GROWTH 171 



amino-acids are not always used as such, is quite evident 

 from Ehrlich's observation that the leucin assimilation 

 by yeast is decreased in the presence of ammonium salt. 



Amino-acid organisms in a strict sense of the word 

 are microorganisms which can utilize nitrogen in no 

 simpler form than that of an amino-acid. This would 

 mean, chemically speaking, that these organisms cannot 

 synthetize the group — NHCHCO — which is so com- 

 mon in the proteins of all living beings. They must 

 obtain this combination ready-made, but can do the 

 rest of the synthesis themselves, eventually only with 

 an especially good source of energy. Most commonly 

 used for experiments of this nature is asparagin, and the 

 good results are perhaps due to the fact that we have 

 here not only the amino-acid group, but, at the same 

 time, the very frequent acid-amid group preformed. 



If we consider the great variety of nitrogenous cleavage 

 compounds of protoplasm, we realize at once that the 

 synthetizing power of a cell that can grow on one amino- 

 acid only, is very remarkable, since all other amino- 

 acids, carbohydrates, and the cell wall are all formed from 

 the one material. 



The term '^ peptone bacteria" is sometimes used and 

 is meant to indicate that these organisms can use nothing 

 simpler than peptones. That such organisms exist, is 

 questionable. If we consider that even the mammals 

 can be kept alive, and probably be made to grow with 

 amino-acids only, and that probably all the body cells 

 are fed with amino-acids and not with peptones, it seems 

 quite probable that this would be true also with bacteria. 

 If they refuse to grow on tyrosin, asparagin or leucin 

 solutions, this is no proof that they require peptone. It 

 shows only that the proper amino-acid had not been 

 offered. Certain species may require a mixture of two 



