114 THE ACTION OF ENZYMES 



Test for indole in each 



(a) With sodium nitrite. Make a small stock 5 per cent, 

 solution of sodium nitrite. Take i c.c. of this and dilute it to 

 100 c.c. Then add i or 2 c.c. of the dilute .05 per cent, solution 

 to the tube to be tested, and pour a few drops of pure concentrated 

 sulphuric or hydrochloric acid down the side of the tube. 



The acid liberates nitrous acid, which reacts with any indole 

 present, producing a pink or purple colour. If pink colour is 

 absent or not distinct, place the tube for thirty to forty minutes 

 in an incubator at 37 C. 



Control tests should be made in uninoculated tubes to show 

 that the reaction is due to the growth of the bacteria. 



(f) With paramidobenzaldehyde. 



i. Marshall's test. 



Dissolve 6 gr. of para-dimethylamidobenzaldehyde in 100 c.c. 

 of water and add 12 c.c. of concentrated sulphuric acid. 



This reagent gives a rose or cherry-red colour when added to cul- 

 tures containing indole, and is more reliable than the former test. 



ii. Ehrlich's test. 



Solution A. Dissolve i gr. of para-dimethylamidobenzaldehyde 

 in 100 c.c. of 96 per cent, alcohol, and add 20 c.c. of concentrated 

 hydrochloric acid. 



Solution B. Saturated solution of potassium persulphate in 

 water. To 10 c.c. of the culture to be tested add 5 c.c. of each 

 of the solutions A and B ; shake and allow to stand for five 

 minutes : a red colour indicates the presence of indole. 



Ex. 64. Grow the organisms mentioned in the previous 

 exercise in slightly alkaline sugar-free bouillon. 



Incubate for eight to ten days. Then test for indole, as above. 



Ex. 65. The Voges-Proskauer Reaction. Prepare a 2 per cent, 

 glucose bouillon : place it in fermentation tubes and sterilize. 



Inoculate tubes with 



1. Bact. coli, 



2. Bact. lactis aerogenes, 



3. Bact. cloaca, 



and incubate at 22 C. for three days. 



