92 VARIATIONS IN VIRULENCE [CH. vi 



bacillus but non-pathogenic. The weight of evidence is against 

 the possibility of rendering such strains virulent by passage. 

 A filtered broth culture, however, of these organisms will 

 provoke the formation of diphtheria antitoxin in the horse 

 (Ark Wright, 1909) so that they must be regarded as true 

 diphtheria bacilli although non-pathogenic. 



The pnetmiococcus is found as a virulent organism in acute 

 lobar pneumonia. It is normally present in the mouths of 

 healthy persons in a form which is non-virulent but which 

 can be made virulent by passage through an animal and in 

 this case is indistinguishable from the pathogenic variety. 

 These two forms are regarded only as varieties of the same 

 species. 



The comparatively harmless B. coli communis, normally 

 present in the human intestine in health, gives place in many 

 unhealthy and inflamed conditions of the intestine to a 

 virulent organism which in every other respect is identical. 

 In this case opinion is divided, some authorities regarding the 

 toxic or pathogenic B. coli as a distinct species from the non- 

 toxic B. coli although they possess no other distinguishing 

 feature apart from virulence and this in the case of the 

 former can be diminished by artificial culture and in the case 

 of the latter increased by " passage." 



The discovery of the amoeba coli in the intestines of 

 healthy individuals in the tropics, where amoebic dysentery 

 is rife, presents a similar problem for solution. Bahr (1912) 

 found the amoeba coli in 30 per cent, of Fijians examined by 

 him and Ashburn and Craig discovered it in 72 out of 100 

 soldiers examined in the Philippines though none of these 

 72 had diarrhoea or dysentery at the time or had ever been 

 on the sick list with either of these diseases. 



The Ba^. anthra^oidss, discovered by Andrewes and de- 

 scribed by Bainbridge (1903), was distinguished from the true 

 B. anthracis by slight differences in the appearance of gelatin 

 and agar cultures, in rate of growth and in motility and by 

 its non-virulence. The observations of Savage and MacConkey, 

 already quoted, as to the frequency with which atypical 

 colonies of some organisms occur on gelatin, shows how 



