IQ2 BACTERIOLOGY. 



brown coating. As a rule, however, they spread 

 out upon the potato in the form of an invisible 

 film the existence of which is made apparent 

 to the observer only through the increased glis- 

 tening of the surface. This growth on potato 

 is usually characteristic, but unfortunately it 

 is not constant and is observed occasionally in 

 other species, so that differential diagnosis of 

 the typhoid bacteria is at times very difficult. 

 Many saprophytic species from water and soil 

 grow, especially upon gelatin, very much in 

 the manner of the typhoid bacillus and can 

 ordinarily be distinguished from it only by 

 such characters as individual form, behavior 

 towards Gram's stain, motility, and growth 

 upon potato. 



It is often extraordinarily difficult to distin- 

 guish the typhoid bacillus from B. coli com- 

 munis. Microscopically and culturally the dif- 

 ference is inconsiderable. The greatest pains 

 have been taken to discover fundamental dif- 

 ferences. As a rule B. coli forms a dirty- 

 yellow incrustation upon a piece of a potato 

 on which the typhoid bacillus grows in the 

 form of an invisible film. B. coli generally 

 coagulates milk and the typhoid bacillus does 

 not ; in glucose solutions vigorous gas-produc- 

 tion is brought about by B. coli, but not by 

 the typhoid bacillus ; B. coli forms indol out of 



