FOOD OF MICROORGANISMS. 97 



grow there at the same time. This peculiar association of aerobic and 

 anaerobic organisms cannot be explained simply by complete exhaustion 

 of the oxygen in the medium by the aerobic species; the latter do not 

 remove the oxygen completely. No other completely satisfactory explan- 

 ation can be given. 



Of the facultative organisms, some prefer to grow in the presence of 

 oxygen, like the yeasts, while others thrive better without air, as certain 

 lactic bacteria. It has been doubted whether facultative organisms 

 can multiply continuously without oxygen. The latest experiments with 

 yeasts indicate that there is a limit to their anaerobic multiplication. They 

 will develop for about 20 to 30 generations; they then need free oxygen 

 again. With bacteria, however, the multiplication seems to have no 

 limit in this particular. 



Facultative organisms thrive without air only in the presence of certain 

 food compounds, preferably carbohydrates. If they cannot get their energy 

 by oxidation, they depend upon anaerobic fermentations, and their ability 

 to ferment is naturally limited to a few compounds. B. coli can grow 

 without air only in the presence of sugars; it is customary to test for the 

 absence of sugar in broth by inoculating with B. coli a fermentation 

 tube which has been filled with the broth. Growth in the closed arm 

 indicates sugar; if there is growth only in the open arm, the broth is sugar- 

 free. Other facultative bacteria may be able to destroy protein under 

 anaerobic conditions, though experiments show that some of them lose the 

 power of liquefying gelatin if grown without oxygen. Some of the facul- 

 tative bacteria can provide for their oxygen by taking it from nitrates or 

 sulphates. 



It is interesting to compare the energy liberated by an aerobic and an 

 anaerobic process. The total energy stored in an organic compound is 

 measured by the amount of heat liberated by its complete combustion. 

 One gram of dextrose produces 3750 calories, if burned to carbon dioxide 

 and water. The plant producing this dextrose from carbon dioxide and 

 water needs, therefore, 3750 calories for every gram made, which are 

 taken from the radiant energy of the sun. The mold oxidizing i g. 

 of dextrose completely gains in this process 3750 calories which may be 

 used in form of chemical energy for its growth and for building up the 

 complex compounds of cell life from small simply constructed molecules. 

 If the mold has ceased growing, these 3750 calories will not be used but will 

 produce a slight rise in the temperature of the nutrient medium. A yeast 

 7 



