400 MITK AND MILK PRODUCTS 
may be between the plate count and the microscopic count. Experi- 
ence 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 isolated, the ratio of the two counts 
would be much closer than where there are present long chains of strep- 
tococci 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 number of colonies shown by 
the plate count, though even here there arc 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 of the other; but a few suggestions will give a fairly satisfactory 
idea as to how the two may be related. 
Grade A raw milk, which should have less than 100,000 bacteria 
per cubic centimeter, will not show more than three to four small clumps 
of bacteria for each 30 fields of the microscope where the diameter of 
the fields is .205 mm. Such milk also ought not to contain more than 
500,000 individual bacteria per cubic centimeter when counted by the 
microscope. For Grade A pasteurized milk (which should have less 
than 200,000 per cubic centimeter by the plate count before pasteuri- 
zation) the microscope should not show more than six to eight clumps 
per 30 microscopic fields, and not more than 1,000,000 individual bac- 
teria when counted with the microscope. 
Grade B milk, which is supposed not to have more than 1,000,000 
bacteria before pasteurization, when counted by the plating method, 
should not show more than 20 individual bacteria per field, where the 
diameter of the fields is .205 mm., and not more than three to four 
groups of bacteria per field. 
While the above relation between the plate count and the microscopic 
counts cannot be relied upon as having a very great amount of accuracy, 
it will serve to give a general idea of the ratio between the two under 
ordinary conditions, and may serve as a guide in the use of direct 
microscopic method. 
The direct microscopic method is not as yet recommended by this 
committee as a method of estimating the numbers of bacteria that are 
present in samples of milk. For this purpose the plate method, which 
has long been in use and is fairly well understood, is still recommended as 
