68 DIFCO MANUAL 



SUPPLEMENTARY MEDIA 



for Dairy and Other Food Products 



The routine bacteriological examination of dairy and other food products may 

 usually be performed upon the standard media listed in the previous section. 

 Special examinations including visual sanitation tests, however, require media 

 for specific purposes. Propagation of pure cultures, and the maintenance of stock 

 cultures also require special media, and the dehydrated culture media listed in 

 this section are prepared for such uses. 



BACTO 



PROTEOSE TRYPTONE AGAR (B23) 



DEHYDRATED 



Bacto-Beef Extract 3 g. 



Proteose Peptone, Difco 5 g. 



Bacto-Tryptone 5 g. 



Sodium Chloride 5 g. 



Bacto-Dextrose 1 g. 



Bacto-Agar 15 g. 



Bacto-Proteose Tryptone Agar is recommended for determining the bacterial 

 plate count of certified milk. This medium is prepared according to the formula 

 specified in "Methods and Standards for the Production of Certified Milk"^ of 

 the American Association of Medical Milk Commissions, Incorporated. 



The combination of Proteose Peptone, Difco, and Bacto-Tryptone in place of 

 a single peptone is excellently suited for the cultivation of bacteria ordinarily en- 

 countered in milk. The larger size of the colonies which develop on Proteose 

 Tryptone Agar during the usual incubation period facilitates counting of the 

 plate. Comparative study of Bacto-Proteose Tryptone Agar by the Committee on 

 Methods and Standards has indicated that this dehydrated medium is satisfactory 

 for determining total counts of bacteria in certified milk. Plate counts are made 

 according to the usual procedure with 0.1 ml. and 0.01 ml. (1 ml. of 1 to 10 and 

 1 to 100 dilution) of certified milk and the plates incubated for 48 hours at 35- 

 37°C. For the bacterial count of milk according to "Standard Methods for the 

 Examination of Dairy Products," Bacto-Tryptone Glucose Extract Agar, as dis- 

 cussed on page 57, should be employed. 



Proteose Tryptone Agar has also been used as a plating medium to determine 

 total counts of food products. Quinn and Garnatz^ used Bacto-Proteose Tryptone 

 Agar for total bacterial count of whole eggs in their study of methods of thaw- 

 ing frozen eggs. 



To rehydrate the medium, suspend 34 grams of Bacto-Proteose Tryptone Agar 

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

 completely. Distribute in tubes or flasks and sterilize in the autoclave for 15 

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

 be pH 7.0. 



One pound of Bacto-Proteose Tryptone Agar will make 13.3 liters of medium. 



1 Methods and Standards for the Production ^ J. Bact., 45:49: i943. 



of Certified Milk, 26:1953-1954. 



