Thermo Scientific™ Desoxycholate Agar

Desoxycholate Agar is a differential medium for the direct count of coliforms in dairy products (American Public Health Association¹).

The medium is used in a `pour-plate’ technique or as a surface inoculated medium. A thin layer of uninoculated desoxycholate agar poured over the surface of a gelled `pour-plate’ assists subsequent counting.

Technique: Enumeration of Coliforms in Milk and Cream (APHA1) 1. Pipette 1-4ml of the sample (or decimal dilution of the sample) into a sterile Petri dish. 2. Cool freshly prepared Desoxycholate Agar to 42-44°C and add 10-20ml to each dish. 3. Mix the contents of the dishes by gentle tilting and rotation. 4. Allow the plates to solidify and pour on an overlay of 3-4ml of uninoculated Desoxycholate Agar. 5. When the overlay has set, invert the plates and incubate them for 18-24 hours at 35°C. 6. Count all dark red colonies measuring at least 0.5mm in diameter and calculate the number of coliform colonies per millilitre or gram of original sample. Isolation of Enterobacteriaceae It is advisable to use Desoxycholate Agar in parallel with other plating media for this purpose. Lightly inoculate a Desoxycholate Agar plate with faeces, rectal swab, or enrichment culture. Incubate for 18-24 hours at 35°C and examine. Non-lactose fermenters of enteric origin form colourless colonies. Non-lactose fermenters which are not of enteric origin are generally inhibited by the sodium desoxycholate in the medium. Identify suspect colonies in the usual manner. The selective agents in this medium may have inhibitory effects on the colony forming ability of Escherichia coli: the degree of inhibition varies with strains. The inhibition may be more pronounced when the cells are enumerated by pour plate rather than surface methods².
 

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