CULTIVATION OF TUBERCLE BACILLUS. 211 



which are necessary to prove the etiological role of the 

 organism in the production of this malady are all ful- 

 filled. 



THE TUBERCLE BACILLUS. Of the three pathogenic 

 organisms of which we are speaking, the tubercle bacillus 

 will give us most difficulty in our efforts at cultivation. 



It is in the strict sense of the word a parasite and 

 finds conditions entirely favorable to its development 

 only in the animal body. On ordinary artificial 

 media the bacilli taken directly from the animal body 

 grow only very imperfectly or in many cases not at all. 

 From this it seems probable that there is a difference in 

 the nature of the individual bacilli of this group some 

 appearing to be capable only of growth in the animal 

 tissues, while others are apparently possessed of the 

 power to lead a limited saprophytic existence. It may be, 

 therefore, that those bacilli which we obtain as artificial 

 cultures from the animal body are offsprings, from the 

 more saprophytic members of the group. At best, one 

 never sees with the tubercle bacillus a saprophytic con- 

 dition in any way comparable to that possessed by many 

 of the other organisms with which we have to deal. 



In efforts to cultivate this organism directly from the 

 tissues of the animal, the method by which one obtains the 

 best results is that recommended by Koch cultivation 

 upon blood-serum. So strictly is this organism a parasite 

 that very limited alterations in the conditions under which 

 it is growing may result in failure to successfully study 

 it. It is, therefore, necessary that the injunctions for 

 obtaining it in pure culture should be carefully observed. 



The blood-serum upon which the organism is to be 

 cultivated should be comparatively freshly prepared 

 that is, it should not be dry. 



