GENERAL FOOD RELATIONSHIPS^METABOLISM 77 



essential for the growth of every bacterium, two of them 

 are of especial importance for the reason that most of the 

 "physiological activities" to be described in the next chap- 

 ters are centered around their acquisition and utilization. 

 These elements are carbon and nitrogen. Some few of the 

 special activities of certain groups have to do with one or 

 the other of the remaining nine, as will be shown later. 

 But generally speaking, when a bacterium under natural con- 

 ditions secures an adequate supply of carbon and nitrogen, the 

 other elements are readily available in sufficient amount. 



Carbon is necessary not only because it is an essential 

 constituent of protoplasm but because its oxidation is the 

 chief source of the energy necessary for the internal life of 

 the cell, though nitrogen and sulphur replace it in this 

 function with a few forms. This latter use of carbon con- 

 stitutes what may be called its respiratory function. Bac- 

 teria like other organisms in their respiration utilize oxygen 

 and give off carbon dioxide. The amount of the latter given 

 off from the cell in this way is very small as compared with 

 that which is frequently produced as an accompaniment of 

 other reactions (see Fermentation, next chapter). But there 

 is no doubt of its formation and it has been determined by a 

 few investigators. On account of this use of carbon, bacteria 

 require relatively large amounts of this element. One group 

 of bacteria concerned in the spontaneous heating of coal 

 seems to be able to use free carbon from this material. 

 Another group is said to be able to oxidize marsh gas, CH4, 

 and use this as its source of carbon. The nitrite, nitrate and 

 sulphur bacteria mentioned later utilize carbon dioxide 

 and carbonates as their carbon supply, and one kind has 

 been described which uses carbon monoxide. With these 

 few exceptions bacteria are dependent on organic compounds 

 for their carbon and cannot use CO2 as green plants do. 



The oxygen requirement is also high partly for the same 

 reason that carbon is, i. e., respiration. Oxygen is alos 

 one of the constituents of protoplasm, and combined with 

 hydrogen forms water which makes up such a large part of 

 the living cell. Anaerobic bacteria are dependent on so- 

 called "molecular respiration" for their energy. That, is. 



