THE BACTERIOLOGIC EXAMINATION OF MILK 423 



smear should not become too hot, as this makes it check, and 

 satisfactory staining is then impossible. As soon as dry, the 

 slides are placed for a short time in a Coplin staining jar contain- 

 ing xylol, which removes the fat. The surplus xylol is then 

 wiped off with filter-paper. The smears are dried and fixed in 

 95 per cent, alcohol. After fixation they are immediately stained 

 for two to three minutes in Loffler's methylene-blue and then 

 decolorized to a light blue in 95 per cent, alcohol. The count- 

 ing is done under an oil-immersion lens. The number of fields 

 composing 1 c.c. is figured, and this is the factor for multipli- 

 cation. It is necessary to count several fields and take the 

 average. 



By direct microscopic enumeration clumps of bacteria can be 

 observed. The individual cells in the clumps can be counted, 

 unless, as happens sometimes, they are so thick that single cells 

 cannot be distinguished. This is an obvious advantage over the 

 plate method, but confusing if results are to be compared with 

 those of the plate method. According to Brew, experience has 

 shown that in milk with low bacterial content the direct count is 

 about forty-four times as high as the plate count, if individual 

 cells are counted, and sixteen to seventeen times as great if clumps 

 are counted as single cells. When the number of bacteria ap- 

 proaches 1,000,000 the direct count is about 5 per cent, higher than 

 the plate count. 



In the investigation by Conn referred to above Dr. Breed 

 made counts by his method of the same samples of milk that 

 were plated in the laboratories. Professor Conn states that (1) 

 in making a comparison of the plate count with the direct count 

 clumps should be counted as individuals, and that then the micro- 

 scopic method gives good results; (2) considerable experience by 

 the person making the count is needed to distinguish between 

 bacteria and dirt particles, particularly when the milk contains 

 minute micrococci; (3) when the microscopic count is made by 

 one who has had sufficient experience, the group count agrees 

 somewhat closely with the plate count, and (4) raw, fresh milk 

 does not contain any appreciable number of dead bacteria which 

 might disclose themselves to the microscope. 



The advantages claimed for the direct microscopic method are 

 several. In the first place, it shows the presence of bacteria which 

 do not grow on agar, and in the second place the work consumes 

 but a short time, so that results can be had quickly; and further- 

 more, the necessary apparatus is simple and less expensive than 

 that required for the plate method. Breed and Brew claim that 

 the microscopic examination of milk is more useful for detecting 

 the causes of a large germ content than the plate method, espe- 



