106 BACTERIOLOGY. 



find it a valuable addition to our means of differentiat- 

 ing bacteria. 



Rosolic acid cannot be used with safety in solutions 

 containing glucose, as the reducing action of the latter 

 deprives it of its color. 



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

 of much use in the differentiation of bacteria is that 

 recommended by Wurtz, consisting of ordinary nutri- 

 ent, slightly alkaline 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 an especially useful aid 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 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 ster- 

 ilization 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 care- 

 fully, to avoid contamination of the tubes during ma- 



