BACTERIA IN MILK 1039 



arranged that the microscopic field covers 1/50 sq. mm. A standard- 

 ized eyepiece micrometer may be used. The average number of 

 bacteria found in such fields may be multiplied by 5,000 to give 

 the number of bacteria contained in 0.01 c.c. of milk. This method 

 nas not yet displaced the one of plating and does not promise to 

 do so for some time. 



For the isolation of special pathogenic bacteria from milk, no 

 rules can be laid down, since, in every case, the method adapted 

 to the particular organism sought for must be chosen. 



Tubercle bacilli can be isolated from milk with success only by 

 guinea-pig injection. The milk is centrifugalized and 5 c.c. of the 

 sediment, together with some of the cream that has risen to the top, 

 is intraperitoneally or subcutaneously injected. 



The control of milk in the market depends upon careful regula- 

 tions, which must include care of cattle, dairy inspection and bac- 

 teriological control of the delivered milk. This is a subject which 

 is too extensive to touch upon in a book of this kind. However, 

 a general idea of the methods employed may be obtained by studying 

 the accompanying table, which is taken from the New York City 

 Department of Health Regulation for the Sale of Milk and Cream. 



Bacteria and Butter. Butter is made from cream separated from 

 milk either by standing or by centrifugalization. After this, the 

 cream is agitated by churning, which brings the small fat-globules 

 into mutual contact, allows them to adhere to each other and form 

 clumps of butter. It has been a matter of common experience, how- 

 ever, that unless the cream is allowed to "ripen" for a considerable 

 period before churning, the resulting butter lacks the particular 

 quality of flavor which gives it its market value. The interval of 

 ripening, at first a necessity upon small farms where cream must 

 be collected and allowed to accumulate, has now been recognized 

 as an essential for the production of the best grades of butter, and 

 it has been shown that the changes taking place in the cream during 

 this period are referable to the action of bacteria. Cream, which 

 before the ripening process contains but 50,000 bacteria to each cubic 

 centimeter, at the end of a period of " ripening" will often contain 

 many millions of microorganisms. At the same time, the cream 

 becomes thick and often sour. 



The species of bacteria which take part in this process and which, 

 therefore, must determine to a large extent the quality of the end 

 product, are various and, as yet, incompletely known. Usually some 



