PHYSIOLOGY OF THE MICROBES 73 



or heat is necessary, and the microbe provides this by burning 

 up a portion pf its food material. Thus, while part of the food 

 is raised to a level of higher organisation, another portion, on 

 the other hand, is reduced to a simpler condition. ■ For 

 example, part becomes protein, part carbonic acid and water, 

 the latter not being incorporated in the living substance. 



Similarly, a microbe, like the Aspergillus niger, burns up 

 with the help of the oxygen of the air the sugar which is 

 furnished by the food, and this combustion puts at its disposal 

 a certain number of calories ; one molecule of sugar weighing 

 i6o grams furnishes 673 calories during its transformation into 

 CO.2 and H2O. The yeast of beer, which decomposes sugar 

 less completely, i.e., into alcohol and carbonic acid, only yields 

 33 calories. If the yeast had the nutritive requirements of the 

 aspergillus it would have to use up nearly twenty times more 

 food. Nutrition of this kind is characteristic of a ferment, 

 and the fermentative power is greater the more of the food- 

 stuff the cell is obliged to break down. 



Lacking chlorophyll as they do almost universally, the 

 microbes cannot take up carbon directly from the air as do 

 the green plants. They demand their food ready made so 

 that they can destroy it, turning it into carbonic acid and 

 water. The heat derived from this destruction takes for them 

 the place of the energy furnished by sunlight to the plants 

 which contain chlorophyll. 



The food of a microbe may then be defined as " every 

 material from which a given microbe, under the conditions of 

 the experiment, can take the material necessary for its develop- 

 ment and the heat necessary to render it independent of solar 

 energy." In calculating the total energy coming into play 

 in the protoplasmic activity a certain amount has to be 

 accounted for as external heat, and it is even the rule for an 

 excess to appear as heat, so that there is a rise of temperature 

 in the medium. Thus a vat at the surface of which acetic 

 fermentation is going on gets notably hotter, and the same is 

 the case in grapes undergoing vinous fermentation. But 

 under certain limited conditions the protoplasm can perfectly 



