362 THE ALCALIGENES— DYSENTERY TYPHOID GROUP 
after eighteen to twenty-four hours' incubation at 37° C. Develop- 
ment in gelatin is less rapid, and the colonies after two to three days' 
incubation at 20° C. are somewhat brownish in color. A uniform 
turbidity is produced in plain broth after eighteen hours' growth at 
37° C. ; development in glucose broth is more intense, but after five 
to seven days it ceases and the organisms die, due to the accmnula- 
tion of acid. Growth is luxuriant in milk, but there is little chemical 
change in the composition of the medium as the result of the growth.^ 
Two types of reaction are observed in litmus milk: (o) The reaction 
becomes slightly acid, turning the litmus to a lilac color which per- 
sists. This is much more common than (6) ; the milk becomes slightly 
acid, as in "a," then it becomes slowly but progressively alkaline. 
Relatively few authentic strains of typhoid bacilli appear to produce 
the transient acidity in this medium. At one time potato was regarded 
as an important differential medium for the recognition of the typhoid 
liosus, flagella stain. 
bacillus. The "invisible growth" described by Gaffky- is now known 
to be dependent largely upon the reaction; potatoes having an acid 
reaction give this invisible growth; old potatoes which usually have 
a slightly alkaline reaction, give a heavy, brownish growth much like 
that of the colon bacillus. The addition of small amounts of alkali, 
as sodium carbonate, to potato prior to inoculation makes the growth 
visible and brown; the adflition of a small amount of organic acid to 
the medium usually results in the development of the invisible type of 
growth. 
The typhoid bacillus is an aerobic, facultatively anaerobic organism, 
whose minimal temperature of growth is about 8° C; development is 
maximal at 37° C, and ceases when the culture is exposed to tem- 
peratures above 43° to 44° C. An exposm-e of ten to twenty minutes 
at 60° C. will kill the naked organisms; a longer exposure at a higher 
1 Kendall, Day and Walker: Jour. Am. Chem. Assn., 1914, 36, 1958. 
2 Loc. cit. 
