20 L. F. RETTGER, N. BERMAN AND W. S. STURGES 



thus far resisted all attempts to isolate or purify them. Hence, 

 it has been impossible to employ all the methods of investiga- 

 tion in a study of their bacteriological-chemical relations which 

 are applicable in connection with certain albumins, as for instance 

 egg albumin. Peptones are now regarded as amino acid com- 

 binations of varying complexities, rather than proteins. Witte's 

 peptone, which is essentially a mixture of albumoses and pep- 

 tones, is far from being made up purely of these nitrogen com- 

 plexes, although it has long been regarded as the standard for 

 bacteriological purposes. The various American brands are 

 undoubtedly even less pure than the Witte product. It does not 

 follow, however, that they are of correspondingly less value as 

 food for bacteria. 



In our study of the behavior of various types of bacteria 

 towards proteoses and peptones the Biuret test for proteins has 

 been employed to great advantage. The method which has 

 been advocated and used by Vernon (1904) for the quantitative 

 estimation of peptone has, with slight modifications been em- 

 ployed by us in the present investigation and in the experiments 

 on bacterial autolysis. A brief description of this method is 

 given here. 



The tests were made in Nessler tubes. One cubic centimeter 

 of the inoculated culture fluid was added to 20 cc. of a 4 per cent 

 solution of sodium hydroxide and 2 cc. of a centinormal solution 

 of copper sulphate. To the same mixture of alkali and copper 

 sulphate in a second tube a standard solution (0.25 per cent) of 

 Witte's peptone was added until the same degree of color was 

 produced as in the test medium. The quantity of peptone 

 required in matching the colors was taken as a measure of the 

 amount of peptone present in the inoculated and incubated cul- 

 ture fluid. For example, if 1 cc. of standard peptone solution 

 was required the value of the biuret test was recorded as 1.0, 

 since both hquids gave the same color in the same concentration. 

 Test fluids and controls contained the same amount of Witte's 

 peptone at the outset as the standards, namely 0.25 per cent. 



Peptone solutions containing from 0.2 to 2.0 per cent of Witte's 

 peptone were at first employed as culture media for the different 



