OBSER VA TION OF INDOL-FORMA TION. 87 



To obtain a " dextrose-free " bouillon it is usual to inoculate ordinary 

 bouillon with some organism, such as B. coli, which is known to 

 ferment dextrose, and allow the latter to act for 48 hours. The bouillon 

 is then filtered and resterilised. A sample is tested for another period 

 of 48 hours with B. coli, to make certain that all the dextrose has been 

 removed. If no fresh gas formation is observed, then to the remainder 

 of the bouillon the sugar to be investigated may be added. It is 

 preferable that the addition should be made in the form of a sterile 

 solution. If the raw crystals be placed in the bouillon and this then 

 sterilised, there is the danger that chemical changes may take place in 

 the sugar, in consequence of its being heated in the presence of sub- 

 stances (such as the alkali) which may act deleteriously upon it. 



The Observation of Indol-formation by Bacteria. The 



formation of indol from albumin by a bacterium sometimes 

 constitutes an important specific characteristic. To observe 

 indol production the bacterium is grown preferably at 

 incubation temperature on a fluid medium containing 

 peptone. The latter may either be ordinary bouillon or 

 peptone solution (see p. 50). Indol production is recog- 

 nised by the fact that when acted on by nitric acid in the 

 presence of nitrites, a nitroso-indol compound is produced, 

 which has a rosy red colour. Some bacteria (e.g. the 

 cholera vibrio) produce nitrites as well as indol, but often, 

 in making the test (e.g. in the case of B. coli), the nitrites 

 must be added. This may be effected by using yellow 

 nitric acid (which of course contains nitrous acid) for the 

 test, or by adding to an ordinary tube of medium i c.c. of 

 a .02 per cent solution of potassium nitrite, and testing 

 with pure nitric or sulphuric acid. In any case only a 

 drop of the acid need be added to say 10 c.c. of medium. 

 If no result be obtained at once it is well to allow the tube 

 to stand for an hour, as sometimes the reaction is very 

 slowly produced. The amount of indol produced by a 

 bacterium seems to vary very much with certain unknown 

 qualities of the peptone. It is well therefore to test a 

 series of peptone with an organism (such as the B. coli) 

 known to produce indol, and noting the sample with which 

 the best reaction is obtained, to reserve it for making media 

 to be used for the detection of this product. 



