254 BARBER. 



The "Shanghai" strain, though evidently less infective, was in 

 one case fatal in a dose of 5 bacilli. These results illustrate how 

 large a part varying susceptibility of animals plays in infection, 

 and emphasize the necessity of inoculating a series of animals in 

 any test of virulence of a microorganism. 



In some guinea pig inoculations, one or few bacilli were 

 washed with salt solution before inoculating and some of these 

 washed doses resulted in fatal infection. Since a most minute 

 quantity, if any at all, of the original body fluid was then 

 inoculated, it does not seem probable that aggressins played any 

 part in these infections. In the experiments on monkeys few 

 succumbed to the small doses, though in two cases fatal infec- 

 tion followed a dose of one bacillus. 



In the rat series the wild gray rat was used. One animal 

 succumbed to a fatal infection following a dose consisting of 

 one chain of four small elements, another to a dose of three 

 bacilli, each divided into two still adherent elements. Two 

 other rats succumbed to doses of 50 and 60, respectively, of 

 such pairs. These results render more plausible the view that 

 sufficient bacilli for infection may enter the abraded skin from 

 the fseces of a flea or a crushed flea. The English Plague Com- 

 mission has shown that flea fseces or flea bodies may contain 

 very large numbers of plague bacilli. 



On the average, animals infected with minute doses survived 

 nearly twice as long as those infected with three-fourths of a 

 million or more. 



In summary, these results show conclusively that the smallest 

 possible dose of virulent plague bacilli may infect fatally the 

 more susceptible guinea pigs, monkeys, or rats. 



