710 BACTERIA IN AIR, SOIL, WATER, AND MILK 



of 1 : 100 is made, quantities ranging from 1 c.c. to 0.1 c.c. of this will 

 furnish 0.01 c.c. to 0.001 c.c. of the milk, respectively. Inoculation of 

 properly cooled tubes of melted neutral agar and gelatin, with varying 

 quantities of these dilutions, are then made and plates poured. After 

 twenty-four to forty-eight hours at room temperature or in the in- 

 cubator, colony counting is done as described upon page 161, and the 

 proper calculation is made. In samples in which few bacteria are ex- 

 pected, direct transference of 1/20 or 1/40 of a c.c. of the whole milk 

 into the agar may be made. This saves time but is less accurate than 

 the method given above. 



Direct methods of counting bacteria in milk have recently been 

 advised, the one most extensively tried being that of Prescott and 

 Breed. By this method a capillary tube is marked to measure accu- 

 rately 0.01 c.c. This amount of the milk is spread over a square cm. 

 on a microscope slide. It is dried in the air and fixed with methyl 

 alcohol, after which the fatty constituents can be solved off with xylo. 

 It can then be stained lightly by the Jenner stain. The bacteria are 

 then counted under an oil immersion lens, the tube length and mag- 

 nification being so arranged that the microscopic field covers 15 sq. mm. 

 The average number of bacteria found in such fields may be multi- 

 plied by 5,000 to give the number of bacteria contained in the 0.01 c.c. 

 of the milk. This method has 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 par- 

 ticular 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 legulations, 

 which must include care of cattle, dairy inspection and bacteriological 

 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 



