Cary-Blair transport medium: Composition, Preparation and Uses 4.8/5 (5)

Cary-Blair transport medium can be used to transport of 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), viability of Vibrio cultures can be maintained for a longer duration & it is the medium of choice for 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 survival of organisms without multiplication. Alkaline pH of the medium minimizes bacterial destruction due to the formation of acid.

Cary-Blair semisolid transport medium
Cary-Blair semisolid transport medium

Other transport media that are similar to Cary-Blair are:

  1. 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.
  2. Alkaline peptone water (APW): may be used to transport cholerae, but this medium is inferior to Cary-Blair and should be used only when the latter medium is not available.
  3. Buffered glycerol saline (BGS):  is used to transport Shigella but unsuitable for transport of V.cholerae.

Contents of Cary-Blair transport medium:

  1. Sodium thioglycollate: provides a low oxidation-reduction potential.
  2. Di-sodium hydrogen phosphate: buffers the medium
  3. Sodium chloride and calcium chloride: maintains the osmotic equilibrium
  4. 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 distilled water (concentration may vary depending on manufacturer).

  1. Prepare as instructed by the manufacturer.
    • Suspend 12.6 grams in 991 ml 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.
  2. 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.)
  3. Sterilize by steaming with caps loosened (do not autoclave) at 100°C for 15 minutes.
  4. When cool, tighten the bottle caps. Label the bottles.
  5. Date the medium and give it a batch number. Record the expiry date (6 months from preparation) on each bottle.
  6. Store in a cool dark place with the bottle tops screwed tightly.

Quality Control of the prepared medium

  1. 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.

Uses:

  1. 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.
  2. For the detection of Campylobacter species from faeces (or rectal swab), the specimen must reach the laboratory within 2 hours. If the delay of more than 2 hour is anticipated, the stool should be placed either in Cary-Blair transport medium or in campy thio medium.

Inoculation:

  1. Immerse a swab of the faecal specimen in a container of sterile Cary-Blair transport medium, breaking off the swab stick to allow bottle top to be replaced tightly.
  2. 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:

  1. Cheesbrough M., District Laboratory Practice in Tropical Countries: Part 2:.Cambridge University Press .
  2. Laboratory Methods for the Diagnosis of Epidemic Dysentery and Cholera; Center for Disease Control and Prevention.

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3 thoughts on “Cary-Blair transport medium: Composition, Preparation and Uses

  1. Hello I am curious as to the detection of Aeromonas spp in the stool and the importance of Cary Blair. My daughter has an infection in her colon of the Aeromonas spp and we were told she has numerous. So I am nervous about the testing procedures. Reason being is the she has had many stool tests showing negative for infection . She enrolled in a clinical trial and did a stool test and this bacteria showed up. I do not know why it showed up now but not on previous tests. She was given an antibiotic then the doctor switched her to a different antibiotic. I want to make sure the infection is gone, so the doctor ordered another stool test to see if it was cleared I am worried if it never showed on the previous tests except for the trial test, it might not show now, which would mean no more antibiotics, but the infection is still there. I really don’t think it could be killed with 5 day dose anyway. Can you help me with the right way to go about this please? My daughter has been ill for quite sometime and she needs the right treatment. From my own research of the bacteria it is very hard to treat because it is resistant to most antibiotics.

     
    1. Tankeshwar Acharya

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      Reply

      Hello June Ghanem, sorry to know about the problem of your child and the difficulties you are having. Yes, Aeromonas can cause gastroenteritis, most frequently in children, but its role in intestinal infection is not always clear. The significance of its isolation in stool specimen should be interpreted with caution. This organism is normally found in fresh water too. Regarding treatment; for gastroenteritis therpay may not be needed. In our settings, this organism is not priority GI tract pathogen for isolation and identification from stool sample.
      I request you to consult with your physician for the signficance of its isolation as well as need of therapy. Wishing prompt recovery of your child.

       
      1. Hello, can you explain to me what is meant by this organism is not priority GI tract pathogen for isolation and identification from stool sample? My daughter was told she has Ulcerative Colitis. I do not believe the immune system is attacking itself. I believe there is an underlying infection which has caused the immune system to attack because it is attacking the Aeronomas, but it cannot kill it so it keeps attacking which in turn causes the ulceration and inflammation. Once the infection is cleared the immune system will stop attacking and the inflammation will stop and then the process of healing the stomach lining will begin. Can you please explain what kind of testing would be done to see if the infection has cleared and what antibiotics would used to clear the infection?

         

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