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Dichloran Rose-Bengal Chloramphenicol (DRBC) (ISO) Agar Base

Code: CM1148

Dichloran Rose-Bengal Chloramphenicol Agar (ISO 21527-1) is a selective medium for yeasts and moulds associated with food spoilage.

* Adjusted as required to meet performance standards

Directions

Suspend 15.75g per 500ml (31.5g/l) of distilled water and heat to dissolve completely. Reconstitute Chloramphenicol Supplement (one vial SR0078E per 500ml or one vial SR0078H per 2 litres medium), as directed. Add the vial contents to the DRBC (ISO) Agar Base. Sterilize by autoclaving at 121°C for 15 minutes. Cool to 50°C, mix well and pour into sterile Petri dishes.

Description

Dichloran Rose-Bengal Chloramphenicol (DRBC) (ISO) is a dehydrated culture medium formulated according to BS ISO 21527-1: 2008 Microbiology of food and animal feeding stuffs – Enumeration of yeasts and moulds Part 1- Colony counting technique in products with >0.95 water activity¹. It is designed as a selective medium for the isolation and enumeration of viable yeasts and moulds in food products with a water activity above 0.95 a_W. DRBC (ISO) is a modification of the original Rose-Bengal Chloramphenicol Medium² and differs as the Rose-Bengal content has been optimized by adding Dichloran and by changing the pH to 5.6. The cumulative effect of these modifications is to further inhibit bacterial growth, inhibit spreading moulds, such as Rhizopus and Mucor, and make the medium capable of supporting the growth of species that cannot be isolated on Rose-Bengal Chloramphenicol Agar or Acidified Potato Dextrose Agar. The inclusion of chloramphenicol in this medium suppresses bacterial growth, allowing moulds and yeasts to grow. The inhibition of spreading moulds and the restriction of their colony size results in improved enumeration and detection. The reduced pH of DRBC (ISO) Agar increases the inhibition of yeasts by the rose-bengal.

Technique

1. Prepare the DRBC (ISO) Medium as directed
2. Add the food sample to 0.1% peptone water and process in a `Stomacher’ for 30 seconds³ or alternatively weigh into 0.1% peptone water and leave for 30 minutes shaking periodically⁴.
3. Inoculate 0.1ml of the prepared sample on DRBC (ISO) Agar and spread using a sterile spreader
4. Incubate the plates at 25°C and examine after 3, 4 and 5 days.
5. Report as number of colony forming units gram of food.

Where bacterial overgrowth occurs from certain foods (raw meats) chlortetracycline hydrochloride at 50mg/L (dissolved in water and filter sterilized) is recommended.

Storage instructions

Store the dehydrated medium at 10–30°C and use before the expiry date

Appearance

Dehydrated medium: Pink coloured, free-flowing powder
Prepared medium: Pink coloured gel

Quality control

* This organism is available as a Culti-Loop®

Precautions

ROSE-BENGAL PHOTO-OXIDIZES TO FORM TOXIC COMPOUNDS. STORE PLATES OF THE MEDIUM IN THE DARK AND AVOID EXPOSURE TO LIGHT⁸.
Some strains of fungi may be inhibited on this medium.
The dichloran compound used in this medium is Botran® 2,6-Dichloro-4-Nitro-Analine (CAS: 99-30-9).

References

1. King D. A. Jr., Hocking A. D. and Pitt J. I. (1979) J. Appl. & Environ. Microbiol. 37. 959-964.
2. Pitt J. I. (1984) Personal Communication.
3. Sharp A. N. and Jackson A. K. (1972) Applied and Environmental Microbiology. 24(2). 175-178.
4. Sharf J. M. (ed) (1966) 2nd ed American Public Health Association, New York.