224 . NUTRITION AND METABOLISM 



nitrates or sulphates, which would be autotrophic, thus providing for 

 their needs. Heterotrophic bacteria are not unusual. In this connec- 

 tion it may be said that it is often difficult to distinguish between oxy- 

 gen needed for cell construction and oxygen needed for energy formation. 

 SOURCES OF MINERALS. The amount of mineral matter necessary 

 for the construction of the cell is very small; potassium and phos- 

 phorus seem to be among the most essential elements. It is customary 

 to consider a tap water with 0.02 per cent of di-potassium hydro- 

 gen phosphate (K 2 HPO 4 ), sufficient in mineral matter of all kinds to 

 provide for fair growth. Some of the common materials used in the 

 preparation of nutrient media, such as meat extract and peptone, also 

 contain considerable amounts of mineral matter. 



FOOD FOR ENERGY 



As all food in its decomposition results in products of some form or 

 other, it may not seem justifiable to separate a paragraph on food 

 from another on products. The essential difference lies in the fact that 

 we consider food from the viewpoint of the cell, while products are 

 commonly considered apart from the construction processes of the cell 

 and only from their application, or, it may be, from the viewpoint of 

 usefulness to man. 



Animals provide for their energy by oxidations, and almost exclu- 

 sively by complete oxidations. Some bacteria, and most molds, do 

 the same. The range of materials which can serve as food for this pur- 

 pose is surprising. With animals, the food is practically limited to 

 plant and animal tissue. With bacteria, we find the strangest sub- 

 stances, such as hydrogen, carbon monoxide, coal, marsh gas, hydrogen 

 sulphide, ammonia, nitrites, formic and oxalic acids, alcohol and thio- 

 sulphates serving this purpose. The fact that many gases are used 

 as food makes us realize that oxygen is not such an extraordinary 

 compound as animal physiology seems to indicate, but that it should be 

 classed merely as one of the many food compounds. This is especially 

 significant since it will be shown later that free oxygen is not necessary 

 for microbial life, and that many organisms can exist without it. 



The oxidations are not always complete. The formation of nitrous 

 acid from ammonia, the oxidation of alcohol to acetic acid are such 

 examples. Some organisms are highly specialized in their food require- 

 ments, especially the mineral-attacking bacteria are usually limited 

 to one source of energy. The microorganisms oxidizing organic com- 



