BACILLUS TYPHOSUS. 407 
' medium appear to be chiefly due to variations in the 
substance, especially in the reaction, of the potato. 
Milk. The typhoid bacillus does not cause coagu- 
lation when grown in sterilized milk. 
Fermentation. It does not produce fermentation in 
either glucose, lactose, saccharose, or glycerin bouillon, 
and evolves no gas as the result of fermentation. 
Lactose-litmus Agar. It grows usually as pale blue 
colonies on lactose-litmus agar, but occasionally causes 
slight reddening of the surrounding medium. 
Indol Reaction. It does not produce indol. This 
test was proposed by Kitasato for differentiating the 
typhoid bacillus from other similar bacilli, such as 
those of the colon group, which, as a rule, give the 
indol reaction. 
The reaction, being a very delicate one, requires 
great care in its performance to arrive at accurate con- 
clusions. (For test of indol, see page 77.) Instead of 
bouillon, the simple peptone-water (which consists of 
dried peptone, 1 part; sodium chloride, 0.5 part, and 
distilled water, 100 parts) is to be preferred for this 
purpose, because its pale color does not mask the reac- 
tion. 
Pathogenic Properties. It has been extremely diffi- 
cult to show experimentally that the bacillus typhosus 
is specifically pathogenic for animals. A great many 
experiments have been made, with the view of repro- 
ducing in the tissues of lower animals the pathological 
lesions of typhoid fever as seen in man, but the results 
have not been completely satisfactory ; nor is this sur- 
prising when one considers that this disease does not 
occur naturally, so far as is known, among animals. 
Sickness or fatal results without the appearance of the 
