Cary-Blair Transport Medium: Introduction, Composition, Types and Use

Introduction of Transport Medium

It uses in the case of delicate organisms (e.g. gonococci) which may not survive the time taken for transit or may be overgrown by non-pathogenic bacteria (e.g. cholera organisms). It maintains only viability. Examples of transport media are Stuart’s transport medium: is a non-nutrient soft agar gel containing a reducing agent to prevent oxidation, and charcoal to neutralize bacterial inhibitors. It may be for organisms such as gonococci. Buffered glycerol saline transport medium for enteric bacilli. Cary-Blair transport medium is a semi-solid medium and uses in the transportation and preservation of clinical specimens mainly stool and rectal swabs. It may use both aerobic and anaerobic bacteria.

Composition of Cary-Blair Transport medium

Sodium Chloride: 5.0 g (maintains the osmotic equilibrium)
Calcium Chloride: 0.1 g (maintains the osmotic equilibrium)
Sodium Thioglycolate: 1.5 g (provides a low oxidation-reduction potential)
Agar: 5.0 g (solidifying agent)
Disodium Phosphate: 1.1 g (buffering agent to prevent overgrowth of enteric pathogens with Escherichia coli and Enterobacter)
Demineralized Water: 1000.0 ml
pH 8.4 ± 0.2 (the alkaline pH minimizes the destruction of bacteria due to acid formation) @ 25°C

Using Method of Cary-Blair Transport Medium

If a stool specimen is received, place approximately one gram of fecal material into Cary Blair Transport Medium. If the specimen is received on a swab, place the swab in Cary Blair Transport Medium and cut off the protruding portion of the swab. Secure the cap tightly and label the specimen. Transport the specimen to the laboratory with a minimal delay at room temperature unless recovery of Shigella is required, in which case the specimen should be maintained at 2-8°C during shipment. Mix the specimen thoroughly prior to inoculation of appropriate media.

Uses of Cary-Blair Transport medium

Cary-Blair transport medium is a semi-solid medium. It is recommended for use in the transportation and preservation of clinical specimens mainly stool and rectal swabs suspected of Shigella, Salmonella, Vibrio cholerae, and Escherichia coli O157:H7. Since this transport medium has a high pH (8.4), the viability of Vibrio cultures can be maintained for a longer duration, and therefore it is the medium of choice for transport and preservation of V.cholerae. But it can also be used both aerobic and anaerobic bacteria. For anaerobic bacteria, there is a need for extra ingredients i.e. resazurin and cysteine.

Types of Cary-Blair Transport Medium

It is of two types-

aerobic bacteria and
anaerobic bacteria

Keynotes

Other transport media that are similar to Cary-Blair are as follows- Alkaline peptone water (APW): This may be used to transport Vibrio cholerae, but inferior to Cary-Blair and should be used only when the absence Cary-Blair medium. Buffered glycerol saline (BGS): It is useful to transport Shigella but unsuitable for the transport of V.cholerae. Amies’ and Stuart’s transport media: acceptable for Shigella and E. coli O157:H7, but they are inferior to Cary-Blair for transport of V.cholerae.
Cary-Blair may be used for up to 1 year as long as there is no loss of volume, contamination, alteration of pH, or color change.
Protect the swab from direct light and excessive heat.
Salmonella species, Shigella species, Vibrio species, and Y. enterocolitica survive well in Cary-Blair medium for at least 2 days ( but several days for Salmonella, Shigella, Vibrio species) and Campylobacter species for up to 6 hours.
Cary-Blair is a good transport medium for Yersinia pestis too.

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