FOOD RELATIONSHIPS OF BACTERIA ' 67 



those found in the intestinal tracts of herbivora, appear to decompose 

 them very readily. 



(c) Hydrogen. Hydrogen is readily obtained by microorganisms 

 from organic compounds containing available carbon, nitrogen, and 

 hydrogen, but not apparently from water. 



(d) Oxygen. Oxygen is indispensable to the life of all living organ- 

 isms as a source of energy and for structural purposes. A few bacteria, 

 the obligately aerobic bacteria, can live only in the presence of free 

 oxygen; another small group, the obligately anaerobic bacteria, live 

 either in the absence of free oxygen or at best in the presence of minimal 

 amounts of it; more than minimal amounts of free oxygen act as 

 specific poisons to them. The majority of bacteria are facultative with 

 respect to their oxygen requirements ; that is, they can either live 

 in the presence of free oxygen or derive their oxygen needs from 

 organic compounds, usually the carbohydrates or proteins. 



(e) Inorganic Salts. Inorganic salts are used by bacteria almost 

 wholly for structural purposes. The requirement for mineral com- 

 pounds is very little, for these substances do not on the average make 

 up more than 7 to 10 per cent, of the solid matter of the bacterial cell. 

 The essential elements and the percentage of them found in the ash 

 of certain bacteria have been referred to previously, and it was stated 

 that the amount of inorganic salts found in the bodies of the bacteria 

 bore a rather direct relationship to the salt concentration of the media. 

 Of the inorganic elements, phosphorus is the most important, for it 

 makes up nearly 50 per cent, of the ash. Phosphorous in contra- 

 distinction to any other inorganic salt is absolutely indispensable to 

 bacterial growth. It is combined organically in nucleo-proteins, 

 glyconucleo-proteins, and nucleic acids, which form the greater part 

 of the protein of the bacterial cell. 



