58 DIFGOMANUAL 



methods specified in "Standard Methods for the Examination of Dairy Prod- 

 ucts."^ In determining the total bacterial counts of certified milk the use of 

 Tryptone Glucose Extract Milk Agar is approved by the Committee on Methods 

 and Standards for the Production of Certified Milk. 2 The Ninth Edition of 

 "Standard Methods for the Examination of Water and Sewage"^ permits the 

 use of Tryptone Glucose Extract Agar in place of Nutrient Agar for standard 

 plate counts of water. 



Bacto-Tryptone Glucose Extract Agar is a modification of the Tryptone Glu- 

 cose Skim Milk Agar of Bowers and Hucker.* Extensive investigations in many 

 widely separated laboratories established the superiority of their medium over 

 Nutrient Agar for estimations of bacteria in milk and other dairy products. 

 This work has been ably summarized by Yale^ in his report on the use of the 

 medium. Robertson^ has employed this medium in a study of the bacterial count 

 of ice cream, and Dennis and Weiser''' employed it in their study of the influence 

 of the incubation temperature on bacterial counts of milk. Prickett^ used a Glu- 

 cose Agar containing Bacto-Tryptone in his study of the thermophilic bacteria 

 in milk which was described in the Sixth Edition of "Standard Methods of Milk 

 Analysis"^ and was prepared in the dehydrated form as Bacto- Yeast Dextrose 

 Agar. "Standard Methods for the Examination of Dairy Products"^^ presently 

 recommends the use of Tryptone Glucose Extract Agar for the detection of 

 thermophilic bacteria. Media similar to that used by Prickett were recommended 

 by Downs, Hammer, Cordes, and Macy^^ in their report on the bacteriological 

 methods for the analysis of dairy products. 



A committee on Standard Methods for the Examination of Dairy Products 

 evaluated, comparatively, the modified medium of Bowers and Kucker to deter- 

 mine whether it was superior to the then standard Nutrient Agar for the plate 

 count of milk. As a result of these studies the committee^^.ia recommended the 

 adoption of Tryptone Glucose Extract Milk Agar as the standard medium for 

 the bacteriological plate count of milk. Details of the "Standard Methods" com- 

 mittee's studies are reported by Abele.^* 



Dilutions of the sample are plated according to Standard Methods procedures 

 on this medium and the plates incubated for 48 hours ± 3 hours, at 32°C. or 

 35 °C. Comparison of total counts with Tryptone Glucose Extract Milk Agar 

 and the former standard Nutrient Agar have indicated greatest differences in 

 low grade milk and milk products. Certified milk and other high grade milks 

 have shown little difference in final counts. 



The dehydrated medium does not contain skim milk since the "Standard 

 Methods" committee has recommended that skim milk be added to the medium 

 only when the dilution of the specimen is greater than 1 to 10. Bacto-Skim Milk, 

 as discussed on page 74, is recommended for the preparation of a standardized 

 skim milk solution to be used with Bacto-Tryptone Glucose Extract Agar in the 

 preparation of the skim milk medium. 



To rehydrate the medium, suspend 24 grams of Bacto-Tryptone Glucose 

 Extract Agar in 1000 ml. of cold distilled water. Tap water, or unsatisfactory 

 distilled water, may give a precipitate in the final medium. 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. When the medium is to be prepared with skim milk, 

 10 ml. of skim milk are added to one liter of medium just before sterilization. 



If directions for the preparation of the medium are not followed carefully, or 

 if the medium is subjected to excessive heat before, during or after the steriliza- 

 tion period, a precipitation may occur. The following recommendations to over- 

 come difficulties in the preparation of the medium are suggested. 



It is necessary to stir the suspended medium during the heating period to 



