Virulence 635 



teria. He deems the best tests to be the inoculation of 

 broth cultures and the subsequent inoculation into animals, 

 which, he advises, should have been previously vaccinated 

 against the streptococcus. Plague bacilli sometimes persist 

 in the urine for a week after convalescence. 



Kolle* has suggested a method valuable both for the 

 diagnosis of the disease and for estimating the virulence of 

 the bacillus. It is as follows: "The skin over a portion of 

 the abdominal wall of the guinea-pig is shaved, care being 

 taken to avoid the slightest injury of the skin. The infective 

 material is carefully rubbed into the shaved skin. Im- 

 portant, in order rightly to understand the occurrence of 

 plague infection, is the fact disclosed here in the case of 

 guinea-pigs, that by this method of inoculation the animals 

 present the picture of true bubonic plague that is to say, 

 the production of nodules in the various organs, principally 

 in the spleen. In this manner guinea-pigs, which would 

 not be affected by large subcutaneous injections, even 

 amounting to 2 mg. of agar culture (equal to a loop) of low- 

 virulence plague bacillus, may be infected and eventually 

 succumb." 



Virulence. By frequent passage through animals of the 

 same species the bacillus can be much increased in virulence. 

 Kolle recommends rats for this purpose, and, indeed, declares 

 that without the use of rats it is impossible to keep cultures 

 at a high grade of virulence. Batzaroff found that the most 

 virulent plague germs were to be obtained from the pneu- 

 monic lungs of rats that had been infected through the nasal 

 aperture with cotton-wool saturated with a culture of the 

 bacillus. This is not, however, a reliable method of inocu- 

 lation. 



Yersin found that when cultivated for any length of time 

 upon culture media, especially agar-agar, the virulence was 

 rapidly lost and the bacillus eventually died. On the other 

 hand, when constantly inoculated from animal to animal, 

 the virulence of the bacillus is much increased. 



Knorr, Yersin, Calmette, and Borrelf have shown that 

 the bacillus made virulent by frequent passage through mice 

 is not increased in virulence for rabbits. 



This no doubt depends upon the sensitivity of the bacillus 

 to the protective substances of the body-juices, immuniza- 



*See Havelburg, "Public Health Reports," Aug. 15, 1902, vol. 

 xvn, No. 33, p. 1863. 



t " Ann. de 1'Inst. Pasteur," July, 1895 



