374 THE TYPHOID BACILLUS 



Tests for indol. To determine whether an organism produces indol or not, 

 a solution of peptone must be used and not ordinary broth. The following 

 is a medium often used for this test : 



Water, - - 100 c.c. 



Witte's, Chapoteaut's or Byla's peptone, 2 grams. 



Sodium chloride, - 0*5 to 1 gram. 



Tube in quantities of about 15 c.c. and autoclave. 



After sowing, incubate for 2-8 days and apply one or other of the following 

 tests : 



(a) Salkowski's reaction. To the culture in peptone water add 1 c.c. of a 

 0'2 per cent, solution of potassium nitrite, then, slowly, 1 c.c. of a 25 per 

 cent, solution of chemically pure sulphuric acid in water. If indol be present 

 a rose tint appears. 



Nonotte and Demanche find that the reaction is more delicate in the warm. To a 

 peptone-water culture add 1 c.c. of a 1 in 1000 solution of nitrite of potassium and 

 8 drops of pure concentrated sulphuric acid and boil the upper part of the liquid. 

 If indol be present a very distinct pink colour appears even when the amount of 

 indol does not exceed 1 part in 4 millions : in the cold, the reaction only takes place 

 if the amount of indol exceeds 1 part, in 75,000. 



(6) Weyl-Legal's reaction. To the culture add 5 to 10 drops of a 5 per 

 cent, solution of sodium nitro-prusside then a few drops of a 30 per cent, 

 solution of washing soda. The solution turns brown. After a few minutes 

 add 10 to 15 drops of glacial acetic acid ; if indol be present a characteristic 

 blue colour appears but often only after some delay. 



(c) Nencki's reaction. To the culture add first a few drops of glacial acetic 

 acid then 2-3 c.c. of alcohol-ether : shake and allow to stand until the ether 

 rises : decant the layer of ether and evaporate it in a porcelain dish. To the 

 residue add 1 to 2 drops of a 0'2 per cent, solution of potassium nitrite and a 

 few drops of pure sulphuric acid. This method is very delicate and the least 

 trace of indol is shown by the appearance of a rose pink colour. 



(d) Fleig's reaction. To 10 c.c. of culture add 10 c.c. of a 1 in 50 alcoholic 

 solution of furfurol, then pure hydrochloric acid drop by drop. If indol be 

 present the solution turns yellow. This method is very delicate. 



[(e) The para-dimethyl-amido-benzaldehyde test. Recommended. Prepare 

 two solutions : 

 Solution I. 



Para-dimethyl-amido-benzaldehyde, - 4 parts. 



Absolute alcohol, - - 380 ; . 



Concentrated hydrochloric acid, - - - - 80 



Solution II. 



Saturated aqueous solution of potassium persulphate. 



[To about 10 c.c. of the broth or peptone-water culture of the organism 

 add 5 c.c. of Solution I. and then 5 c.c. of Solution II., shake the mixture and 

 the presence of indol is indicated by the appearance, in a very short time, of 

 a red colour, which gradually becomes darker on standing. The reaction 

 may be accelerated by heating the mixture. | 



Some peptones contain a trace of indol and to avoid all possibility of mistake 

 Sicre recommends using a 1 per cent, solution of Byla's peptone and when testing 

 for indol to test at the same time a tube of sterilized peptone- water as a control. 



Growth on Synthetic media. A number of synthetic media have been 

 prepared on which the typhoid bacillus grows slowly and feebly while closely 

 related organisms with which it may be confused grow freely. 



Too much importance should not be attached to the differentiating function of 

 these media, but, generally speaking, if growth be absent or delayed on any one of 



