SPECIAL MEDIA. 135 



biuret reaction), which remains permanent even after 

 boiling for five minutes. If, instead of a violet color, 

 there appears a red or reddish-yellow precipitate, the 

 peptone should be discarded, as in his experience no 

 indol is produced from peptone giving this reaction. 

 Both the peptone solution and that of the copper (par- 

 ticularly the latter) should be relatively dilute in order 

 for the reaction to be successful. 



Lactose litmus-agar, or litmus-gelatin of Wurtz. A 

 medium of much use in the differentiation of bacteria 

 is that recommended by Wurtz, consisting of slightly 

 alkaline nutrient agar-agar, to which from 2 to 3 per 

 cent, of lactose and sufficient litmus tincture to give 

 it a pale-blue color have been added. Bacteria capable 

 of causing fermentation of lactose when grown on this 

 medium develop into colonies of a pale-pink color and 

 cause, likewise, a reddening of the surrounding medium, 

 owing to the production of acid as a result of their 

 action upon the lactose ; while other bacteria, incapable 

 of such fermentative activities, grow as pale-blue colonies 

 and cause no reddening of the surrounding medium. 

 It is especially useful in the differentiation of the bacillus 

 of typhoid fever, which does not possess the property 

 of bringing about fermentation of lactose, from other 

 organisms that simulate it in many other respects, but 

 which do possess this property. 



Its preparation is as follows : to nutrient agar-agar 

 or gelatin, the alkalinity of which is such that 1 c.c. 

 will require 0.1 c.c. of a 1 : 20 normal sulphuric-acid 

 solution to neutralize it, lactose is added in the propor- 

 tion of 2 or 3 per cent. ; it is then decanted into test- 

 tubes and sterilized in the usual way. When ster- 

 ilization is complete enough sterilized litmus tincture 



