98 NUTRITION AND METABOLISM. 



fermenting the dextrose to alcohol and carbon dioxide does not use the 

 entire energy of the dextrose; there is considerable available energy 

 left in the alcohol; carbon dioxide contains no available energy. One 

 g. of dextrose gives about 0.48 g. of alcohol and 0.52 g. of carbon 

 dioxide. The heat of combustion of alcohol is 7183 calories per g. or 

 3476 calories for 0.48 g. The heat of combustion of carbon dioxide 

 is nil. The dextrose before fermentation represents 3750 calories, the 

 products of fermentation contain 3476 calories; the yeast therefore gains 

 by the fermentative process only " 247 calories from i g. of dextrose. 

 This is a very small gain, compared with that of oxidation; the ratio is 

 274:3750 or 1:14. This means that a yeast, in order to get the same 

 supply of energy as an oxidizing mold, has to ferment fourteen times as 

 much sugar as the mold oxidizes. 



A very good example of this is the different power of nitrogen fixation 

 with aerobic and anaerobic bacteria. The aerobic Azotobacter fixes about 

 15 mg. of nitrogen for each g. of sugar, while the anaerobic B. (Clostridium) 

 butyricus fixes only 2 mg. for the same amount of sugar, when this sugar is 

 fermented to butyric and acetic acid, carbon dioxide and hydrogen. 

 The nitrogen numbers indicate the rate of growth of aerobic and 

 anaerobic organisms with the same amount of food. 



Occasionally, attention is called to the enormous destructive power 

 of microorganisms, the proportion of food to growth being entirely 

 different from that of animals. One reason for this difference can be 

 plainly seen from the above discussion. The higher animals oxidize 

 their food almost completely and consequently need only a fraction of 

 what a fermenting organism would require. 



The oxygen requirements of protozoa have not been investigated 

 until recently. It is probable that some parasitic protozoa can live and 

 multiply without free oxygen for a considerable time. 



ADDITIONAL REMARKS ON MICROBIAL FOOD. 



PHYSIOLOGICAL GROUPS. It is customary to divide the microorgan- 

 isms into physiological groups according to the kind of food and the 

 metabolic products formed. This is convenient and helpful in describing 

 certain characters though the classification and nomenclature has been 

 accomplished as far as possible with strictly morphological characteristics, 

 as is the custom in all classification of plants and animals. The so- 



