CONDITIONS OF GROWTH. 53 



dance of water ; salts are also indispensable, and there 

 must be organic material as a source of carbon and 

 nitrogen. The greater number of important bacteria 

 and all the pathogenic species thrive best in media 

 containing albuminoid substances and of a slightly 

 alkaline reaction. The demands of bacteria in the 

 composition of the culture media vary very consid- 

 erably. There are some species of water bacteria, 

 for instance, which require so little organic material 

 that they will grow in water that has been twice 

 distilled. In such cases development probably takes 

 place owing to some contamination of the water, or 

 else through the decomposition of the ammonia and 

 carbonic acid in the air. A certain species will 

 grow abundantly in water containing ammonium car- 

 bonate in solution and no other source of carbon and 

 nitrogen. This shows the power of some bacteria of 

 producing cell substance from the simplest materials a 

 power which belongs to the higher plants which obtain 

 their nourishment from the air through their chlorophyl 

 and the assistance of sunlight. Few bacteria, how- 

 ever, of any importance in medicine are so easily satis- 

 fied, though there are many species which are able 

 to develop without the presence of albumin and in 

 comparatively simple culture media, such as the culture 

 liquid proposed by Uschinsky, or the simpler one of 

 Voges and Fraenkel, which consists of: Water, 1000; 

 sodium chloride, 5 ; neutral sodium phosphate, 2 ; 

 ammonium acetate, 6; and asparagin, 4. In these 

 media many bacteria grow well. 



When we consider in detail the source of the more 

 important chemical ingredients of bacteria we find that 

 their nitrogen is most readily obtained from diffusible 



