LACTOSE LITMUS-AGAR 129 



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 (particularly 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 organ- 

 isms 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 neu- 

 tralize it, lactose is added in the proportion of 2 or 3 per 

 cent.; it is then decanted into test-tubes and sterilized in 

 9 



