Cary-Blair Transport Medium: Composition, Principle, Preparation, and Uses

A health worker at a rural health post in the Terai collects a rectal swab from a patient with profuse watery diarrhoea during monsoon season. The nearest laboratory with TCBS agar and a CO2 incubator is four hours away by road. By the time the specimen reaches the laboratory on a plain swab in a dry tube, Vibrio cholerae — if present — will almost certainly be dead.

The solution is Cary-Blair transport medium: a semisolid, alkaline, low-nutrient medium that keeps enteric pathogens viable during transport without allowing them to multiply, overgrow each other, or generate the acid metabolites that would otherwise kill fragile organisms like Shigella within hours. In settings where specimens cannot be processed immediately — which describes most district laboratories in resource-limited settings — correct transport medium selection is as important to the final culture result as any step that follows.

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.

Figure: Cary-Blair semisolid transport medium

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

1. 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.
2. 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.
3. Buffered glycerol saline (BGS) is used to transport Shigella but unsuitable for the transport of V. cholerae.

Principle

Transport media are fundamentally different from culture media in their design objective. Culture media are formulated to support and encourage bacterial growth. Transport media are formulated to do the opposite — to maintain bacterial viability while actively preventing growth, multiplication, and metabolic activity that would alter the specimen's original microbial composition.

Cary-Blair medium achieves this through four design features working together:

1. Minimal nutrients — preventing multiplication: The medium contains no carbohydrates, no blood, and no growth-promoting supplements. Organisms present in the specimen cannot multiply because the nutritional substrates required for cell division are absent. This ensures the specimen reflects the original microbial burden — a high count of Salmonella remains high; a low count of Shigella is not diluted by overgrowth of competing coliforms.

2. Sodium thioglycollate — preventing oxidative killing: Sodium thioglycollate is a reducing agent that lowers the oxidation-reduction (redox) potential of the medium. This is particularly important for two reasons: (a) anaerobic and microaerophilic organisms such as Campylobacter are killed by prolonged oxygen exposure — thioglycollate creates a low-oxygen microenvironment that extends their viability; (b) the reducing conditions prevent the accumulation of reactive oxygen species that damage bacterial cell membranes during transport.

3. Alkaline pH (8.4) — protecting Vibrio and suppressing acid formation: At neutral or acidic pH, metabolically active bacteria produce organic acids that rapidly lower the local pH, killing acid-sensitive organisms (particularly Shigella) and reducing overall recovery. The alkaline pH of 8.4 neutralises these acids as they form, maintaining a protective environment. The alkaline pH is also specifically important for V. cholerae, which thrives at pH 8.0–9.6 and survives transport far better in alkaline conditions than in neutral media.

4. Semisolid consistency (0.5% agar) — immobilising the specimen: The low agar concentration creates a semisolid matrix that keeps swabs and specimen material distributed throughout the medium rather than settling to the bottom. This ensures even contact between organisms and the protective medium components, and prevents the swab from drying out — desiccation being the most common cause of organism death during transport.

Key teaching point: Transport media preserve; they do not enrich. An organism present in very low numbers in a stool specimen will still be present in very low numbers after Cary-Blair transport. If enrichment is needed (e.g., carrier screening, convalescent specimens), a separate enrichment step — selenite broth for Salmonella, APW for Vibrio — must be performed after the specimen arrives at the laboratory.

Composition of Cary-Blair Transport Medium

Final pH: 8.4 ± 0.2 at 25°C

Why no carbohydrates? The deliberate absence of any fermentable carbohydrate is essential. If glucose or lactose were present, fermenting organisms would produce acid during transport, dropping the pH and killing acid-sensitive pathogens such as Shigella. The "minimal nutrient" formulation is not a limitation of Cary-Blair — it is its defining design feature.

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).

1. 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.

1. 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.)
2. Sterilize by steaming with caps loosened (do not autoclave) at 100°C for 15 minutes.
3. When cool, tighten the bottle caps. Label the bottles.
4. Date the medium and give it a batch number. Record the expiry date (6 months from preparation) on each bottle.
5. 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 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.

Choosing the Right Transport Medium for Enteric Specimens

Different transport media have different organism-specific performance characteristics. No single medium is optimal for all enteric pathogens.

Decision rule:

• Suspected enteric outbreak (mixed pathogens possible): Cary-Blair
• Suspected cholera, Cary-Blair unavailable: APW — subculture within 6–8 hours
• Suspected Shigella dysentery only, Cary-Blair unavailable: BGS or Amies
• Any specimen requiring >48 hours transport: Cary-Blair with refrigeration — the only transport medium validated for extended periods

Inoculation

1. 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.
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.

How to Remember

Transport media preserve; they do not enrich — and three design choices explain everything:

1. No carbohydrates → no acid production → pH stays stable → acid-sensitive organisms (Shigella) survive
2. Sodium thioglycollate → low oxygen environment → microaerophilic organisms (Campylobacter) survive
3. pH 8.4 → alkaline environment → Vibrio cholerae survives

Each design feature maps directly to one pathogen group it was designed to protect.

The organism survival time table as a clinical decision tool:

The Vibrio/cholera diagnostic workflow — where Cary-Blair fits:

Specimen collection → Cary-Blair (transport) → Lab receipt → APW (6–8 hrs enrichment) → TCBS agar (18–24 hrs incubation) → Yellow colonies → V. cholerae suspect

Cary-Blair starts the chain. APW amplifies. TCBS reveals. Each step exists because of the biological fragility of V. cholerae at different stages of the diagnostic process.

Why NOT to use APW as a transport medium unless Cary-Blair is unavailable: APW is an enrichment broth — it actively multiplies Vibrio while suppressing other enteric pathogens. If used for transport, it will destroy the original specimen's microbial composition (killing Salmonella and Shigella while multiplying Vibrio). Cary-Blair preserves all enteric pathogens neutrally; APW is selective enrichment only.

References and further readings

1. Cheesbrough, M. (2006). District Laboratory Practice in Tropical Countries, Part 2 (2nd ed.). Cambridge University Press.
2. Centers for Disease Control and Prevention (CDC). (2011). Laboratory Methods for the Diagnosis of Epidemic Dysentery and Cholera. Atlanta: CDC.
3. Tille, P. M. (2017). Bailey and Scott's Diagnostic Microbiology (14th ed.). Elsevier.
4. World Health Organization. (2004). Laboratory Methods for the Diagnosis of Meningitis caused by Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae (2nd ed.). Geneva: WHO. (For Cary-Blair use in meningitis specimen transport)