CHAPTER VII 



VAEIATIONS IN PATHOGENICITY 



We have shown that morphology, fermenting properties and 

 virulence are all variable features. There remains to be 

 considered one other character of bacteria which is of great 

 value in their classification, namely their " pathogenicity " or 

 their power to cause specific disease. 



Under this head are to be considered, firstly, the kind of 

 animal in which a particular organism can develop disease, 

 secondly, the kind of symptoms caused, and thirdly, the kind 

 of lesions produced by that organism's invasion of the living 

 body. 



The pathogenicity of an organism is something quite 

 distinct from its other characters. Two organisms may possess 

 the same morphology, the same fermenting power and the 

 same degree of virulence and yet show a wide divergence in 

 their pathogenicity, giving rise in the body to a totally 

 different train of symptoms and lesions. 



This character of pathogenicity derives particular value 

 and importance from the fact that it is generally regarded as 

 being more " fixed " than the other characters we have been 

 discussing. Great reliance is, for this reason, placed upon 

 resemblances and differences in pathogenicity in determining 

 whether two organisms do or do not belong to the same 

 species, — ^in fact it is regarded as constituting a final appeal 

 in doubtful cases. 



For example, Clark (1910) maintains that Hofinann's 

 bacillus and the Klebs-LoefiSer bacillus represent diflerent 

 species on the ground that the former when rendered virulent 

 gives rise to different symptoms in the body. 



Again, the gonococcus and the meningococcus show close 

 resemblances in many of their characters but are readily 



