1 88 PATHOGENIC BACTERIA. 



boiled and filtered and the solid ingredients added and dis- 

 solved subsequently. This observation referred especially 

 to bouillon intended for the culture of diphtheria bacilli 

 for toxin. The reaction is then carefully titrated accord- 

 ing to the directions already given. For rough work in 

 students' classes litmus-paper is commonly used as an 

 indicator, the alkaline solution being added drop by drop 

 until a faint blue appears on the red paper. The method 

 of using phenolphthalein suggested by Timpe is to con- 

 tinue the addition of the alkaline solution until a drop of 

 the bouillon produces a red spot upon phenolphthalein- 

 paper. Such a paper can easily be made by using a solu- 

 tion of 5 grams of phenolphthalein to i liter of 50 per 

 cent, alcohol. The bibulous paper is cut into strips, 

 moistened with the solution, and then hung up to dry. 

 It keeps quite well. Acids do not change the appearance 

 of the paper, but small traces of alkali turn it red. 



The bouillon thus prepared is a clear fluid of a straw 

 color, much resembling normal urine in appearance. It 

 is dispensed in previously sterilized tubes with cotton 

 plugs — about 10 c.cm. to each — and is then sterilized by 

 steam three successive days for fifteen to twenty minutes 

 each, according to the directions already given for frac- 

 tional sterilization. (See p. 167.) 



When it is desirable to prepare the bouillon from beef- 

 extract, the method is very simple. To 1000 c.cm. of 

 clean water 10 grams of Witte's dried beef-peptone, 5 

 grams of sodium chlorid, and about 2 grams of beef- 

 extract are added. The solution is boiled until the con- 

 stituents are dissolved, titrated, and filtered when cold. 

 If it is filtered while hot, there is always a subsequent 

 precipitation of meat-salts, which clouds it. 



Bouillon and other liquid culture-media are best dis- 

 pensed and kept in small receptacles — test-tubes or flasks 

 — in order that a single contaminating organism, should 

 it enter, may not spoil the entire bulk. A very con- 

 venient simple apparatus used by bacteriologists for fill- 

 ing tubes with liquid media is shown in Figure 21. It 



