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 
