154 LABORATORY GUIDE IN BACTERIOLOGY 



SECTION i 



PREPARATION OF CULTURE MEDIA AND OF DILU- 

 TION FLASKS 



The same culture media will be used as in the bac- 

 teriological examination of water (see p. 144), with 

 this difference, that dextrose agar only is to be prepared, 

 instead of lactose agar. In addition about ten tubes 

 of whey agar are to be prepared. 



SECTION 2 



QUANTITATIVE BACTERIOLOGICAL EXAMINATION 

 OF MILK 



1 . Secure samples of raw market milk, of pasteurized 

 market milk, of certified milk, and of cream. 



2. Prepare dilutions as follows: 



Raw market milk, i : 1,000 and i : 10,000. 

 Pasteurized market milk, i : 100 and i : 1,000. 

 Certified milk, i : 100. 

 Cream, i : 1,000 and i : 10,000. 



3. Plate these in dextrose litmus agar, making 

 duplicate plates from each dilution. 



4. Incubate one set at 37 C, the other set at 20 C. 

 The incubation period at 37 C. is 48 hours, at 20 C. 

 72 hours. 



5. Count the colonies and express the results in 

 numbers of bacteria per cubic centimeter of milk. 



6. Make a differential count of acid-forming colonies 

 and non-acid-forming colonies. 



