286 BACTERIA PATHOGENIC TO MAN 



bacillus is pink, and the surrounding medium becomes red; while the 

 colonies of the typhoid bacillus are blue, and there is little or no red- 

 dening of the surrounding medium. The same points hold true on 

 the Drigalski-Conradi medium. 



7. The colon bacillus possesses the property of producing indol in 

 cultures of bouillon or peptone; the characteristic typhoid bacillus 

 does not produce indol in these solutions. 



8. The colon bacillus rarely produces thread outgrowths in properly 

 prepared Hiss plate medium. The typhoid bacillus produces thread 

 outgrowths and smaller colonies in this medium. In the Hiss tube 

 medium the colon bacillus produces either a growth limited to the 

 area inoculated or a diffuse growth streaked with clear lines and spaces. 

 The typhoid bacillus produces a diffuse growth, evenly clouding the 

 entire medium. 



9. On the Capaldi medium the colon colonies are more granular 

 and darker than those of the typhoid bacilli. 



10. Finally, on testing the bacilli in the hanging drop with the 

 serum of animals immunized to the typhoid bacillus, the typhoid bacilli 

 become agglutinated in high dilutions of the serum, while the colon 

 bacilli do not. 



None of these tests alone can be depended upon for making a differ- 

 ential diagnosis of the colon bacillus from the typhoid bacillus or other 

 similar bacilli. 



Unfortunately, also, in most of these characteristics certain degrees 

 of variation may often be observed and these may lead to confusion. 

 For instance, the morphology may vary considerably, at times even 

 when grown on the same culture media, and the motility is not always 

 equally pronounced; the flagella may vary; the rapidity of growth 

 may differ, especially between freshly made and old cultures; the 

 grape-leaf appearance of the surface colonies on gelatin, which is 

 usually characteristic, may vary with the composition of the gelatin, 

 at times no typical colonies at all being presented; the threads in the 

 Hiss media may be lacking; in rare instances the typhoid bacillus pro- 

 duces indol; the growth on potato is not to be depended on, often 

 being visible and not characteristic; the virulence of both the bacilli 

 is so little characteristic that it can hardly be used for diagnostic pur- 

 poses; and finally, the serum test is not to be depended on unless the 

 agglutinins in the serum have been properly tested, for there is 

 abundant agglutinin for some of the colon bacilli in the serum of 

 many untreated animals. This is less true of rabbits than of horses 

 and of young than older animals. 



In spite, however, of these difficulties it is very easy to sufficiently 

 identify the typhoid bacillus for all practical purposes. A bacillus 

 which grows typically in the Hiss tube media, and shows agglutination 

 with a high dilution of the serum of an animal immunized to the typhoid 

 bacillus, is in all probability the typhoid bacillus. If this bacillus 

 absorbs the specific typhoid agglutinins it is undoubtedly the typhoid 

 bacillus. The same could probably be said of a bacillus which grew 



