102 BACTERIOLOGY. 



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 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 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 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 

 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 

 carefully, to avoid contamination of the tubes during 

 manipulation. It is better not to add the litmus tinc- 

 ture before sterilizing the tubes, as its color character- 

 istics are in some way altered by its contact with organic 

 matters under the influence of heat. 



When ready it may be used as ordinary agar-agar or 

 gelatin, either for plates or slant cultures. 



Lceffler's blood-serum mixture. Loeffler's blood-serum 



