INDOL PRODUCTION BY BACTERIA 473 



We have used three media. (1) Dunham's pepton solution. 

 One per cent of pepton (Armour's) and 0.5 per cent sodium chloride 

 were dissolved in distilled water and the reaction adjusted to +1 

 to phenolphthalein. (2) Rivas' trypsinized pepton (Rivas 

 1912). Ten grams of pepton (Armour's) were dissolved in 200 

 cc. distilled water. To this was added a solution of 0.5 gram 

 trypsin in 10 cc. of water (trypsin dissolved by shaking and gentle 

 heating not over 40°C.) and digestion allowed to continue for three 

 hours at 37°C., with frequent stirring. The solution was then 

 made up to 1 liter and reaction adjusted to + 1 to phenolphtha- 

 lein. (3) Cannon's casein medium (Cannon, 1916). Twenty 

 grams of chemically pure casein were added to 250 cc. distilled 

 water and the whole made alkaline to phenolphthalein with 

 sodium carbonate. One-half gram of trypsin was added and the 

 casein allowed to digest for six hours. The medium was then 

 autoclaved and 5 grams each of asparagin and ammonium lactate, 

 2 grams of dipotassium phosphate and 0.2 gram magnesium 

 sulphate were added. The solution was made up to 1 liter and 

 reaction adjusted to +1 to phenolphthalein. 



A large number of comparative tests were made on these three 

 media. In no case did the final indol test vary but a positive 

 reaction was obtained more quickly, and the color tests were 

 stronger, with the trypsinized casein or pepton than with 

 Dunham's pepton solution. Positive tests with the trypsinized 

 pepton were noted after six hours incubation with Bad. coli and 

 color production with Ehrlich's reagent was at its maximum at the 

 end of twenty-four hours. With Dunham's solution the max- 

 imum was obtained only after four days. After six days the 

 indol began to disappear. As most of our tests were made 

 simultaneously on all three media we used the four day period, 

 although forty-eight hours is sufficient for the trypsinized media. 



The influence of oxygen supply on indol formation has been 

 studied by Porcher and Panisset (1911). They found that 

 growing cultures of the colon bacillus and of proteus anaerobi- 

 cally decreased the amount of indol formed, while if a current of 

 oxygen was kept going through the flask, the amount was in- 

 creased. However, they were unable to provoke the formation 



