248 



DlSEASE-PltODUCINci BaCTEKIA. 



The most important disease-producing germ that is found 

 in milk is the bacillus tuberculosis. This germ may gain 

 admission to the milk from a tuberculous udder ; it may 

 occasionally get into the milk with the dust that has been 

 infected by the expectoration of tuberculous persons ^nd 

 tuberculous cattle. Tubercle bacilli do not grow or multiply 

 to any appreciable degree in commercial milk, because the 

 growing temperature limits* are between 80 and 104 degrees 

 F., and, when under the most favorable growing conditions, 

 they grow very slowly. In the mixed milk from a herd or in 

 the milk from a single cow the number of tubercle bacilli are 

 so few that it is very difficult to find them' by microscopic 

 examination. Generally not more than one cow in a heid will 

 have tuberculosis of the udder or of the lymphatic glands 

 near the udder ; consequently, in the mixed milk of a herd, 

 the tubercle bacilli are so few that it is almost impossible to 

 detect them. In some cases the germs in milk may be thrown 

 to the bottom of a small vessel and then examined. The fol- 

 lowing method,t described by Hammond, a student of the 

 McGill Veterinary College, is one of the most practicable: 



" Taking milk to which (preferably in order to arrest the 

 growth of other bacteria which are apt to hide the tubercle 

 bacilli) 5 per cent, of glacial carbolic acid has been added, put 

 15 c.c. of the milk into each of the two tubes, then centrifu- 

 galize it for 25 minutes (preferably in the hand centrifugal 

 machine manufactured by Bausch and Lorab, Rochester, N. 

 Y.); the supernating fluid is poured off; the precipitated 

 debris, bacteria, etc., which contains the bacilli, is then 

 treated with about 3 c.c. of a 5 per cent, caustic potash solu- 

 tion, is mixed up thoroughly by giving a good shake and is 

 left for two or three minutes. The tube is then filled up to 

 the 15 c.c. mark with distilled water and centrifugalized for 



* The variety of tubercle bacilli in fish may grow at a much lower 

 temperature, and the variety of tubercle bacilli in birds may grow at a 

 higher temperature. 



t American Veterinary Review, Aug., 1898, p. 322. 



