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Dehydrated Culture Media

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

Code: CM1150

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

Typical Formula*

gm/litre

Casein Enzymatic digest

5.0

Glucose

10.0

Potassium dihydrogen phosphate

1.0

Magnesium sulphate

0.5

Dichloran (2,6-dichloro-4-nitroaniline)

0.002

Agar

15

pH 5.6 ± 0.2 @ 25°C

 
* Adjusted as required to meet performance standards

Directions
Suspend 15.75g per 500ml (31.5g/l) of distilled water and heat to dissolve completely. Reconstitute Chloramphenicol Supplement (one vial SR0078E per 500ml or one vial SR0078H per 2 litres medium), as directed. Add the vial contents to the DG18 (ISO) Agar Base. Before autoclaving add 220g of Glycerol per 1 litre of medium. Heat the complete solution gently to aid is 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
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 (aW) from 0.999 to 0.95. 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 aW) 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

Control organism

Expected results

Saccaromyces cerevisiae ATCC®9763

1-3mm Cream colonies

Wallemia sebi ATCC® 42694

0.5-5mm Brown mycelia with or without brown spores

Aspergillus caesiellus ATCC® 42693 0.5-6mm White mycelia with or without green spores
Eurotium rubrum ATCC® 42690  
Bacillus subtilis ATCC® 6633* No growth
Escherichia coli ATCC® 25922* No growth
* 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).

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.

 
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