DETERMINATION OF BACTERIA IN MILK 579 



To take the fullest advantage of the medium the counting 

 procedure should be as follows : 



1. Make a total count. 



2. Count strong acid-forming colonies. Those with a cloudy zone 

 about them or a slight hazy edge. 



3. Count weak acid-forming colonies. This is done by flowing the 

 plate with brom-cresol purple and counting the total acid colonies and 

 subtracting the number of strong acid-forming colonies. 



4. Count peptonizers. Flow plate with a 5 per cent solution of acetic 

 acid before counting. 



5. Calculate the number of alkali formers and inert colonies. To do 

 this add the total number of acid colonies to the number of peptonizers 

 and subtract from the total count. 



MILK-POWDER MEDIUM "b" 



Feeling that there might be some objection to the use of a 

 medium which was slightly cloudy, it was decided to work out 

 a medium w T hich w r ould be clear. This milk-powder medium 

 differs from the first merely in the amounts of peptone and meat 

 extract used. To make this medium, simply substitute in the 

 formula for medium "A" the following: 2 grams of peptone and 

 1 gram meat extract instead of 5 grams peptone and 3 grams 

 extract. The medium in other respects is made exactly the 

 same. No adjustment of reaction is necessary, as it will be 

 about pH 7.0 to pH 7.1, brown or slightly red brown with phenol 

 red. 



The counts obtained on this medium and other media are 

 shown in table 2. It will be seen that they are just as high on 

 the whole as on milk-powder medium "A" and in many cases 

 higher. The colonies of strong acid-forming bacteria can be 

 seen on this medium with a small white zone about them. They 

 are not so distinct, however, as on the milk-powder medium 

 "A." In order to distinguish colonies of peptonizing bacteria 

 it is necessary to flow the plate with 5 per cent acetic acid. This 

 precipitates the casein and causes the clear medium to have 

 a w r hite, opaque appearance. After flowing with acid, clear 

 zones may be seen about the colonies of the peptonizers. 



