270 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



all, while a Grade C milk will show the field crowded with 

 bacteria. In all doubtful cases, however, counting should 

 be done, and there should never be less than thirty fields 

 counted in order to have reliable results. Counting thirty 

 fields is not so tedious a task as it would seem to be, since 

 in ordinary milk the number of bacteria in each field is 

 small, and the counting may be done very rapidly." 



Counting. " Counting the bacteria in such a smear 

 may be done in two ways: 1. The number of groups of 

 one or more bacteria present. 2. The number of indi- 

 viduals. The second, of course, is really the correct count 

 of the number of bacteria, but the former will give a 

 count much closer to that obtained by the plate count, 

 since the colonies upon the plate represent groups of bac- 

 teria rather than individuals, each group growing into 

 a single colony only. Extensive tests have shown that 

 there is a fair correspondence between the number of 

 groups reported by experienced observers and the num- 

 ber of colonies that may grow in plates made from the 

 same milk, although there are occasionally discrepancies 

 of considerable extent. These discrepancies are caused 

 by variations in judgment as to what constitutes a 

 group, variations in the extent to which groups break 

 up in the dilution waters when the smears are made, 

 and the presence of dead bacteria or of bacteria which do 

 not grow on the plates. Some experience is needed by 

 the microscopist in determining just what should be 

 counted. In high-grade milks an inexperienced person 

 is apt to fail to recognize differences between bacteria 

 and other minute objects. This results as a rule in an 

 overcount by inexperienced men. In milk containing 

 many readily recognizable bacteria in each field the in- 



