DETERMINATION OF BACTERIA IN MILK 573 



with phenol red, and purple with brom-cresol purple. From 

 this computation it is easy to figure the amount of N HC1 required 

 to bring the total volume of solution to the proper pH. If the 

 color is greenish or yellow with brom-cresol purple add ■£> NaOH 

 and calculate the amount of N NaOH necessary to bring the 

 total volume of solution to a point where it is purple with brom- 

 cresol purple and yellow with phenol red. This is a very rough 

 way of obtaining a reaction of about pH 6.8 but is accurate 

 enough for this medium. This process need only be carried out, 

 as previously stated, with each new lot of ingredients used. 

 The addition of washed agar does not appreciably affect the 

 reaction. 



To prepare the stock solution of phenol red indicator take 

 0.2 gram of the powdered phenol red and add 11.4 cc. of ^ NaOH. 

 Warm and agitate until dissolved, then make up to 100 cc.with 

 distilled water. Take one volume of this stock solution and 9 

 volumes of distilled water to make the test solution. Use 5 

 drops of this test solution to 10 cc. of medium at 45°C. in a 

 test tube. 



The stock solution of brom-cresol purple is made by adding 

 14.8 cc. w NaOH to 0.4 gram of powdered brom-cresol purple. 

 This is warmed and when dissolved is made up to 100 cc. with 

 distilled water. To make the test indicator solution mix one 

 volume of stock solution with 9 volumes of distilled water. 

 Use 5 drops to a tube with 10 cc. of medium at 45°C. to indicate 

 the hydrogen-ion concentration. 



COUNTS OBTAINED WITH THE MEDIUM 



Comparisons were made of the counts on standard extract 

 agar, standard infusion agar (1910 standard methods) and milk- 

 powder medium. The plates were incubated at 37°C. for forty- 

 eight hours and a hand lens was used in counting. Various kinds 

 of milk were examined, such as raw, pasteurized, and certified 

 milk. The results of this work are shown in table 1. It will 

 be seen that with three exceptions the counts on milk-powder 

 agar were higher than on the standard extract agar. With the 



