326 THE ALCAL1GENES DYSENTERY TYPHOID 



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. 1 

 Two types of reaction are observed in litmus milk: (a) 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 

 bacillus. The "invisible growth" described by Gaffky 2 is now known 

 to be dependent largely upon the reaction; potatoes having an acid 

 reaction give this invisible growth; old potatoes which usually have 



FIG. 46. Bacillus typhosus, flagella stain. 



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 addition 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 exposure of ten to twenty minutes 

 at 60 C. will kill the naked organisms; a longer exposure at a higher 

 temperature is required to kill them when they are suspended in organic 



1 Kendall, Day and Walker, Jour. Am. Chem. Assn., 1914, xxxvi, 1958, 



2 Loc. cit. 



