62 BACTERIOLOGY. 



the simpler amido acids, or from strictly inorganic com- 

 pounds, such as ammonium salts, nitrates or, in rare cases, 

 free nitrogen. The fact that some species can thrive better 

 than others upon one of these sources of nitrogen can be 

 made use of in differentiating one form from another. Thus j 

 the typhoid bacillus cannot utilize the nitrogen in ammonium 

 salts, whereas the colon bacillus can. Artificial culture 

 media (see Uschinsky's medium) can be prepared which con- 

 tain chiefly inorganic constituents and only simple organic 

 compounds, such as lactic, or tartartic acids, or glycerin, or 

 glucose. The latter compound is especially useful as a 

 source of carbon under these conditions of growth. 



The hydrogen which enters into the composition of pro 

 toplasmis probably derived along with the carbon from the 

 organic food constituents. Whether it can be obtained from 

 inorganic compounds, such as ammonia and water is not 

 known. Water, as such, is taken up by the cell from the 

 surrounding medium and, moreover, may be formed, in part, 

 like carbonic acid, by oxidation changes within the organ- 

 ism. 



In addition to carbon, nitrogen and hydrogen every liv- 

 ing organism must be supplied with oxygen. The higher 

 animal obtains its oxygen from the air, whereas higher 

 plants are obliged to depend upon the oxygen contained in 

 water, nitrous and nitric acids. The majority of bacteria 

 can probably utilize the oxygen of the air, which is seen in 

 the fact that they grow only in the presence of air. Other 

 bacteria, as will presently be seen, thrive only in the ab- 

 sence of air. In such cases the necessary oxygen is undoubt- 

 edly obtained from organic compounds such as proteins, or 

 carbohydrates. 



Certain inorganic salts or mineral constituents are like- 

 wise necessary to the well-being of living organisms. Bacte - 

 ria may obtain their sulphur and phosphorus from preformed 

 organic compounds such as the proteins. They can, however, 



