78 METHODS OF CULTIVATION OF BACTERIA 



production the bacterium is grown, preferably at incubation 

 temperature, in a fluid medium containing peptone. The latter 

 may either be ordinary bouillon or preferably peptone solution 

 (see p. 38). Indol production is recognised 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 usually in making the test (e.g. in the case of b. coli) the 

 nitrites must be added. This is effected by adding to an ordinary 

 tube of medium 1 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. In many instances incubation at 37 C. for 

 several days may be necessary before the presence of indol is 

 demonstrable. 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 peptones 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. 



The Drying of Substances in vacuo. As many substances, 

 for example toxins and antitoxins, with which bacteriology is 

 concerned would be destroyed by drying with heat as is done in 

 ordinary chemical work, it is necessary to remove the water at 

 the ordinary room temperature. This is most quickly effected 

 by drying in vacuo in the presence of some substance such as 

 strong sulphuric acid which readily takes up water vapour. The 

 vacuum produced by a water-pump is here not available, as in 

 such a vacuum there must always be water vapour present. An 

 air-pump is therefore to be employed. Here we have found the 

 Geryk pump most efficient, and it has this further advantage, 

 that its internal parts are lubricated with an oil of very low 

 vapour density so that almost a perfect vacuum is obtainable. 

 The apparatus is shown in Fig. 39. The vacuum chamber 

 consists of a bell-jar set on a brass plate. A perforation in the 

 centre of the latter leads into the pipe a, which can be connected 

 by strong- walled rubber-tubing with the air-pump, and which 

 can be cut off from the latter by a stop-cock 6. In using the 

 apparatus the substance to be dried is poured out in flat dishes 

 (one-half of a Petri capsule does very well), and these are stacked 

 alternately with similar dishes of strong sulphuric acid on a 



