DG-18 (DICHLORAN GLYCEROL) AGAR

Cat. no. W85 DG-18 Agar, 15x100mm Plate, 26ml 10 plates/bag

INTENDED USE

Hardy Diagnostics DG-18 Agar is recommended as a selective medium for the isolation and cultivation of xerophilic molds.

This product is not intended to be used for the diagnosis of human disease.

SUMMARY

Hardy Diagnostics DG-18, or Dichloran Glycerol, Agar is based on a formulation by Hocking and Pitt and is recommended for the enumeration of xerophilic yeasts and molds from dried and semi-dried foods. (8) Examples include fruits and spices, confectionery products, cereals and nuts, and dried meat and fish. This medium is highly selective, has a high osmotic pressure, and allows for the enumeration of fungal growth. A parallel study using DG-18 Agar and DRBC, or Dichloran Rose Bengal Chloramphenicol, Agar showed that DG-18 Agar exhibited a greater recovery and more robust growth of two molds, Aspergillus penicilloides and Wallemia sebi, commonly found in dried foods. (8)

Hardy Diagnostics DG-18 Agar contains peptones which provide nitrogen, vitamins and amino acids required for microbial growth. Glucose provides an energy source. Phosphate acts as a buffering agent. The inorganic salt, magnesium sulfate, stimulates fungal growth and enhances sporulation. Dichloran inhibits the spreading of mucoraceous fungi and restricts the colony size of other genera commonly present in food samples. Chloramphenicol inhibits typical bacterial colonies present in environmental and food samples. Agar acts as the solidifying agent. Glycerol lowers the water activity (a W ) of the media from approximately 0.999 to 0.95 and provides an additional carbon source.

FORMULA

Ingredients per liter of deionized water:*

Glucose 10.0gm
Peptone 5.0gm
Potassium Dihydrogen Phosphate 1.0gm
Magnesium Sulfate 0.5gm
Chloramphenicol 0.1gm
Dichloran 0.002gm
Glycerol 220.0ml
Agar 15.0gm

Final pH 5.6 +/- 0.2 at 25ºC.

* Adjusted and/or supplemented as required to meet performance criteria.

STORAGE AND SHELF LIFE

Storage: Upon receipt store at 2-8ºC. away from direct light. Media should not be used if there are any signs of deterioration (shrinking, cracking, or discoloration), contamination, or if the expiration date has passed. Product is light and temperature sensitive; protect from light, excessive heat, moisture, and freezing.

PRECAUTIONS

PROCEDURE

Refer to the appropriate references for applications using DG-18 Agar for yeast and mold testing. (1,2,7-9,11)

1. Process the sample in a stomacher bag, adding 40gm of sample to 200ml of 0.1% peptone water (Cat. no. U201). Note: For powdered products, shake periodically for 30 minutes using 0.1% peptone water.

2. Dilute the sample 1:10 in 0.1% peptone water.

3. Plate 0.1ml of the prepared sample per plate.

4. Incubate plates at 15-30ºC. and examine for growth for up to 7 days.

INTERPRETATION OF RESULTS

Observe and record the number of yeasts and/or molds present and report as the number of xerophilic colonies per gram of food.

LIMITATIONS

Complete classification of yeasts and molds is dependent upon microscopic examination of direct and/or slide culture preparations, in addition to biochemical and serological analysis.

Due to nutritional variation, some strains may grow poorly or fail to grow entirely on this medium.

MATERIALS REQUIRED BUT NOT PROVIDED

Standard microbiological supplies and equipment such as loops, swabs, applicator sticks, other culture media, 0.1% peptone water (Cat. no. U201), incinerators, and incubators, etc., as well as serological and biochemical reagents, are not provided.

QUALITY CONTROL

Test Organisms Inoculation Method* Incubation Results
Time Temperature Atmosphere
Saccharomyces cerevisiae
ATCC® 9763
A 1-7 days 15-30°C Aerobic Growth
Escherichia coli
ATCC® 25922
A 1-7 days 15-30°C Aerobic Partial to complete inhibition
Bacillus subtilis
ATCC® 6633
A 1-7 days 15-30°C Aerobic Partial to complete inhibition

USER QUALITY CONTROL

REFERENCES

1. American Public Health Association. Standard Methods for the Examination of Dairy Products, APHA, Washington, D.C.

2. APHA Technical Committee on Microbiological Methods for Foods. Compendium of Methods for the Microbiological Examination of Foods, APHA, Washington, D.C.

3. Anderson, N.L., et al. Cumitech 3B; Quality Systems in the Clinical Microbiology Laboratory, Coordinating ed., A.S. Weissfeld. American Society for Microbiology, Washington, D.C.

4. U.S. Food and Drug Administration. Bacteriological Analytical Manual. AOAC, Arlington, VA.
http://www.fda.gov/Food/FoodScienceResearch/LaboratoryMethods/ucm2006949.htm.

5. Beckers, H.J. et al. 1982. Inter. Stand. Org. Document ISO/TC34/SC9/N151.

6. Beuchat, L.R. and C.A. Hwang. 1996. Evaluation of Modified Dichloran 18% Glycerol (DG18) Agar for Enumerating Fungi in Wheat Flour: A Collaborative Study. Int. J. Food Microbiol.; 29:161-166.

7. Tille, P., et al. Bailey and Scott's Diagnostic Microbiology, C.V. Mosby Company, St. Louis, MO.

8. Hocking, A.D. and J.I. Pitt. 1980. Dichloran-glycerol Medium for Enumeration of Xerophilic Fungi from Low Moisture Foods. Appl. and Environ. Microbiol.; 39:488-492.

9. Isenberg, H.D. Clinical Microbiology Procedures Handbook, Vol. I, II & III. American Society for Microbiology, Washington, D.C.

10. Quality Assurance for Commercially Prepared Microbiological Culture Media, M22. Clinical and Laboratory Standards Institute (CLSI - formerly NCCLS), Wayne, PA.

11. U.S. Food and Drug Administration. Bacteriological Analytical Manual. AOAC, Arlington, VA.
http://www.fda.gov/Food/FoodScienceResearch/LaboratoryMethods/ucm2006949.htm.


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