200 BIOLOGY OF PNEUMOCOCCUS 



for pneumococci of Type IV the killing dose varied from 1 to 100 

 to 1 to 1,000. One of the most virulent strains tested, a Type IV 

 culture, was fatal in a dose of 10 8 cubic centimeters. 



Petrie and Morgan 1085 in the same year reported the results ob- 

 tained in an investigation of the factors influencing the lethal 

 power for mice of a virulent culture of Type I Pneumococcus. 

 Stocks of mice from different sources appeared to show some va- 

 riation in susceptibility, and a small proportion — from 5 to 10 

 per cent — exhibited an innate resistance to small doses of pneu- 

 mococci. The weight factor of the test animals failed to affect the 

 mortality, although it modified the survival time. However, the au- 

 thors decided that the density and virulence of the culture deter- 

 mined its lethal power, and that this power could be calculated 

 when a reasonable number of mice were used. The criteria, by 

 means of which the minimal fatal dose of any culture of Type I 

 Pneumococcus could be specified, were the percentage fatality, the 

 mean death-time, and the distribution of deaths at appropriate 

 time intervals in a group of mice receiving a definite dose of the 

 test culture. 



AVENUE OF INOCULATION 



In general, the virulence of a given organism varies with the 

 tissue into which it is introduced. Direct inoculation into the blood 

 stream is the most rapidly and most surely lethal route, followed 

 in order of effectiveness by injection into the anterior chamber of 

 the eye, into the peritoneum, under the skin, into the skin, into the 

 lung either by direct puncture or through the bronchi, and lastly 

 by way of the mouth. 



DETERMINATION OF VIRULENCE 



While the intraperitoneal injection of white mice is the pre- 

 ferred method for testing the virulence of pneumococcal cultures, 

 other methods have been described. The rabbit, because of cost and 

 individual and special idiosyncrasies, is less valuable. The relation 



