1038 BACTERIA IN AIR, SOIL, WATER, AND MILK 



changed, at least not to an extent which renders it less valuable 

 as a food. Statistics by Park and Holt have shown strikingly the 

 advantages of pasteurized over raw milk in infant feeding. Of 

 fifty-one children fed with raw milk during the summer months, 

 thirty-three had diarrhea, two died, and only seventeen remained 

 entirely well. Of forty-one receiving pasteurized milk, but ten had 

 diarrhea, one died, and thirty-one remained entirely well throughout 

 the summer. The actual diminution of the living bacterial contents 

 of milk by pasteurization is enormous, the milk so treated often 

 containing not more than one thousand, usually less than fifteen 

 thousand, living bacteria to each cubic centimeter. 



Methods of Estimating the Number of Bacteria in Milk. In es- 

 timating the number of bacteria in milk, colony counting in agar 

 or gelatin plates is resorted to. Great care must be exercised in 

 obtaining the specimens. If taken from a can, the contents of the 

 can should be thoroughly mixed, since the cream usually contains 

 many more bacteria than the rest of the milk. The specimen is then 

 taken into a sterile test tube or flask. If the milk is supplied in an 

 ordinary milk bottle, this should be thoroughly shaken before being 

 opened, and the specimen for examination taken out with a sterile 

 pipette. Dilutions of the specimen can then be made in sterile broth 

 or salt solution. If an initial dilution 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 incubator, colony 

 counting is done, and the proper calculation is made. In samples 

 in which few bacteria are expected, direct transference of 1/20 or 

 1/40 of a c.c. of the whole milk into the agar may be made. This 

 method saves time but is less accurate. 



Direct Methods of Counting Bacteria. Direct methods of count- 

 ing 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 accurately 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 dissolved with xylol. It can then be stained 

 lightly with the Jenner stain. The bacteria are counted under an 

 oil immersion lens, the tube length and magnification being so 



