ENERGY SUPPLY OF THE CELL 79 



while they obtain their energy from the oxidation of 

 hydrogen sulfide. 



On the other hand, the Streptococci are not able 

 to utilize oxygen (p. 20), but are not especially sensitive 

 to it; we do not ordinarily consider them as anaerobes. 

 Further, the sensitivity of bacteria to oxygen depends 

 upon the number of cells present and upon their physio- 

 logical condition. Large inocula of anaerobes will 

 frequently grow when small inocula of the same culture 

 in the same medium will not. 



Finally, the kind of medium is of great influence. 

 It has been shown that by oxidizing a medium, or by 

 adding appropriate hydrogen acceptors, aerobic bacteria 

 may develop without oxygen gas. It has also been 

 shown that the addition of pyruvic acid or cysteine to a 

 medium will permit growth of some strictly anaerobic 

 species without special protection from oxygen. 



Our differentiation between aerobic, anaerobic and 

 facultative organisms is quite useful as a working 

 definition, but it involves a number of different unrelated 

 principles, such as the need of oxygen for energy libera- 

 tion, the possibility of substitutes for this free oxygen 

 by organic or inorganic hydrogen acceptors, the require- 

 ment of oxygen for growth when it is not required for 

 energy, the toxicity of oxygen to the cell, and the over- 

 coming of toxicity by special media. There are no 

 clear-cut groups set off distinctly by their reaction 

 towards oxygen; all are allied in varying degrees, accord- 

 ing to the composition of the culture medium. 



In discussing oxygen requirements, it should be 

 remembered that the solubility of oxygen is very small, 

 as Table 10 shows. There is only between 0.0007 and 

 0.0010% of oxygen in solution, and considering oxygen 

 as a food, a medium with such a small amount of food 



