ACID BEEF BROTH 69 



If the rose colour does not develop with cresol-red, but 

 only a brown shade, the medium is on the acid side ; 

 if a pink colour appears with the phenolphthalein, the 

 medium is too alkaline. The error in either direction can 



N 

 be corrected by the addition of a few drops of NaOH or 



HC1, as the case may be, to the medium in bulk and the 

 reaction is again controlled. As mentioned, a reaction of 

 P H == 7-6 is aimed at ; in the adjusted medium it will 

 be somewhat over this (i.e. it is too alkaline), but in 

 the process of sterilisation the acidity is increased so 

 that the sterilised medium will have about the right 

 reaction. 1 



The reaction of media standardised in this way may 

 be designated by the P H value obtaining. 



Preparation of Culture Media 



Nutrient broth, nutrient gelatin and nutrient agar-agar 

 are the more important culture media for the cultivation 

 of the pathogenic micro-organisms ; they may also be 

 employed for the cultivation of a number of saprophytic 

 forms. These may be prepared either from an infusion of 

 meat, usually beef, termed acid beef-broth, or with one 

 of the commercial meat extracts such as " Lemco." 



The constituents composing the medium are weighed 

 out or measured and are introduced into a glass flask, 

 a beaker or an enamelled iron jug or saucepan. 



Acid beef -broth. Constituents : 1,000 c.c. of water, 

 preferably distilled ; 500 grm. of finely minced fresh 

 gravy beef, free from fat. Heat over a water-bath at 

 40-45 C. for twenty minutes with frequent agitation, 

 then boil for ten minutes, strain through cotton-wool, and 



1 See Journ. of Bacteriology, vol. iv, 1919, p. 119, and Mclntosh and 

 Smart, Lancet, 1919, vol. ii. p. 723. 



