PEPTONE BO UILL ON MEDIA . 43 



extract .5 per cent sodium chloride and i per cent 

 peptone albumin. Boil till both are quite dissolved, and 

 neutralise with a saturated solution of sodium carbonate. 

 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. This neutralisation 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 carbonate must therefore 

 be added till red litmus is turned slightly but distinctly 

 blue, and blue litmus is not at all tinted red. After alkali- 

 nisation, 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. 



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, Chap. V.) 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 substi- 

 tuted. The reason for the alkalinisation is that it is found 

 that most bacteria grow best on a slightly alkaline medium. 

 Some, e.g. the cholera vibrio, will not grow at all on even a 

 slightly acid medium. 



i (fi). Glucose Broth. To the other constituents of i (a) 

 there is added i or 2 per cent of grape sugar. The steps 

 in the preparation are the same. Glucose being a reducing 

 agent, no free oxygen can exist in a medium containing it, 

 and therefore glucose broth is used as a culture fluid for 

 anaerobic organisms. 



i (c). Glycerine Broth. The initial steps are the same 



