SPECIAL MEDIA. 123 



The tubes are then to be sterilized in the usual way by 

 steam. When sterilization is completed and the tubes 

 cooled the solution will be of a very pale rose color, 

 which disappears entirely under the action of acids, but 

 becomes much more intense under the influence of alka- 

 lies. We have used this solution for some time for the 

 study of the reactions produced by different organisms, 

 and find it a valuable addition to our means of differen- 

 tiating bacteria. 



Rosolic acid cannot be used in solutions containing 

 glucose, as the reducing action of the latter deprives it 

 of its color. 



Lactose Utmus-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 



