BOUILLON MEDIA 33 



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 1 . . . . 1000 c.c. 

 Sodium chloride . . 5 grms. 



Peptone albumin 2 . . . 10 „ 



Boil till the ingredients are quite dissolved, and make slightly 

 alkaline to litmus as directed below. After alkalinisation, 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. 28, 29). 



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

 by peptone, a soluble protein 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. 191) 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, meningococcus, are very sensitive 

 to the reaction of their surroundings. 



Adjustment of the Reaction of Media. — The adjustment of 

 the reaction of bacteriological 'media is a matter of great com- 

 plexity. The method usually adopted with meat extract, which 

 as prepared above is ordinarily slightly acid, is to add saturated 

 sodium carbonate or sodium hydrate solution till the medium is 

 slightly but distinctly alkaline to red litmus paper and' no longer 

 affects blue litmus paper. The occurrence of an amphoteric 

 reaction — i.e., one where red litmus is turned blue, and blue, 

 red — is thus avoided. The test paper must be immersed in the 

 liquid — on no account is the sampling to be done by trans- 

 ferring drops to the paper by means of a glass rod. The 

 disadvantages of this method are that ordinary litmus is not a 

 delicate indicator and, further, no standardisation of the proper 

 tint to be aimed at is possible. The latter difficulty can be 



1 Some workers, instead of meat extract as made above, use Liebig's 

 extract of beef, 2 grammes to the litre. "While a medium made up in this way 

 suffices for most of the commonly occurring pathogenic bacteria, it is advisable, 

 in the case of the less robust organisms, to use media freshly made with meat. 



2 Whatever kind of peptone is used, it ought always to be tested for the 

 absence of sugars and indol before b^Mused for a medium. Chapetaut's, 

 and also Savory and Moore's, peptone^^^^ecommended at present. 



