190 DIFGO MANUAL 



pounds pressure (121°C.). The minimum amount of heat required for steriliza- 

 tion is to be desired, so as to avoid hydrolysis of the carbohydrate. By packing 

 the tubes loosely in the autoclave to allow free circulation of steam, the time 

 required may be appreciably shortened, provided the temperature in the auto- 

 clave is actually 121 °C. The final reaction of the medium without added carbo- 

 hydrate, will be pH 6.8. 



The addition of some carbohydrates may result in an acid reaction. In this case 

 it is suggested that O.IN sodium hydroxide be added drop by drop to restore 

 original color. 



One pound of Bacto-Purple Broth Base will make 28.3 liters of medium. 



BACTO 



PURPLE AGAR BASE (B228) 



DEHYDRATED 



Bacto-Beef Extract 1 g. 



Proteose Peptone No. 3, Difco ... 10 g. 



Sodium Chloride 5 g. 



Bacto-Agar 15 g. 



Bacto-Brom Cresol Purple 0.02 g. 



Bacto-Purple Agar Base is recommended for the preparation of carbohydrate 

 agars used in fermentation studies for the cultural identification of pure cultures 

 of microorganisms, particularly members of the enteric group. Although broth 

 media are generally employed in studying the fermentation reactions of micro- 

 organisms, many bacteriologists prefer a solid medium for this purpose. Bacto- 

 Purple Agar Base is available for those bacteriologists preferring a fermentation 

 medium of slightly acid reaction (pH 6.8). This medium supports luxuriant 

 growth of organisms and with the indicator brom cresol purple gives clear-cut 

 reactions from slight changes in the pH of the medium. This medium is free 

 from fermentable carbohydrates which could give erroneous results. 



To rehydrate the medium, suspend 31 grams of Bacto-Purple Agar Base in 

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

 pletely. To this solution add 5-10 grams of the test carbohydrate, and, when 

 dissolved, dispense into culture tubes and sterilize in the autoclave for not more 

 than 15 minutes at 15 pounds pressure (121°C.). The final reaction of the 

 medium without added carbohydrate will be pH 6.8. 



The addition of some carbohydrates may result in an acid reaction. In this 

 case, it is suggested that O.IN sodium hydroxide be added drop by drop to restore 

 the original color. 



One pound of Bacto-Purple Agar Base will make 14.6 liters of medium. 



BACTO 



SNYDER TEST AGAR (B247) 



DEHYDRATED 



Bacto-Tryptone 20 g. 



Bacto-Dextrose 20 g. 



Sodium Chloride 5 g. 



Bacto-Agar 20 g. 



Bacto-Brom Cresol Green 0.02 g. 



Bacto-Snyder test Agar is prepared for the colorimetric diagnosis of caries 

 activity as described by Snyder.^'^ The method is based on acid production in a 



