BACTERIOLOGICAL EXAMINATION OF MILK. 1 15 



organisms are observed, plate out on agar. The colonies showing 

 morphologically characteristic organisms should be tested as to ag- 

 glutination and bacteriolysis. Inasmuch as the true cholera spirillum 

 shows a marked cholera-red reaction it is well to inoculate a tube of 

 peptone solution from such a colony and add a drop of concentrated 

 sulphuric acid after incubating for 18 hours. The rose-pink colora- 

 tion is given by the cholera spirillum with the acid alone the nitroso 

 factor in the reaction being produced by the organism. 



BACTERIOLOGICAL EXAMINATION OF MILK. 



A bacterial milk count is of comparatively little value as showing 

 whether a milk is dangerous or not. As a matter of fact, a milk which 

 contains several million of bacteria per c.c. might be less dangerous 

 than one containing only a few thousand, especially if in the latter 

 there were numerous liquefiers and gas producers present. There is, 

 however, one point of importance in connection with the quantitative 

 estimation of bacteria in milk, and that is the fact that in order to keep 

 the development of the bacteria within the limits of 10,000 to 50,000 

 per c.c., it is necessary that the requirements of cleanliness in milking 

 and the rapid cooling of the milk after obtaining it and the keeping 

 of the temperature below 50 C. be rigidly observed. If a milk has a 

 high count it shows some error in the handling of the milk. In making 

 a quantitative bacteriological examination, the principle is the same 

 as with water. 



Make a known dilution of the milk with sterile water; add definite 

 quantities of this diluted milk to tubes of melted agar or gelatin and 

 pour into plates. The diluted milk may also be delivered in the center 

 of the plate and the melted agar or gelatin poured directly on it, 

 mixing thoroughly. . Always shake the bottle well before taking 

 sample. 



Example: Added i c.c. of milk to 199 c.c. of sterile water in a 

 large flask (500 to 1000 c.c.). After shaking thoroughly, take i c.c. 

 of this i : 200 dilution and add it to 99 c.c. of sterile water. Shak- 

 ing thoroughly, w r e have a dilution of i : 20,000. Of this we added 

 .5 c.c. to a tube of gelatin or agar. After incubation the plate showed 



