181 



Danysz'^ therefore conceived the notion that it would be of great 

 interest first to extend the field of action of one of these organisms by 

 increasing its virulence so that it would attack other species of rodents, 

 and then maintain this increased virulence at its highest point. 



In 1900 Danysz isolated a bacillus from a spontaneous epidemic 

 among harvest mice. This organism was a cocco-bacillus presenting 

 the general characteristics of the colon-typhoid group and resembling 

 the bacillus of LoefHer — B. typhi murium. From the first this bacillus 

 showed a slight pathogenicity for gray rats (M. decumanus). Out of 

 10 animals fed with a culture of this microbe 2 or 3 would die; several 

 others would sicken and recover; others appeared completely refrac- 

 tory. The fact that a certain number of the rats fed with these 

 cultures always succumbed led to thp hope that it would be possible 

 to increase the virulence of this particular microbe by the generally 

 accepted methods-^that is to say, by a certain number of passages 

 from rat to rat. 



Danysz first tried to increase the virulence of the organism by this 

 means, but he found that successive passages from rat to rat, whether 

 by feeding or by subcutaneous injection, ended by enfeebling rather 

 than increasing the virulence of the microbe. He found that it was 

 rarely possible to go beyond 10 to 12 passages. Sometimes the series 

 was stopped at the fifth passage, or even sooner, by the survival of all 

 the animals undergoing experiment. The result was exactly the 

 same if, instead of alternating each passage through the animal by a 

 culture in bouillon or agar, the bodies of animals dead of a preceding 

 passage were fed to others. 



It was therefore plain that in the evolution of an epidemic caused 

 by this microbe it was necessary to take account of the indisputable 

 diminution of the virulence of the microbe, as well as the natural 

 resistance of the survivors. 



Passage of cultures in collodion sacs inclosed in the peritoneal 

 cavities of rats was tried, both in interrupted series and by alternating 

 each sac culture with a culture in bouillon or on agar, but the end was 

 invariably a notable diminution of virulence when administered by 

 the digestive tract. 



Finally, after long and painstaking procedures, Danysz obtained a 

 very virulent culture that, contained in flasks and kept from the 

 ■ influence of light and air, preserved its virulence for several months. 

 Planted on agar it preserved its virulence without appreciable dimi- 

 nution for two months under laboratory conditions. In bouillon, in 

 flasks, or in tubes stoppered with cotton it altered very rapidly. 



a Danysz, J.: Un microbe pathogfene pour les rats {Mus decumanus et Mus rattus) 

 et son application ^la destruction de ces animaux. Ann. Inst. Pasteur, vol. 14, 1900, 

 p. 193. 



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