Cary-Blair transport medium can be used to transport clinical specimens suspected to contain enteric pathogens, including 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. Cary-Blair is the medium of choice for the transport and preservation of V. cholerae.
Cary-Blair’s semisolid consistency provides for ease of transport, and the prepared medium can be stored after preparation at room temperature for up to 1 year. Cary-Blair medium base is prepared with minimal nutrients to facilitate the survival of organisms without multiplication. The alkaline pH of the medium minimizes bacterial destruction due to the formation of acid.
Other transport media that are similar to Cary-Blair are:
- Amies’ and Stuart’s transport media are acceptable for Shigella and E. coli O157:H7, but they are inferior to Cary-Blair for transport of V .cholerae.
- Alkaline peptone water (APW) may be used to transport V. cholerae, but this medium is inferior to Cary-Blair and should be used only when the latter medium is not available.
- Buffered glycerol saline (BGS) is used to transport Shigella but unsuitable for the transport of V. cholerae.
Composition of Cary-Blair transport medium
- Sodium thioglycollate: provides a low oxidation-reduction potential.
- Di-sodium hydrogen phosphate: buffers the medium
- Sodium chloride and calcium chloride: maintains the osmotic equilibrium
- Agar: solidifying agent
Preparation of Cary-Blair transport medium
This medium is best prepared from ready-to-use dehydrated powder available from most suppliers of culture media. The medium is usually used at a concentration of 1.3 g in every 100 ml of distilled water (concentration may vary depending on the manufacturer).
- Prepare as instructed by the manufacturer.
- Suspend 12.6 grams in 991 ml of distilled water.
- Heat to boiling to dissolve the medium completely.
- Cool to 50°C and aseptically add 9 ml of 1% aqueous calcium chloride solution. (Note: Several commercially available dehydrated formulations of Cary-Blair are available. Some require the addition of calcium chloride and some do not. )
- Adjust pH to 8.4 if necessary.
- Dispense the medium in 7ml amounts in screw-cap bottles of 9 ml capacity (large size Bijou bottles). (Sufficient volume of Cary-Blair medium is dispensed into containers so that swabs will be covered by at least 4 cm of medium.)
- Sterilize by steaming with caps loosened (do not autoclave) at 100°C for 15 minutes.
- When cool, tighten the bottle caps. Label the bottles.
- Date the medium and give it a batch number. Record the expiry date (6 months from preparation) on each bottle.
- Store in a cool dark place with the bottle tops screwed tightly.
Quality Control of the prepared medium
- pH of medium: This should be within the range pH 8.3 -8.5 at room temperatures.
Shelf life: Cary-Blair transport medium is quite stable if stored in tightly-sealed containers in a cool dark place so that the medium does not dry out. 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.
- To transport enteric pathogens, including Shigella, Salmonella, Vibrio cholerae, and Escherichia coli O157:H7. Prompt plating, refrigeration, or freezing of specimens in Cary-Blair medium is particularly
important for the isolation of Shigella which is comparatively more fragile than other enteric organisms.
- For the detection of Campylobacter species from feces (or rectal swab), the specimen must reach the laboratory within 2 hours. If a delay of more than 2 hours is anticipated, the stool should be placed either in Cary-Blair transport medium or in campy thioglycollate medium.
- Immerse a swab of the fecal specimen in a container of sterile Cary-Blair transport medium, breaking off the swab stick to allow the bottle top to be replaced tightly.
- Protect the swab from direct light and excessive heat.
Note: Salmonella, Shigella, Vibrio, and Y. enterocolitica survive well in Cary-Blair medium for at least 48 hours (several days for Salmonella, Shigella, Vibrio species) and Campylobacter species for up to 6 hours. It is also a good transport medium for Y.pestis.
References and further readings
- Cheesbrough M., District Laboratory Practice in Tropical Countries: Part 2:.Cambridge University Press.
- Laboratory Methods for the Diagnosis of Epidemic Dysentery and Cholera; Center for Disease Control and Prevention.