DG 18 Metal Agar

Description

The DG18 Agar formulation used is that proposed by Hocking & Pitt in 1980, and it includes dichloran, which limits the size of fungal colonies more efficiently than rose bengal. Chloramphenicol inhibits bacterial growth and its thermostability allows it to be included in the medium before sterilization.

The inclusion of 18% (w/w) of glycerine gives the medium a water activity (aw) of 0.955 without causing any of the problems that generally occur when this water activity is provided by sodium chloride or sugar.

The trace metals complement the inhibition and sporulation.

Technique

Mass inoculation is recommended by spread plating using an inoculation loop, a swab or by spreading the sample with a Drigalski loop. Never use an inoculum volume greater than 0.1 mL.

According to the standardized technique, plates must be incubated at 22-25 °C, with partial readings after 3 and 5 days, and definitive readings after 7-8 days. Results are expressed in xerophiles-CFU/g or mL of food sample or CFU/m³ of air.

Plates of DG18 Agar in bags will keep for up to one week at (5 ± 3) °C in the dark. Due to its extreme water activity (aw = 0.955), the plates must be rejected if any kind of dehydration is suspected.

Each laboratory must evaluate the results according to their specifications.

Presumptive isolation of any xerophile must be confirmed by further microbiological and biochemical tests.

(Final volume: 1220 mL)

Physical/chemical control

Microbiological control

Inoculate: practical range 100 ± 20 CFU; min. 50 CFU (productivity) / 10⁴-10⁶ (selectivity).

Aerobic. Incubation at 22.5 ± 2 °C up to 5 days (moulds and yeast).

Sterility control

Incubation 48 hours at 30-35 °C and 48 hours at 20-25 °C: NO GROWTH.

Check at 7 days after incubation in same conditions.

• Atlas, R.M., et al. (1993) Handbook of Microbiological Media. CRC Press Inc. London.
• BEUCHAT, L.R. and C.A. HWANG (1995) Evaluation of modified dichloran 18% glycerol (DG18) agar for enumerating fungi in wheat flour. Int. J. Food Microbiol. 29:161-166.
• HOCKING, A.D. and J.I. PITT (1980) Dichloran-glycerol medium for enumeration of xerophilic fungi from low-moisture food. Appl. Environm. Microbiol. 39:488-492.
• ISO 11133:2014/Amd 1:2018. Microbiology of food, animal feed and water. Preparation, production, storage and performance testing of culture media.
• ISO 16000-17 Standard. (2008) Indoor air - Part 17: Detection and enumeration of moulds - Culture-based method.
• ISO 21527-2:2008. 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.
• PITT, J.I., and A.D. HOCKING (1985) Fungi and Food Spoilage. Academic Press. Sydney.
• PITT, J.I., A.D. HOCKING and D.R. GLENN (1983) An improved medium for the detection of Aspergillus flavus and A. parasiticus. J. appl. Bacteriol. 54:109-114.
• SAMSON, R.A., E.S. HOEKSTRA, J.C. FRISVAD and O. FILTENBORG (2002) Introduction to the Food Borne Fungi. 6th ed. Centraalbureau voor Schimmelcultures. Utrecht.
• TAPIA de DAZA, M.S. and L.R. BEUCHAT. (1992) Suitability of modified dichloran glycerol (DG18) agar for enumerating unstressed and stressed xerophilic molds. Food Microbiol. 9:319-333.