INDOL TEST IN TRYPTOPHANE SOLUTION 87 



hydrogen ion concentration increases during the steriHzation. 

 Furthermoi'e, it follows from this that some alkali has been 

 leached from the glass of the ordinary tubes, because the values 

 of the pH of the Jena glass tubes are, consistently, lower (from 

 0.14 to 0.32) than for the others. 



With these four sterilized solutions, indol tests were carried 

 out with eight different species of bacteria, of which three are 

 known to be strong indol liberators, namely Bad. coli, Bad. 

 vulgare and Vibrio cholerae.* The other five are not indole 

 liberators. These species were Bad. aerogenes, a variety of 

 Bad. Zopfii, isolated from soil of the northern coast of Green- 

 land, a motile non-sporeforming rod, isolated from the faeces 

 of a crow and finally a yellow, non-motile non-sporeforming 

 short rod isolated from the faeces of the musk ox. The two last 

 were also obtained from Northern Greenland. 



Of all these strains, one platinum loop from a twenty-four 

 hour broth culture was inoculated in each of the above mentioned 

 solutions. After incubation for twenty-four hours at 37°C., 

 they were examined for growth (turbidity), as well as for the 

 setting free of indol, by adding 5 cc. of the p-dimethylamidoben- 

 zaldehyde. According to Zipfel's w^ork, which I can confirm in 

 this point, it is quite unnecessary to let the tryptophane cultures 

 stand longer than twenty-four hours at ST'^C. If there is no 

 growth in this time, it is of no use to continue the observation. 



The results of these series are given in table 2. 



If we consider at first onlj^ the influence of the hydrogen ion 

 concentration, we find our suspicions confirmed that the non- 

 neutralized solution is too acid always to permit the growth of 

 the organism which is to be examined for indol liberation. Bad. 

 vulgare and Vibrio diolerae do not grow and therefore naturally 

 cannot give the indol reaction in the solutions which are not 

 neutralized. 



If we consider the results from the solutions with and without 

 ammoniimi lactate, we may conclude from these experiments 



* I wish to thank Prof. C. Kling, director of Statens Bakteriologiska Labora- 

 torium, for his kindness in supplying me with the cultures of V. cholerae and Bad. 

 lyphi. 



