360 PATHOGENIC MICROORGANISMS 



or not the virulence of pneumococci is attenuated by sojourn within the 

 human body during disease is uncertain. The attenuation of virulent 

 pneumococci on artificial media may be hastened, according to Frankel, 1 

 by cultivation of the organism at or above a temperature of 41 C. 



The virulence of attenuated cultures may be rapidly enhanced by 

 passage of the organisms through the bodies of susceptible animals. 

 The virulence of strains may be so enhanced that one one-millionth of 

 a c.c. will kill a mouse. 



Among the domestic animals white mice and rabbits are most sus- 

 ceptible. Guinea-pigs, dogs, rats, and cats are much more resistant. 

 Birds are practically immune. 



The results of pneumococcus inoculation into susceptible animals 

 vary according to the size of the dose, the virulence of the introduced 

 bacteria, the mode of administration, and the susceptibility of the 

 subject of the inoculation. Subcutaneous inoculation of virulent 

 pneumococci into mice and rabbits usually results in an edematous 

 exudation at the point of inoculation, which leads to septicemia and 

 death within twenty-four to seventy-two or more hours. When the 

 dose has been extremely small or the culture unusually attenuated, a 

 localized abscess may be the only result. Intravenous inoculation is 

 usually more rapidly fatal in these animals than the subcutaneous 

 method. Intraperitoneal inoculation in rabbits results in the formation 

 of a rapidly spreading peritonitis in which the exudates are apt to be 

 accompanied by a deposit of fibrin, and to lack the transparent red color 

 so often caused by the hemolyzing streptococci. With very virulent 

 strains, these differences are less marked. In almost all of these infec- 

 tions death is preceded by septicemia and the microorganisms can be 

 recovered from the heart's blood of the victims. 



The production in animals of lesions comparable to the lobar pneu- 

 monia of human subjects has been the aim of many investigators. 

 Wadsworth, 2 recognizing that such lesions probably depended upon the 

 partial immunity which enabled the infected subjects to localize the 

 pneumococcus processes in the lungs after infection by way of the 

 respiratory passages, succeeded in producing typical lobar pneumonia 

 in rabbits by partially immunizing these animals and inoculating them 

 intratracheally with pneumococci of varying virulence. Lamar and 

 Meltzer 3 produced lobar pneumonia in dogs in 1912 by injecting cul- 



1 Frankel, Deut. med. Woch., 13, 1886. 



2 Wadsworth, Amer. Jour. Med. Sci., May, 1904. 



3 Lamar and Meltzer, Jour. Exp. Med., xv, 1912. 



