﻿STUDIES IN PLAGUE IMMUNITY. 169 



oesen of this culture constituted a lethal dose for guinea pigs of 250 to 

 300 grams weight, when inoculated subcutaneously, the animals usually 

 dying from subacute plague infection. The virulence of this organism 

 has still further been reduced by growing it at a temperature of from 

 41° to 43° C. in flasks of alcoholic bouillon for three weeks at a 

 time as recommended by Hetsch. 26 (See p. 310, "Virulence of Pest 

 Bacillus.") By this means a considerable reduction in virulence has 

 occurred, since at the present time guinea pigs of 250 to 300 grams 

 weight are usually able to withstand the subcutaneous inoculation of 

 one entire 24-hour agar slant culture of this organism. However, occa- 

 sionally they succumb to pest infection from a such dose, although 

 the course of the disease in these instances is always prolonged. This 

 organism also shows the typical morphology of Bacillus pestis and 

 agglutinates with a standard pest serum; animals vaccinated with it also 

 acquire pest immunity. As this strain was known formerly to have 

 possessed a greater virulence in nature and had been artificially attenuated, 

 experiments were performed with it in monkeys to ascertain whether 

 it would be possible to cause it to regain its original virulence. However, 

 from this standpoint these experiments were unsuccessful. (See p. 301.) 



w Ztschr. f. Hyg. u. Infectionskrankh., Leipz. (1904), 48, 442. 



