Carbon and Nitrogen 33 



tories. Pathogenic bacteria, in particular, seem to re- 

 quire protein compounds as their source of carbon, and 

 some species will not develop without it. 



Generally speaking, therefore, different species of 

 bacteria are not alike in their preference for one or 

 another source of carbon. Some species will develop in 

 solutions of cane-sugar fully as well as in solutions of 

 grape-sugar, while others will grow in solutions of 

 grape-sugar, but not in solutions of cane-sugar. Analo- 

 gous relations may be observed in the case of other 

 sugars. Again, there are species, like certain kinds of 

 denitrifying bacteria, which will develop perfectly in 

 culture solutions containing salts of citric acid as the 

 only source of carbon, while other organisms will not 

 grow in such solutions at all. Similarly, there are bac- 

 teria that will utilize pure cellulose, when the vast 

 majority of microorganisms will utterly fail to develop. 

 These examples will suffice to show that there are deep- 

 seated differences in the chemical machinery ■ of the 

 bacterial cells and in the methods by which the carbon 

 compounds are transformed and assiniilated. 



The source of nitrogen. — Relations like those just 

 described exist also in the assimilation of nitrogen food 

 by bacteria. Just as in the case of field crops, some plants 

 prefer ammonia and others nitrate, as a source of 

 nitrogen, so aihong bacteria, different species show 

 analogous preferences. Certain species reject both am- 

 monia and nitrate nitrogen, and demand for their 

 growth some organic nitrogenous compound (prefer- 

 ably protein), or substances derived from protein and 

 known as amino-compounds. The so-called nitrogen- 



