IS4 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. i44^j 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 : i,ooo and i : 10,000. 

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

 Certified milk, i : 100. 

 Cream, 1 : 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. 



