DESCRIPTIONS OF PATHOGENIC BACTERIA. 193 



peptone, while the typhoid bacillus does not, 

 so that when potassium nitrite and sulphuric 

 acid are added to both cultures a rose-red colo- 

 ration appears in the culture of the former, and 

 not in that of the latter. B. coli forms dextro- 

 rotatory, the typhoid bacillus laevo-rotatory 

 lactic acid out of milk-sugar. The reduction 

 of nitrates to ammonia is accomplished by both 

 species. However, many of these statements 

 are positively contradicted by some observ- 

 ers. 



There are in fact at present two opposing 

 views. The one which to me seems to be the 

 better founded is that the bacteria of typhoid 

 fever and B. coli communis are two distinct 

 species. The other view is that the common 

 intestinal saprophyte, B. coli communis, is an 

 ceco-parasite which, under special conditions, 

 may become able to invade the body, and 

 penetrate into the living organism where it 

 undergoes transformation into the typhoid bac- 

 terium. 



Both kinds of bacteria are usually not in- 

 fectious for the lower animals, but as a rule 

 merely toxic ; only when the bacteria are very 

 virulent does a multiplication take place in 

 living rabbits and mice. The " true " typhoid 

 bacteria produces typhoid fever in man, pene- 

 trating from the intestine into the Peyer's 

 13 



