PATHOGENIC BACTERIA. 



303 



tose-litmus-gelatin or -agar of VVurtz makes use of the blue 

 tinge possessed by colonies of the typhoid bacillus on this 

 medium to distinguish them from colonies of the colon 

 bacillus and other bacteria which form acids from lactose. 

 Neutral red has been used in the same manner, as it is said 

 not to be altered by the typhoid bacillus, but to be changed 

 by the colon bacillus to a yellow color. (To neutral, plain 

 agar add i per cent, of a saturated aqueous solution of 

 neutral red, and some also add .3 per cent, dextrose.) 



FIG. 86. 







' 



' 



, 



-K&- 



i ' 



~f- 



Bacillus of typhoid fever, stained by Loffler's method to show flagella. 



(X 1000.) 



Ill Dunham's peptone solution indol is not formed, as a 

 rule. On potato it usually forms what is called an invisible 

 growth; that is, although no development is apparent to the 

 eye, numerous bacilli may be shown under the microscope 

 in smear preparations made from the surface of potato 

 inoculated about forty-eight hours previously. Occasionally 

 a slight visible growth is seen on potato. 



