CONTROL OF THE PUBLIC MILK SUPPLY 



439 



Methods of Making the Bacterial Count. There are three methods of 

 making bacterial counts, namely: (1) by the plate method of Koch; 

 (2) by the direct method of counting the bacteria in a smear prepared 

 according to the Breed method, or by the less accurate method of Slack; 

 and (3) according to the stained incubated slide method of Frost. The 

 Koch method is the standard one; it consists of growing the organisms 

 at a chosen temperature and for a definite time usually at 68F. for 

 48 hr. in specially prepared nutrient media in petri dishes and counting 

 the organisms that develop. The summary of the advantages and dis- 

 advantages of the Breed and of the Koch plate methods as given by 

 Brew appears in Table 117. 



TABLE 117. THE DISADVANTAGES AND ADVANTAGES OP THE MICROSCOPICAL AND 

 METHODS OP PLATE MAKING BACTERIAL COUNTS COMPARED (BREW) 



Disadvantages 

 Breed method 



1. Difficult to measure so small a quan- 

 tity of milk accurately. 



2. The sample measured is too small to 

 be representative. 



3. Dead bacteria may be counted. 



4. Error of count is great where bac- 

 teria are very few or many. 



5. Cannot be used for quantitative 

 work when the bacteria are few in 

 number. 



6. Many fields must be counted, be- 

 cause of the uneven distribution, if an 

 accurate count is required. 



7. Large compact clumps cannot be 

 counted. 



8. Bacteria may be lost in process of 

 preparing slides. 



Plate method 



1. All bacteria do not grow on the 

 plates because of changes in food, tem- 

 perature relations or other conditions of 

 environment. 



2. The difficulty of breaking up the 

 clumps in the milk affects the accuracy 

 of the count. 



3. Requires from 2 to 5 days' incuba- 

 tion period. 



4. Different species require different 

 incubation temperatures. 



5. Gives no idea of the morphology of 

 the organisms present. 



6. More apparatus is required; there- 

 fore, is more expensive. Technique 

 complicated and difficult for trained 

 bacteriologists to use in such a way as to 

 get consistent results. 



Advantages 



1. Less apparatus required, therefore 

 less expensive. Technique simple. 



2. The results on a given sample can be 

 reported within a few minutes. 



3. Shows the cell content, the presence 

 or absence of streptococci and other im- 

 portant things necessary in estimating 

 the sanitary quality of milk. 



4. Gives a better idea of the actual 

 number of germs present. 



1. Is necessary for isolation of pure 

 cultures. 



2. Gelatin shows the liquefiers and if 

 litmus is used, the acid producing bac- 

 teria. 



3. Shows character of growth. 



4. Shows living organisms only. 



There are two ways of counting the Breed smears. By one method each 

 cell in a chain of streptococci or in a clump of bacteria is counted, with 

 the result that roughly about 10 times the number of bacteria shown by 



