XXI] PATHOGENIC ORGANISMS 537 



has indeed been shown by experimental investigation, that 

 there are a good many microbes, not derived from, or 

 associated with, any infectious disease of man or animals, 

 but generally carrying on a saprophytic existence, which 

 under certain conditions are capable of producing decided 

 pathogenic action in the animal body. This question has 

 to be considered under two aspects : (a) is there any 

 evidence to show that a true saprophyte can, owing to 

 alteration in the conditions of its growth in outside nature, 

 acquire pathogenic action ? and {b) can a true saprophyte, 

 previously non -pathogenic, become pathogenic in the animal 

 body owing to conditions within the animal body ? It 

 must be clear that for the sanitarian the first aspect is of the 

 first importance, for if a true saprophyte could so alter in 

 outside nature as to be capable of eventually starting an 

 epidemic of infectious disease his views of the specific 

 nature of infectious diseases will have to undergo a com- 

 plete alteration. We will illustrate this by the following 

 instances : G. Roux and Bordet have asserted that the 

 bacillus coli, which as we have shown. is a common sapro- 

 phyte in the human and animal intestine, when sojourning 

 in sewage — into which it naturally and commonly finds its 

 way — is capable of becoming changed into the typhoid 

 bacillus. This assertion was made on quite insufficient 

 bacteriological evidence ; moreover it was made at a time 

 when the distinction between bacillus coli and bacillus of 

 typhoid fever was not as easy or as well established as it 

 now is. We can now dismiss this statement, viz., the 

 conversion into and interchange of bacillus coli and bacillus 

 of typhoid, as contrary to bacteriological experience. 



Another instance of this kind is that adduced by Buchner 

 on the experimental conversion of bacillus subtilis or hay 

 bacillus into the bacillus anthracis (" Ueber die experim. 



