64 DIFCOMANUAL 



BACTO 



DESOXYGHOLATE LACTOSE AGAR (B420) 



DEHYDRATED 



Bacto-Peptone 10 g 



Bacto-Lactose 10 g 



Sodium Desoxycholate 0.5 g. 



Sodium Chloride 5.0 g 



Sodium Citrate 2.0 g 



Bacto-Agar 15 g 



Bacto-Neutral Red 0.03 g 



Bacto-Desoxycholate Lactose Agar is used for the direct enumeration of coli- 

 form organisms in milk, water and other materials. It is a modification of the 

 medium described by Leifson,^ and is prepared according to the formula speci- 

 fied for Desoxycholate Lactose Agar in "Standard Methods for the Examination 

 of Water and Sewage"^ and "Methods and Standards for the Production of 

 Certified Milk."^ 



Counts of coliform organisms may be made directly using Desoxycholate 

 Lactose Agar. Gram-positive cocci and rods including the spore-forming bacilli 

 are generally inhibited. Lactose fermenting coliform organisms grow unrestricted 

 and form typical red subsurface colonies, surrounded by a zone of precipitated 

 bile and indicator. Incubation periods should be 24 hours or less since other 

 organisms may develop, on longer incubation causing confusion. 



A thin layer of sterile medium may be used as a cover, thus having only sub- 

 surface colonies. Following solidification of the inoculated plate a small amount 

 of sterile medium (about 4—5 ml. per 95 mm. plate) is added and allowed to 

 solidify forming a thin cover layer. 



Bacto-Desoxycholate Lactose Agar differs from Bacto-Desoxycholate Agar 

 essentially in that it contains less sodium desoxycholate, and is accordingly 

 slightly less selective against Gram-positive organisms. It is suggested that the 

 medium be sterilized in the autoclave if not used immediately following heating 

 to boiling to effect complete solution. 



To rehydrate the medium suspend 42.5 grams Bacto-Desoxycholate Lactose 

 Agar in 1000 ml. of cold distilled water and heat to boiling to dissolve the 

 medium completely. The medium requires no further sterilization if used at 

 once. If it is to be stored, distribute in tubes or flasks and sterilize in the auto- 

 clave for 15 minutes at 15 pounds pressure (121°C.). Avoid overheating of the 

 medium. Final reaction of the medium will be pH 7.1. 



One pound of Bacto-Desoxycholate Lactose Agar will make 10.7 liters of 

 medium. 



1 J. Path. Bact., 40:581:1935. a Methods and Standards for the Production 



2 Standard Methods for the Examination of Certified Milk, 26:1953-1954. 

 of Water and Sewage, 9th Edition: 228: 1946. 



BACTO 



POTATO DEXTROSE AGAR (B13) 



DEHYDRATED 



Potatoes, Infusion from 200 g. 



Bacto-Dextrose 20 g. 



Bacto-Agar 15 g. 



Bacto-Potato Dextrose Agar is recommended for the determination of yeasts 

 and molds in butter and in frozen dessert ingredients according to the procedure 



