168 ESSAYS ON BACTERIOLOGY. 



the mimber of bacteria. This is more readily clinic- 

 ally recognized with those diseases, and experiment- 

 ally -with those bacteria, susceptibility to which in the 

 animals experimented with is not great. 



And thus we have established the bacteriological 

 doctrine that the number of bacteria in an importa!nt 

 way influences infection, confirming the clinical doc- 

 trine that quantity of poison and great exposure have 

 much to do in determining the development of an in- 

 fectious disease. 



Examining our culture plates again, we may dis- 

 cover a third phenomenon: Upon some of these 

 plates the germs of diphtheria have fallen. We make 

 inoctilation experiments and find that some of these 

 diphtheria bacilli are of great virulence, others pro- 

 duce but mild results, while others cannot be made to 

 produce either typical lesions or general symptoms of 

 diphtheria. Inoculating animals with these various 

 types of diphtheria bacilli, we find that somp produce 

 pronounced local lesions with equally pronounced 

 constitutional symptoms. Evidently these germs pos- 

 sess their full biological characteristics. Others pro- 

 duce fairly typical local lesions, but little or no consti- 

 tutional disturbance, while others cause but slight 

 local disturbance. Yet these are all genuine diph- 

 theria bacilli. Evidently they differ biologically in 

 this, that some are possessed of their full powers, oth- 

 ers are weakened, while some have lost all or almosl 

 all pathogenic property. And this is the reason for 

 the different effects upon the inoculated animals. 



