THE BACTERIUM TUBERCULOSIS 417 



in the steam sterilizer just before using (Ghriskey). This 

 precaution is necessary because 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 re- 

 tained 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 a growth 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 body temperature. Once having ob- 

 tained the organism in pure culture, its subsequent culti- 

 vation may be conducted upon the glycerin-agar-agar 

 mixture ordinary neutral nutrient agar-agar to which 

 from 4 to 6 per cent, of glycerin has been added. This is 

 a very favorable medium for the growth of this organism 

 after it has accommodated itself to its saprophytic mode 

 of existence, though blood-serum is perhaps the best medium 

 to be employed in obtaining the first generation of the 

 organism from tuberculous 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 from 4 to 6 per cent, of glycerin. 



Cultures of the tubercle bacillus are characteristic in 

 appearance once having seen them there is little proba- 

 bility of subsequent mistake. They appear as dry masses, 

 which may develop upon the surface of the medium either 

 as flat scales or as coarse, heaped up, granular masses. 

 They are never moist, and frequently have the appearance 

 27 



