NOTE ON THE INDOL TEST IN TRYPTOPHANE 



SOLUTION 



CHR. BARTHEL 



Department of Bacteriology, Central Agricultural Experiment Station, Expert- 



mentalfaltet, Stockholm 



Received for publication, July 22, 1920 



The application of the indol test to tryptophane sokitions by 

 H. Zipfel was, without doubt a great advance. The theoretical 

 basis of this reaction is so generally^ known that it is superfluous 

 to give an explanation here. 



In applying the method of Zipfel it has happened on different 

 occasions, that I have failed to obtain growth (turbidity) in the 

 tryptophane solution even with bacteria, which are known as 

 strong indol liberators,- as for example Bad. vulgare. Of course 

 in this case there is also no indol reaction with the reagent of 

 Ehrlich (p-dimethylamidobenzaldehyde) . It occured to me that 

 this failure might be due to the hydrogen ion concentration in 

 the solution in question. 



Zipfel says nothing about the neutralization of the solution 

 in either of his two publications on this subject and so far as I 

 know, this fact has never been pointed out by any other worker. 

 In an electrical determination of the hydrogen ion concentration, 

 which I undertook on a tryptophane solution of the composition 

 prescribed by ZipfeP I found the value of the pH = 5.41. 



1 Centralbl. flirBakteriol., Abt. I., Orig., 64, 1912, 65; Centralbl. fiir Bakteriol. 

 .Abt. I., Orig., 67, 1913, 572. 



2 The term "liberation" is better than "formation," as the action is merely 

 a splitting up of the tryptophane molecule, with liberation of the indol group. 



3 



per cent 



Asparagin 0.5 



Ammonium lactate 0.5 



Potassium diphosphate 0.2 



Magnesium sulfate 0.02 



Tryptophane 0.03 



85 



