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Material Safety Data Sheet

Organisms

Organisms this product works with:

Dehydrated Culture Media

DICHLORAN-GLYCEROL (DG18) AGAR BASE

Code: CM0729

a selective low water activity (aw ) medium for xerophilic moulds from dried and semi-dried foods as well as a general purpose medium for counting yeasts and moulds in foodstuffs

Typical Formula*

gm/litre

Peptone

5.0

Glucose

10.0

Potassium dihydrogen phosphate

1.0

Magnesium sulphate

0.5

Dichloran

0.002

Agar

15.0

Final pH 5.6 ± 0.2 @ 25°C

 
* Adjusted as required to meet performance standards

CHLORAMPHENICOL SELECTIVE SUPPLEMENT

Code: SR0078

Vial contents

 SR0078E
(1 vial per 500ml medium)

SR0078H
(1 vial per 2 litres medium)

per litre

Chloramphenicol

50mg

200mg

100mg

 

Directions
Suspend 15.75g in 500ml (31.5g/l) of distilled water and heat to dissolve completely. Add 110g per 500ml (220g/l) of Glycerol (Analytical Reagent grade). Reconstitute one vial SR0078E per 500ml medium or one vial SR0078H per 2 litres medium, as directed. Add the vial contents to the DG18 Agar Base. Sterilise by autoclaving at 121°C for 15 minutes. Cool to 50°C, mix well and pour into sterile Petri dishes.

Description
Dichloran-Glycerol (DG18) Agar Base is based on the formulation described by Hocking and Pitt1 and 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 16% (w/w) which lowers the water activity (aW) from 0.999 to 0.95. Glycerol was chosen because of advantages it showed over sodium chloride and sugars which have traditionally been used to formulate media of reduced aW1. 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 unobscured growth of organisms that ordinarily form small colonies.
A modification to the formula has been described in which the addition of Triton-X to DG18 agar increases the inhibition of vigorously-spreading fungi2.

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 and Wallemia sebi, grow very poorly or not at all on DRBC.

Technique

  1. Prepare the DG18 medium as directed using CM0729, SR0078 and glycerol.
  2. Process the food sample in a Seward `Stomacher’ adding 40g to 200ml of 0.1% peptone water. For powdered products shake periodically for 30 minutes with 0.1% peptone water.
  3. Dilute the sample 1:10 in 0.1% peptone water.
  4. Surface plate 0.1ml of the prepared sample per plate.
  5. Incubate at 25°C and examine after 4, 5 and 6 days.
  6. Report as number of xerophilic colonies per gram of food.

    Further experience with this medium has shown it to be a good general purpose medium. In a collaborative exercise in Holland the DG18 medium gave the best results for yeasts and moulds isolated from foodstuffs2.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared plates at 2-8°C.

Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Straw coloured gel

Quality control

Positive controls:

Expected results

Saccharomyces cerevisiae ATCC® 9763*Good growth; cream coloured colonies
Aspergillus niger ATCC®9642*

White / Yellow mycelium, black spores

Negative controls:

 
Escherichia coli ATCC® 25922 *No growth

Bacillus subtilis ATCC® 6633*

No growth

* This organism is available as a Culti-Loop®

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

References
1. Hocking A. D. and Pitt J. I. (1980) J. Appl. & Env. Microbiol. 39. 488-492.
2. Beuchat L. R. and Hwang C. A. (1996) Int. J. Food Microbiol. 29. 161-166.
3. Beckers H. J., Boer E., van Eikelenboom C., Hartog B. J., Kuik D., Mol N., Nooitgedagt A. J., Northolt M. O. and Samson R. A. (1982) Intern. Stand. Org. Document ISO/TC34/SC9/N151.

 
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