428 BACTERIOLOGY. 



tricts. (Fig. 72.) These colonies have sometimes the 

 appearance of flattened pellicles of glass-wool, and 

 usually a pearl-like lustre. 



On AGAR-AGAR the colonies present nothing typical. 



STAB-CULTURES. In stab-cultures the growth is 

 mostly on the surface, there being only a very limited 

 development down the track made by the needle. The 

 surface-growth has the same appearance in general as 

 that given for the colonies. 



POTATO. The growth on potato is usually described 

 as luxuriant but invisible, making its presence evident 

 only by the production of a slight increase of moisture 

 at the inoculated point, and by a limited resistance 

 offered to a needle when it is scraped across the track 

 of growth. While this is so in many cases, yet it cannot 

 be considered as invariable, for at times this organism 

 develops more or less visibly on potato. 



POTATO-GELATIN. The growth is similar to that 

 upon ordinary nutrient gelatin. 



MILK. It does not cause coagulation when grown 

 in sterilized milk. 



BOUILLON. It causes uniform clouding of the bouil- 

 lon and brings about a slightly acid reaction. 



INDOL FORMATION. It is customary to regard this 

 organism as devoid of the power of forming indol ; 

 in fact, this has hitherto been considered one of its 

 important differential peculiarities. By the usual meth- 

 ods of cultivation and testing the indol reaction is not 

 observed in cultures of the typhoid bacillus. It has 

 recently been shown, however, by Peckham, that by 

 repeated transplantation, at short intervals, into either 

 Dunham's peptone solution, or, preferably, a freshly 

 prepared alkali-tryptone solution, made from trypton- 



