Transformation., etc., of Pathogenic 31icrobes. 87 



incidentally become pathogenic and determine the 

 development of morbid troubles in man or animals. 



II. Attenuation of pathogenic microbes. 



The diminution of virulence of pathogenic microbes 

 is occasionally, not always, connected with a diminu- 

 tion in their general nutritive activity ; it may occur 

 under quite varied conditions, either spontaneously 

 or from certain definite influences purposely brought 

 into play by the experimenter; we will here study 

 the different means by which attenuation may be ob- 

 tained. 



1. Attenuation by the normal atmosphere. — "We al- 

 ready know that the majority of the atmospheric 

 germs are dead. It is logical to admit that the loss 

 of their vitality did not take place abruptly, but, on 

 the contrary, was gradually produced, and that what 

 virulence they may originally have possessed also dis- 

 appeared gradually. In short, the germs of the air 

 are attenuated before being destroyed. The atmos- 

 pheric conditions which determine these changes in 

 microbic life are far from being simple : oxygen, 

 light, the electrical condition, desiccation, probably 

 all act in concert. 



But the external air, of itself alone, can bring 

 about the attenuation of pathogenic germs. Very 

 active cultures may lose their virulence in some days, 

 weeks, or occasionally months. 



Cultures of fowl cholera, abandoned to the air, 

 gradually diminish in virulence so as to completely 

 lose it at the end of a time varying from six weeks 

 to two months, occasionally much less. By re-sowing 

 these germs, in way of attenuation, at periods more 



