METHODS OF EXAMINING MILK 271 



experienced man is apt to overlook some of them, giving 

 an undercount. These difficulties are overcome, how- 

 ever, by training and experience. 



COMPARISON OF RESULTS OBTAINED BY THE TWO METHODS 



" It must be recognized that the results obtained from 

 the microscopic record give a closer approximation to 

 the actual number of bacteria present in the milk than 

 those obtained by the plate method, since the plate 

 method will count as one either a single bacterium or a 

 group which may sometimes contain a hundred or even 

 more individuals. Inasmuch, however, as the plate count 

 has become a method of analysis that is well known and 

 commonly applied, it becomes desirable to know as closely 

 as possible what relations there may be between the plate 

 count and the microscopic count. Experience has shown 

 that the count of individual bacteria is ordinarily 1.5 to 8 

 times as great as the plate count, the ratio between the 

 two being largely dependent upon the size of the clumps 

 of bacteria present. Where the bacteria are mostly iso- 

 lated, the ratio of the two counts would be much closer 

 than where there are present long chains of streptococci 

 or masses of cocci. After one has had a little experience 

 in counting clumps it is found that the number of groups 

 shown by the microscope agrees fairly well with the num- 

 ber of colonies shown by the plate count, though even here 

 there are occasionally discrepancies, due among other 

 things to the appearance in the microscope of kinds of 

 bacteria which fail to grow in the culture media used in 

 making plates. In all cases, however, the direct count of 

 raw milk will give a much closer approximation to the 

 actual numbers of bacteria than the plate count. In view 

 of these facts it is difficult to interpret one count in terms 



