CHAPTER XX. 



THE TYPHOID BACILLUS (BACILLUS TYPHOSUS). 



THIS organism was first observed by Eberth, and independently by 

 Koch, in 1880, in the spleen and diseased areas of the intestine in 

 typhoid cadavers, but was not obtained in pure culture or its principal 

 biological cultures described until the researches of Gaffky, in 1884. 

 The methods of identification employed by Gaffky were found insuffi- 

 cient to separate the typhoid bacillus from other bacilli of the colon- 

 typhoid group. Every known cultural characteristic of the typhoid 

 bacillus was found to be duplicated in some member of this group, and 

 it was only when a bacillus combined all the characteristics of a typical 

 variety that it could be assumed that it was in all probability the typhoid 

 bacillus. The absolute identification of the bacillus only became pos- 

 sible with the increase of our knowledge concerning the specific immune 

 substances developed in the bodies of immunized animals. Its etiological 

 relationship to typhoid fever has been particularly difficult of demon- 

 stration, for, although pathogenic for many animals when subcutaneously 

 or intravenously inoculated, it has been impossible to produce infection 

 in the natural way or produce gross lesions corresponding closely to 

 those occurring generally in man. It has been recently shown, however, 

 that animals under certain conditions, when their power of resistance 

 has been reduced, may be rendered susceptible to infection, with the 

 production of more or less characteristic lesions. These results, together 

 with the specific reactions of the blood serum of typhoid patients, the 

 constant presence of the bacillus typhosus in the intestines and some 

 of the organs of the typhoid cadavers, the frequent isolation of this 

 bacillus from the roseola, spleen, blood, and excretions of the sick 

 during life, the absence of the bacilli in healthy persons, unless they 

 have been directly exposed to or are convalescent from typhoid infec- 

 tion, all these have demonstrated scientifically that this bacillus is the 

 chief etiological factor in the production of the great majority of cases 

 designated as typhoid fever. 



Morphology and Staining. Typhoid bacilli are short, rather plump 

 rods of about 1, to 3/* in length by 0.5^ to 0.8/* in diameter, having 

 rounded ends, and often growing into long threads. They are longer 

 and somewhat more slender in form than most of the members of the 

 colon group of bacilli (Figs. 88 and 89). 



The typhoid bacilli stain with the ordinary aniline colors, but a little 

 less readily than do most other bacteria. Like the bacilli of the colon 

 and paratyphoid groups, they are decolorized by Gram's method. 



