154 BACTERIOLOGICAL EXAMINATION OF MILK. 



pigs and sickens those with slight lesions. Others, presumably 

 non-tubercular, are chloroformed. At autopsy, all organs 

 showing deviations from the normal are examined for tuber- 

 culosis by smears, cultures and sections.* Care must be taken 

 to differentiate tuberculosis from the lesions of Bacillus pseudo 

 tuberculosis. This organism causes peritonitis with adhesions 

 and nodules in organs like the liver or spleen, which show 

 a tendency to central necrosis. In section these nodules are 

 seen to consist of lymphoid elements, very few epithelioid and 

 multinuclear cells. The giant cells typical of tuberculosis do 

 not occur. Cultures grow more rapidly than those of //. 

 tuberculosis. 



The older method of direct microscopic examination of milk 

 sediments for tubercle bacilli is quite inadmissible, and all con- 

 clusions based upon it have to be discarded. Other organisms 

 having the staining peculiarities of B. tuberculosis (acid-fast) 

 are frequently found in dairy products. Johne's disease, 

 affecting the intestines of cattle, is caused by an organism 

 similar in morphology to B '. tuberculosis. 



Determination of streptococci. Streptococci may be recog- 

 nized on the plates made in connection with the standard 

 method for numerical determination. The colony is small, 

 consisting of a slightly elevated center surrounded by a thin 

 spreading border. Their significance in a quantitative sense is 

 discussed in Chap. VII. 



Qualitative determinations. The examination of milk by 

 methods permitting the determination of the percentage of 

 various groups of organisms has been quite extensively used 

 in the study of dairy problems, and has yielded valuable 

 information (2). Certain characteristics of growth on plates, 

 such as formation of acid, character of colony, and liquefaction 

 of gelatin, have been made the basis for the identification of 

 groups of organisms for the' study of their growth under var- 

 ious conditions. The relative proportion in which certain 



* Dead tubercle bacilli, killed for instance by pasteurization, may lead 

 to the formation of small tubercles, with no tendency to further multi- 

 plication. Some workers consider it well to inoculate a second pig from 

 these lesions, to settle doubt as to their nature. 



