BACILLUS TYPHOSUS. 403 
unless they are convalescent from typhoid infection, 
have demonstrated, on a scientific basis, that this bacil- 
lus is the chief etiological factor in the production of 
typhoid fever. 
Morphological Characters. The typhoid bacilli are 
rods of about 1p to 3y in length by 0.5y to 08y in 
diameter, with rounded ends, often growing into long 
threads. They are usually longer and somewhat more 
slender in form than the bacilli coli communis under 
similar conditions. The typhoid bacilli vary, however, 
in shape when grown in different culture media. (See 
Figs. 49, 50, and Fig. 6, page 39.) 
Typhoid bacilli from nutrient agar. Typhoid bacilli from nutrient gelatin. 
X 1100 diameters. X 1100 diameters. 
The typhoid bacilli stain with the ordinary aniline 
colors, but a little less readily than do most other bac- 
teria, though there is no constant difference in staining 
characteristics between these and other bacilli of this 
group—the colon bacilli, They are decolorized by 
Gram’s iodine solution. Not infrequently, particu- 
larly when grown on potato, refractive granules may 
