PEPTONE BOUILLON MEDIA. 35 



reaction. We have now to see how, by the addition of proteid 

 and other matter, it may be transformed into proper culture 

 media. Another and equally reliable method of making meat 

 infusion, in which a great saving of time is accomplished, is by 

 dissolving 2^ grammes of Liebig's extract of meat in looo c.c. 

 of boiling water. 



I. Bouillon Media. — These consist of meat infusion with 

 the addition of Certain substances to render them suitable for 

 the growth of bacteria. 



I (a). Peptone Broth or Bouillon. — This has the composi- 

 tion : — 



Meat infusion .... looo c.c. 



Sodium chloride ... 5 grms. 



Peptone albumin . . . 10 „ 



Boil till the ingredients are quite dissolved, and neutralise with 

 a 4 per cent solution of sodium hydrate. This is done by add- 

 ing cautiously a cubic centimetre or two of the sodium hydrate 

 solution, stirring well the while, and testing the reaction by 

 means of phenol-phthaleine paper, proceeding until the latter 

 strikes a well-defined rose-pink colour, thus indicating the point 

 of beginning alkalinity. Should, however, the color be deeper, 

 approaching a madder, then the alkalinity can be decreased by 

 adding enough of a 5 per cent hydrochloric acid solution until 

 the desired colour tint is acquired. To prevent the subsequent 

 precipitation of phosphates and other matters in the broth after 

 autoclaving in tubes, it is recommended to autoclave the whole 

 of the broth after adjusting the reaction, allowing it to become 

 cold, and then filter it, when it may be tubed and again auto- 

 claved with no fear of any subsequent clouding of the medium. 

 This method of neutralisation is to be recommended for all 

 ordinary work. 



In this medium the place of the original albumins of the meat is taken by 

 peptone, a soluble proteid not coagulated by heat. Here it may be remarked 

 that the commercial peptone albumin is not pure peptone, but a mixture of 

 albumoses (see footnote, p. 172) with a variable amount of pure peptone. The 

 addition of the sodium chloride is necessitated by the fact that alkalinisation 

 precipitates somp of the phosphates and carbonates present. Experience has 

 shown that sodium chloride can quite well be substituted. The reason for the 

 alkalinisation is that it is found that most bacteria grow best on a medium 

 slightly alkaline to litmus. Some, e.g: the cholera vibrio, will not grow at all 

 on even a slightly acid medium. 



