THE BACTERIUM TUBERCULOSIS 415 



into the body of a susceptible animal. From the tissues 

 of this animal the bacilli may be obtained and cultivated 

 artificially, and these cultures are capable of again produc- 

 ing the disease when further inoculated. Thus are fulfilled 

 the postulates formulated by Koch for proving the etio- 

 logical role of an organism in the production of a malady. 



THE BACTERIUM TUBERCULOSIS. 



Of the three pathogenic organisms liable to occur in the 

 sputum of a tuberculous subject, the tubercle bacillus gives 

 us the greatest difficulty in our efforts at cultivation. 



It is almost an obligate 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 individual tubercle 

 bacilli some appearing to be capable of growth in the 

 animal tissues only, 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 of 

 the more saprophytic varieties. At best, one never sees 

 with the tubercle bacillus a saprophytic condition in any 

 degree comparable to that possessed by many of the other 

 organisms with which we have to deal. 



For the cultivation of bacillus tuberculosis directly from 

 the tissues of the animal, the best method is that recom- 

 mended by Koch, viz., cultivation upon blood-serum. Its 

 parastitic tendencies are so pronounced that even very 

 slight variations in the conditions under which one endeavors 



