PRE-SUPPLEMENTED DICHLORAN-GLYCEROL 18% (DG18) (ISO) AGAR BASE

Code: CM1151

Pre-supplemented Dichloran-Glycerol 18% (DG18) (ISO) is a selective medium for yeasts and moulds associated with food spoilage.

* Adjusted as required to meet performance standards

Directions
Suspend 15.8g per 500ml (31.6g/l) of distilled water and heat to dissolve completely. Before autoclaving add 220g of Glycerol per 1 litre of medium. Heat the complete solution gently to aid its dissolution of the glycerol. This has been found to be a key procedure to follow to allow proper dissolution of the glycerol. Sterilize by autoclaving at 121°C for 15 minutes. Cool to 50°C, mix well and pour into sterile Petri dishes.

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

Description
Pre-supplemented Dichloran-Glycerol (DG18) (ISO) Agar is a dehydrated culture medium formulated according to BS ISO 21527-2: 2008 Microbiology of food and animal feeding stuffs – Enumeration of yeasts and moulds Part 1- Colony counting technique in products with less than or equal to 0.95 water activity¹. It is recommended for the enumeration and isolation of xerophilic moulds from dried and semi-dried foods. Examples of these are dried fruits, spices, confectionery, cereals, nuts and dried meat and fish products.

The medium formulation contains glycerol at 18% (w/w) which lowers the water activity (a_W) from 0.999 to 0.95. The medium also contains Chloramphenicol at 0.1g/L as per the ISO 21527-2 formulation. The addition of this antibiotic into the powder aids in reducing hamful chemical handling in the laboratory and provides an agar medium that only needs glycerol adding. The medium also contains Dichloran which inhibits spreading of mucoraceous fungi and restricts the colony size of other genera. This restrictive characteristic makes the medium especially suitable for enumeration because it allows growth of background organisms, which form only small colonies, not obscuring mould or yeast identification and enumeration. In a comparative study carried out between DG18 and DRBC, (a medium of higher a_W) greater recovery of xerophilic moulds was achieved on the DG18 medium¹. In this study it was found that two of the fungi commonly isolated from dried foods in high numbers, Aspergillus penicilloides grow very poorly or not at all on DRBC.

Technique

1. Prepare the DG18 (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 DG18 (ISO) Agar.
4. Incubate the plates at 25°C and examine after 3, 4 and 5 days.
5. Report as number of colonies per 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: Straw coloured, free-flowing powder
Prepared medium: Pinkj coloured gel

Quality control

* This organism is available as a Thermo Scientific Culti-Loop® Quality Control Organism

Precautions
The dichloran compound used in this medium is Botran® 2,6-Dichloro-4-Nitro-Analine (CAS: 99-30-9).

Pre-supplemented DG18 (ISO) Agar (CM1151) contains Chloramphenicol in the base powder and is therefore considered a toxic compound which should be handled in accordance with good industrial hygiene and safety practice. Always consult the MSDS before handling the powder.

References
1. BS ISO 21527-2: 2008 Microbiology of food and animal feeding stuffs - Enumeration of yeasts and moulds Part 1- Colony counting technique in products with 2. Hocking A. D. and Pitt J. I. (1980) J. Appl. & Env. Microbiol. 39. 488-492.
3. Kramer C. L and Pady S. M. (1961) Trans. Kan. Acad. Sci. 64. 100-116.