174 



We might at first think that the formation of indol would 

 be largely dependent on whether one or other of these four 

 very different products from which it could be formed was 

 forthcoming. A very marked uniformity however was obser- 

 ved on the whole, as the cultures either formed indol in all 

 the four media or in none of them. Occasionally the forma- 

 tion of indol, present in the other media, failed. But this 

 was chiefly the case in the Chapotaut peptone, which (as 

 it was used as a pure peptone solution without broth) proved 

 to be a very bad nutritive medium for Pfeiffers bacillus, and this 

 also explains why the positive reactions in this medium were 

 weak and considerably more irregular than in the other media. 

 In a few instances the reaction was also absent in the broth 

 containing Witte peptone, which is quite intelligible as the 

 positive reactions were usually weak on this medium also, and 

 in those cases where the reaction failed it was comparatively 

 weak on the other media. In Witte peptone tryptophane is not 

 easily accessible because it is not sufficiently liberated from 

 the albumose complex. 



The formation of indol in erepton and casein-tryptone 

 media (IX and X) however, agree excellently with one another. 

 Most of the strains seem to break down all the tryptophane 

 (4 strains of Bacillus coli, inoculated in the casein-tryptone 

 medium, gave an indol reaction of exactly the same intensity 

 as the vigorously indol-producing Pfeiffer's bacilli), and thus 

 the reactions „4" and „7" in the erepton and casein-tryptonie 

 media respectively make their appearance^ Two or three strains 

 proved to be weak indol-formers: intensity in erepton, 1; in 

 casein-tryptone, 3 — 4. 



In the case of all the strains H 260—313, which came from the 

 same series of inoculations, and were all uniformly good growers 

 and can therefore be compared with one another, those strains 

 are noted where the reactions on the gelatine medium (tested by 

 mixing the melted gelatine culture with the reagents) were weak. 

 It will be seen that weak reactions on the 2 different media re- 

 gulary coincide. 



In a number of the cultures examined the growth was only 

 weak or uncertain and it might be thought that the negative 

 result of the indol test could be due in some cases to lack 

 of growth. In order to test this I inoculated, in November 

 1920, all the strains which had reacted negative in the pre- 



