468 APPLICATION OF METHODS OF BACTERIOLOGY 



evidence of any reaction at all. These exhibitions of the 

 extremes of its pathogenic properties, viz., death of the 

 animal, on the one hand, and only very slight local effects 

 on the other, was at one time thought to indicate the existence 

 of two separate and distinct organisms that were alike in 

 cultural and morphological peculiarities, but which differed 

 in their disease-producing power. Further studies on this 

 point have, however, shown that genuine bacillus diph- 

 therias may possess almost all grades of virulence, and that 

 absence of or diminution in virulence can hardly serve to 

 distinguish as separate species those, varieties that are other- 

 wise alike; moreover, the histological conditions found at 

 the site of inoculation in animals that have not succumbed, 

 but in which only the local reaction has appeared, are in 

 most cases characterized by the same changes that are seen 

 at autopsy in animals in which inoculation has proved fatal. 



In the course of their observations upon a large number 

 of cases Roux and Yersin found that it was not diflScult to 

 detect, in the diphtheritic deposits of the same individual, 

 bacteria of identical cultural and morphological peculiarities, 

 but of very different degrees of virulence, and that with the' 

 progress of the disease toward recovery the less virulent 

 varieties often became quite frequent.^ 



There is, moreover, a mild form of diphtheria, etiologically 

 speaking, affecting only the mucous membrane of the nares, 

 known as membranous rhinitis, from which it is very com- 

 mon to obtain cultures in all respects identical with those 

 from typical diphtheria, save for their inability to kill sus- 



1 It must not be assumed from this that the bacteria lose their viru- 

 lence entirely, or that they all become attenuated with the establishment of 

 convalescence, for this is contrary to what experience has shown to be the 

 case. 



