138 Manual of Veterinary 31icrobiology . 



depend also upon the special conditions in which 

 they are placed (aerobic and anaerobic). The medium 

 becomes impoverished in alimentary substances, and, 

 at the same time, becomes charged with excretory 

 products. These we have already described ; we will, 

 however, repeat that they may be toxic for the germs 

 and arrest their multiplication, and that they often 

 communicate to the media in which they are dif- 

 fused, certain pathogenic properties which the germs 

 themselves possess. 



Preservation of cultures. 



The exhaustion or contamination of the media 

 speedily brings the germs to the condition of latent 

 life; from that time they cease to multiply ; the com- 

 bined action of the air and light more or less quickly 

 destroys them. The virulence of a culture also pro- 

 gressively diminishes from the same cause. 



This annihilation of virulence is more or less 

 quickly produced according to the germs concerned, 

 and also according to the composition of the culture 

 medium. Thus, we have seen cultures of fowl chol- 

 era deprived of all pathogenic action for adult rab- 

 bits after one day, although, usually, this is preserved 

 for sixty days, e^iposed to the air. 



For the longer preservation of cultures of germs 

 they may be inclosed in small sterilized tubes which 

 are sealed by the flame so as to include as little air as 

 possible. These tubes are kept in the dark or in 

 wooden cases, as we are in the habit of employing. 

 In this way their preservation is lengthened, but it 

 is not indefinite; after a variable time, months or 

 years, the germs die ; in order to preserve the seed, it 



