32 METHODS OF CULTIVATION OF BACTERIA 



immediately required is put into a large flask, the neck plugged 

 with cotton wool, and the whole sterilised by methods B 

 (2) or (3). This extract contains very little albuminous matter, 

 and consists chiefly of the soluble salts of the muscle, certain 

 extractives, and altered colouring 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. 



1. Bouillon Media. These consist of meat extract with the 

 addition of certain substances to render them suitable for the 

 growth of bacteria. 



(1) (a). Peptone Broth or Bouillon. This has the com- 

 position : 



Meat extract .... 1000 c.c. 

 Sodium chloride ... 5 grms. 



Peptone albumin . . . 10 



Boil till the ingredients are quite dissolved, and neutralise 

 with a saturated 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. 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 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. 27, 29). 

 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. 165) 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 

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

 on even a slightly acid medium. 



