BACTERIAL NUTRITION 539 



is less readily attacked and utilized as food by bacteria than the 

 remainder of the mixture. On the other hand, the real peptone 

 fraction is undoubtedly a more heterogeneous mixture of poly- 

 peptides, some of which are indeed quite simple, and are easily 

 broken up by bacteria or their enzymes. In the proteolysis 

 which takes place slowly under most favorable conditions, the 

 simpler polypeptides are attacked. When these have been ex- 

 hausted so-called "peptolysis" ceases. 



The above explanation appears all the more probable from the 

 fact that as bacterial decomposition continues there is a change in 

 the biuret-giving ingredients of the peptone which is indicated 

 by a difference in the color obtained. While the original pep- 

 tone medium gives a pink color, the reaction changes if there is 

 appreciable peptolysis, and the color becomes distinctly violet 

 and indistinguishable from the biuret color obtained with pro- 

 teoses and some of the higher proteins. 



The peptolysis which is brought about by Bacillus coli and 

 its close allies is undoubtedly the result of an ereptic enzyme 

 (erepsin) . This enzyme differs, however, from erepsin of animal 

 origin in that it does not attack casein. In comparison with 

 intestinal erepsin. and with the proteolytic enzymes of B. sub- 

 tilis, B. prodigiosus and other strong gelatin-liquefjdng bacteria, 

 in so far as peptolytic action is concerned, the erepsin elaborated 

 by the Coli-typhi-paratyphi group of organisms is decidedly 

 weak. 



REFERENCES 



(1) Rettger, L. F., Berman, N. and Sturges, W. S.: This Journal, The 



utilization of proteid and non-proteid nitrogen. 1, 15-33, 1916. 



(2) Hammarsten and Hedin: Textbook of physiological chemistry. English 



translation by Mandel, pp. 86-91, 1914. 



