426 MICROORGANISMS 



mon disease, chicken cholera. He had labored with his 

 experiments so strenuously that he was finally forced on ac- 

 count of ill health to take a short vacation. This he dreaded 

 to do for he feared that lack of care would cause all his valuable 

 cultures to die. When he returned to his laboratory, you can 

 imagine his dismay when he found that all his cultures were 

 apparently ruined. This would have meant that much of 

 his labor had been lost and that the final result which he 

 hoped for would have been delayed. He made every effort 

 to revive his cultures by transferring them to new culture 

 media and among other things, he inoculated some chickens 

 with one of these old cultures of the bacteria that cause chicken 

 cholera. These chickens failed to develop any symptoms of 

 the disease; Pasteur considered the cultures lost. He was 

 greatly surprised, however, to find that later when he inoculated 

 these same chickens from fresh virulent cultures, they failed to 

 take the disease. He quickly prepared other cultures and al- 

 lowed them to stand in test tubes as these original ones had done 

 and then repeated the experiment of first inoculating chickens 

 with the weakened cultures and then later inoculating the same 

 chickens from fresh virulent cultures. He was thus soon able 

 to prepare weakened, or ATTENUATED, cultures, as he called 

 them, which would regularly serve to prevent the chickens 

 from taking the disease when inoculated from strong virulent 

 cultures. 



This method of treating, or VACCINATING, chickens, as the 

 process has come to be called, has never come into general 

 use, for later experience showed that some fowls are really given 

 the cholera by it. It served, however, to give Pasteur a 

 principle to work on, and he was soon able to produce a 

 VACCINE for anthrax which has proved to be of inestimable 

 value. 



After numerous experiments in which Pasteur grew the 

 anthrax bacillus under almost all possible conditions and 

 then used them to inoculate susceptible animals, he finally 

 discovered that, when the bacteria are grown at a temperature 



