathogens alive in swabs. The charcoal in Amies Transport media to extends the viability of pathogenic organisms. The charcoal absorbs harmful toxic materials and nutrilizes it. Therefore, sensitive organisms like Neisseria gonorrhoeae remain alive. At the same time it doesn’t allow the microbes to multiply. This media is designed in such a way that it maintains the viability of microorganisms and doesn’t allow overgrowth.

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