SPECIAL MEDIA. 95 



it a valuable addition to our means of differentiation of 

 bacteria. 



Eosalic acid cannot be used M'ith safety in solutions 

 containing glucose, as the reducing action of the latter 

 deprives it of its color. 



Ladose-litmus-agar, or gelatin of Wurtz. A medium- 

 of much use in the differentiation of bacteria is that 

 recommended by Wurtz, consisting of ordinary nutrient, 

 slightly alkaline agar-agar to which from 2 to 3 per 

 cent, of lactose and sufficient litmus tincture to give it 

 a 25ale 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 for 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 possess this property. 



Its preparation is as follows : To nutrient agar-agar 

 or gelatin, the. alkalinity of which is such that one cubic 

 centimetre will require 0.1 c.c. of a 1 : 20 normal sul- 

 phuric acid solution to neutralize it, lactose is added in 

 the proportion of 2 or 3 per cent. ; it is then decanted 

 into test-tubes and sterilized in the usual way. When 

 sterilization is complete there is to be added to each 

 tube enough sterilized litmus tincture to give a decided 

 though not very intense blue color. This must be done 

 careftiUy, to avoid contamination of the tubes during 



