CULTURES OF TUBERCLE BACILLUS. 261 



is necessary because of the slow growth of the organism. 

 Under the most favorable conditions tubercle bacilli 

 directly from the animal body show no evidence of 

 growth for about twelve days after inoculation upon 

 blood-serum, and, as they must be retained during this 

 time at the body temperature — 37.5 C. — evaporation 

 would take place very rapidly and the medium would 

 become too dry for their development. 



If these primary efforts result in the appearance of a 

 culture of the bacilli, further cultivations may be made 

 by taking up a bit of the colony, preferably a moderately 

 large quantity, and transferring it to fresh serum, and 

 this in turn is sealed up and retained at the same tem- 

 perature. Once having obtained the organism in pure 

 culture its subsequent cultivation may be conducted 

 upon the glycerin-agar- agar mixture — ordinary neutral 

 nutrient agar-agar to which 6 or 7 per cent, of glycerin 

 has been added. This is a very favorable medium for 

 the growth of this organism after once having estab- 

 lished its saprophytic form of existence, though blood- 

 serum is perhaps the best medium to be employed in 

 obtaining the first generation of the organism from the 

 tubercular tissues. 



The organism may be cultivated also on neutral 

 milk to which 1 per cent, of agar-agar has been added, 

 also upon the surface of potato, and likewise in meat 

 infusion bouillon containing 6 or 7 per cent of 

 glycerin. 



Cultures of the tubercle bacillus are characteristic 

 in appearance — after once having seen them there is 

 but little probability of subsequent mistake. 



They appear as dry masses, which may develop upon 

 the surface of the medium either as flat scales or as 



12* 



