IS2 NUTRITION AND METABOLISM. 



Sources of Hydrogen and Oxygen. — The sources of hydrogen are 

 hardly ever discussed with bacteria since hydrogen bears such a close 

 and pecuUar relation in water and organic food supplies. The ulti- 

 mate association of hydrogen with oxygen in the molecule of water 

 (H2O) and with carbon in organic substances (CH4") establishes its 

 importance in all life processes. There are many prototrophic bacteria, 

 -using oxygen as such; others are able to reduce such compounds as 

 nitrates or sulphates, which would' be autotrophic, thus providihg 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, K2HPO4, sufficient in mineral mattey 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 rhatter. 



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 reaUze that oxygen is not such an extraordinary 

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



