Dichloran Glycerol Agar (DG 18) ISO

Glycerol reduces the water activity from 0.999 to 0.95, thereby reducing bacterial growth, and is also the carbon source.

Chloramphenicol is an antibiotic which aids in isolating pathogenic fungi from heavily contaminated material, as it inhibits most contaminating bacteria. It is a recommended antibiotic for use with media due to its heat stability and wide bacterial spectrum. Inhibition of bacterial growth and restriction of the spreading of more rapidly growing molds aid in the isolation of slow-growing fungi by preventing their overgrowth by more rapidly growing species. Dichloran prevents the fast spreading of mucoraceous fungi, improving the colony count.

Peptone provides nitrogen, vitamins, minerals and amino acids essential for growth. Dextrose is the fermentable carbohydrate providing carbon and energy. Potassium phosphate acts as a buffer system. Magnesium sulfate provides sulfur and other trace elements. Bacteriological agar is the solidifying agent.

Preparation

Suspend 31.6 grams of the medium in one liter of distilled water. Add 175 mL of glycerol. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Distribute and sterilize in autoclave at 121 °C for 15 minutes. Cool to 45-50 °C and pour into Petri dishes.

Instructions for use

1. Use two different plates.
2. Onto one DG 18 plate, transfer 0.1 mL of the test sample if the sample is liquid, or 0.1 mL of the initial suspension if the sample is not liquid.
3. Onto a second DG 18 plate, transfer 0.1 mL of the first decimal dilution (10⁻¹) if liquid, or 0.1 mL of the second decimal dilution (10⁻²) if not.
4. Inoculate and incubate at 25 ± 1 °C.
5. Examine for growth after 5-7 days. If the presence of Xeromyces bisporus is suspected, incubate the plates for 10 days.
6. Select the dishes containing < 150 colonies and count them. If fast-growing molds are a problem, count colonies after 2 days and again after 5-7 days of incubation.
7. This number can be reported as number of xerophilic colonies per gram of food.

• ISO 21527-2: Microbiology of food and animal feeding stuffs - Horizontal method for the enumeration of yeasts and moulds - Part 2: Colony count technique in products with water activity less than or equal to 0.95.
• Hocking, A.D. and Pitt, J.I. (1980) Dichloran-glycerol medium for enumeration of xerophilic fungi from low moisture foods. Appl. Environm. Microbiol. 39, 488-492.