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 l/i to 3/2 in length by 0.5// to Sfjt 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.) 



FIG. 49. 



FIG. 50. 



Typhoid bacilli from nutrient agar. 

 X 1100 diameters. 



Typhoid bacilli from nutrient gelatin. 

 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 



