DEHYDRATED CULTURE MEDIA 63 



BACTO 



DESOXYGHOLATE AGAR (B273) 



DEHYDRATED 



Bacto-Peptone 10 g. 



Bacto-Lactose 10 g. 



Sodium Desoxycholate 1 g. 



Sodium Chloride 5 g. 



Dipotassium Phosphate 2 g. 



Ferric Citrate 1 g. 



Sodium Citrate 1 g. 



Bacto-Agar 15 g. 



Bacto-Neutral Red 0.03 g. 



Bacto-Desoxycholate Agar is used for the direct enumeration of coliform bac- 

 teria in dairy products as specified in "Standard Methods for the Examination 

 of Dairy Products."^ It may also be employed as a non-selective primary plating 

 medium for the isolation of enteric pathogens. The formula of Bacto-Desoxycho- 

 late Agar is essentially that described by Leifson^ and is recommended for all 

 procedures specifying the use of a Desoxycholate Agar. 



For presumptive determination of coliforms in dairy products a poured plate 

 of Desoxycholate Agar is prepared. After the medium has solidified, many in- 

 vestigators prefer to add a thin cover layer of uninoculated medium. Plates are 

 incubated at 37 °C. for 18-24 hours. Organisms other than those of the enteric 

 group are inhibited. Coliform colonies are red in contrast to light colorless col- 

 onies produced by enteric organisms not capable of attacking lactose. 



For the isolation of enteric pathogens streak or smear plates are prepared. 

 Plates should be dry before inoculation for best results. On this medium coli- 

 form organisms are not inhibited and develop red opaque colonies. Colonies of 

 Salmonella are colorless and raised with margins varying from smooth to irregu- 

 lar. Shigella colonies are colorless or opaque presenting somewhat of a ground 

 glass appearance. 



Saccharose in 1 per cent concentration may be added to isolation media, such 

 as Bacto-Desoxycholate Agar to permit the detection of certain members of the 

 coliform group which ferment saccharose more readily than lactose. This prin- 

 ciple was described by Holt-Harris and Teague^ and has been employed by 

 many other bacteriologists. In some laboratories pathogenic significance is as- 

 signed to these organisms, and under such conditions, saccharose should not be 

 added to the medium. 



To rehydrate the medium, suspend 45 grams of Bacto-Desoxycholate Agar in 

 1000 ml. of cold distilled water and heat to boiling to dissolve the medium com- 

 pletely. Do not sterilize in the autoclave. Cool the dissolved liquid medium to 

 45-50 °C. before using it in making poured plates for the coliform count. After 

 the inoculated medium in the petri dish has solidified, cover the surface with a 

 thin layer of the medium cooled to 45-50° C. The final reaction of the medium 

 will be pH 7.3. 



One pound of Bacto-Desoxycholate Agar will make 10 liters of medium. 



-^Standard Methods for the Examination 2 j. Path. Bact., 40:581:1935. 



of Dairy Products, 9th Edition: 132: 1948. 3 J. Infectious Diseases, 18:596:1916. 



