PATHOGENIC MICROBES— INFECTION 119 



the fowls became ill but did not die, and further, they resisted 

 a second inoculation of a very virulent strain which killed the 

 controls. This was the first demonstration of an attenua- 

 tion produced by keeping in contact with air at incubator 

 temperature. By taking cultures of different ages, a scale of 

 virulence could be produced —a series of " vaccines." 



If on re-inoculating an enfeebled culture, one obtained a 

 virulent culture, it would not be correct to speak of attenua- 

 tion, but merely of enfeeblement — transitory lowering of the 

 virulence. 



Attenuation is only applied to a permanent enfeeblement, 

 one which passes from one generation to the next. One 

 cannot say hereditary because, strictly speaking, heredity only 

 occurs when there is sexual reproduction. In the experiments 

 on fowl cholera the new cultures showed themselves to be 

 weakened in series ; it was thus a true attenuation. 



" If you take each one of the cultures whose virulence has 

 been attenuated as the starting point of successive cultures 

 and without a perceptible interval in the starting of the 

 cultures, the whole series will reproduce the attenuated 

 virulence of the one serving as starting point. Similarly, a 

 culture of zero virulence reproduces another of the same" 

 (Pasteur). 



A similar process of attenuation appeared at first inapplic- 

 able to the anthrax bacillus ; as the culture grows older the 

 bacterium sporulates and the spores are not affected by the 

 conditions which act upon the bacillary form. One could only 

 therefore expect attenuation from an anthrax bacillus which 

 did not produce spores. 



Now at 42°'5 the anthrax bacillus does not sporulate. 

 Pasteur cultivated it therefore at 42°-^ in order to diminish its 

 virulence by the action of the heat and the air. 



It was then found that sporulation, instead of being an 

 obstacle to attenuation, was a condition entirely favourable ; 

 reinoculated at 35°, a bacillus attenuated at 42°-5 produced 

 sporulating bacilli, but the bacilli germinating from these 

 spores possessed the same degree of virulence as the bacilli 



