Desoxycholate agar

Desoxycholate Agar is recommended by APHA for the counting of coliforms in dairy products. APHA recommend placing a sample (1 ml) in the petri dish and preparing a pour plate of medium. Once this has set, a further 5 ml of uninoculated medium is poured over the plate. Plates are incubated at 35 °G for 18-24 hours and coliform numbers can be estimated by counting all dark red colonies. Enteric bacteria which are non-lactose fermenters form colourless colonies, whilst non-enteric bacteria are inhibited by the desoxycholate. 

Xylose lysine desoxycholate agar is also useful for distinguishing Salmonella and Shigella, Salmonella producing red colonies with black centres, and Shigella producing red colonies without black centres. Non-pathogenic coliforms produce yellow colonies. 

Vibrio species are difficult to isolate from natural sources as they tend to be present with coliforms, which outgrow them on many media. Several selective media are available, including DCLS Agar which contains sodium desoxycholate, lactose, sucrose, and the indicator Neutral Red, the desoxycholate suppressing the growth of coliforms. Cholera medium works in a similar manner, in that high levels of ox bile are used to suppress the growth of the normal gut flora. 

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