112 EXPERIMENTAL DAIRY BACTERIOLOGY 



It is useless to examine milk for the presence of the organ- 

 isms of diphtheria or typhoid fever. They do not find favor- 

 able conditions for luxuriant growth in milk, in competition 

 with the ordinary milk forms. Again, by the time the milk 

 supply is suspected of being the agent of transmission, the 

 source of contamination has often disappeared and the milk 

 is no longer dangerous. In order to determine the source 

 of an epidemic, a careful historical survey of all the condi- 

 tions surrounding the outbreak must be made, rather than 

 to seek the causal organism in the water or milk supply by 

 bacteriological methods. 



Tubercle bacilli cannot grow in milk after withdrawal from 

 the animal, but they persist for considerable periods in sour 

 milk, butter, and cheese. Milk may be examined for the 

 presence of tubercle bacilli by microscopical methods and by 

 the inoculation of experimental animals. 



Microscopical examination for tubercle bacilli. The organ- 

 ism of tuberculosis cannot be stained by the use of ordinary 

 stains. A solution having an intense staining power must be 

 employed. The cells once stained, retain the color even when 

 treated with dilute mineral acids. Thus a preparation may 

 be stained, the color removed from all the material except 

 the tubercle bacilli, and then the preparation restained with 

 a contrast stain. 



The microscopical examination of milk for tubercle bacilli 

 is not satisfactory, since these are likely to be present in 

 such small numbers as to be very difficult or impossible 

 to find. 



Procedure. The milk should be heated to 70 C. for a few 

 moments to break up the clusters of fat globules and prevent, 

 as far as possible, the bacteria being carried with the cream 

 in the subsequent process of centrif ugalization. Ten to fifteen 



