PEPTONE BOUILLON MEDIA. 43 



matters, along with any slight traces of soluble proteid 

 not coagulated by heat. It is of acid reaction. We have 

 now to see how, by the addition of proteid and other 

 matter, it may be transformed into proper culture media. 



r. Bouillon Media. These consist of meat extract with 

 the addition of certain substances to render them suitable 

 for the growth of bacteria. 



i (a). Peptone Broth or Bouillon. Add to the meat 

 extract .5 percent sodium chloride and i per cent peptone 

 albumin. Boil till both are quite dissolved, and neutralise 

 with a 4 per cent solution of sodium hydrate. Add the 

 latter drop by drop, shaking thoroughly between each drop 

 and testing the reaction by means of litmus paper. Go on 

 till the reaction is slightly but distinctly alkaline. Neutral- 

 isation must be practised with great care, as under certain 

 circumstances, depending on the relative proportions of the 

 different phosphates of sodium and potassium, what is 

 known as the amphoteric reaction is obtained, i.e. red 

 litmus is turned blue, and blue red, by the same solution. 

 The sodium hydrate must be added till red litmus is turned 

 slightly but distinctly blue, and blue litmus is not at all 

 tinted red. After alkalinisation, allow the fluid to become 

 cold, filter through Swedish filter paper into flasks, make 

 up to original volume with distilled water, plug the flasks 

 with cotton wool, and sterilise by methods B (2) or (3), 

 PP- 37> 39- This method of neutralisation is to be recom- 

 mended' 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. 154) with a 

 variable amount of pure peptone. The addition of the sodium chloride 

 is necessitated by the fact that alkalinisation precipitates some 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 alk'aline to litmus. Some, e.g. the cholera vibrio, 

 will not grow at all on even a slightly acid medium. 



Standardisation of Reaction of Media. While the 



