Chloramphenicol Yeast Glucose Agar is a selective medium recommended for isolation and enumeration of fungi-yeasts and moulds in milk and milk products (1, 2, 3). Recently this medium has been recommended by ISO committee for the enumeration of yeasts and moulds (4). The medium contains yeast extract, which provides nitrogenous nutrients and vitamin B complex. Dextrose is the energy source. Chloramphenicol, a thermostable antibiotic, suppresses accompanying bacterial flora. This improves shelflife of the prepared medium and the prepared medium can be used over a period of at least 4 months (5).
Technique: Take two sterile Petri plates and transfer 1 ml of sample (if liquid) or 1 ml of the initial suspension in case of other products. Further take another two sterile plates and transfer 1 ml of 10-1 dilution to each sterile Petri plate or 1 ml of 10-2 dilution for other products. Repeat the procedure using further dilutions if necessary. Pour about 15 ml of Chloramphenicol Yeast Glucose Agar (5) previously melted and maintained at 45 ± 1°C. The time elapsing between the end of the preparation of the initial suspension and the moment when the medium is poured into the dishes shall not exceed 15 minutes. Carefully mix the inoculum with the medium and allow it to solidify. Prepare control plate to check the sterility. Incubate the plates at 25 ± 1°C. Count the colonies on each plate after 3, 4 and 5 days incubation. It is necessary to carry out a microscopic examination in order to distinguish, according to their morphology, the colonies of yeast and molds from colonies of bacteria.
It is advisable to examine the plates at the end of three days for yeast colonies, as they are likely to be overgrown by mold growth. Make a separate count of yeast colonies, which are characterized, as smooth, moist, elevated surface colonies. Count mold colonies, which are recognized by their profused growth of hyphae. If only yeast counts are required, add 0.25% of sterile sodium propionate solution to the medium at the time of preparation of plates to inhibit the growth of molds (6).
Storage and Shelf-life:
Store powder medium and prepared medium at 2 - 8°C. Use before expiry date on the label.
1. DIN Deutsches Institut fur Normung e.v. Reterenzverfahren DIN 10186.
2. International Organization for Standardization (ISO), Draft ISO/DIS 6611.
3. Internationaler Milchwirtschaftsverband: Internationaler IMV-Standard 94 1980.
4. International Organization for Standardization (ISO), 1987, Draft ISO/DIS 7954.
5. Engel G., 1982, Milchwiss, 37:727.
6. International Organization for Standardization (ISO), 1999, ISO 5403 :1999.