274 BACILLUS TYPHOSUS. 



to the same point as before, filtered, and 1 per cent, of potas- 

 sium iodide added. The typhoid and colon bacillus are the 

 only organisms that will grow on this medium, the colonies 

 of the colon bacillus appearing some time before those of the 

 typhoid bacillus (Figs. 126 and 127). 



Another method for isolating the typhoid bacillus from the 

 feces is proposed by Drigalski and Conradi: Agar containing 

 2 per cent, of peptone and 1 per cent, of riutrose is used. To 

 this are added 130 c.c. of litmus solution, containing 15 

 grams of milk-sugar, 2 c.c. of a 10 per cent, soda solution, 

 and 10 c.c. per liter of " Krystall violett-Hoechst " solution. 

 This medium is plated in Petri dishes. After it has hardened, 

 the material is spread over the surface of the plate in a thin 

 film. After this film has dried thoroughly the plates are 

 placed in the incubator. In about fifteen hours the colonies 

 are distinctly visible. Those of the colon bacillus are from 2 

 to 6 mm. in diameter and red in color. The typhoid colonies 

 are about half that size, smooth, dew-drop-like, and blue. 

 The colonies are examined microscopically and the results 

 verified. 



Pathogenesis : Typhoid fever is unquestionably due to 

 Bacillus typhosu8 y although thus far it has been impossible 

 to produce typhoid fever experimentally in animals. It is 

 never found in the tissues of a healthy individual, and it is 

 constant in the lesions of typhoid fever. During the conva- 

 lescent period it can also be found in the excreta, especially 

 the urine. When an animal is inoculated with a pure culture 

 of the germ, it dies with symptoms of intoxication. In man, 

 ingestion of the bacillus is followed by the typical disease, 

 providing the conditions are favorable to the development of 

 the organism. To all appearances, man possesses a certain 

 degree of immunity to typhoid, and a special predisposition, 

 either general or local, such as convalescence from some other 

 disease, or an acute intestinal catarrh, is necessary before 

 infection can take place. 



Infection : Although infection may occur by way of the 

 respiratory tract by inhaling dust containing the typhoid 

 germ, yet the digestive tract is the usual portal of entry for the 

 germ. The natural resistance of the germ to acids enables it 



