3G0 PATHOGENIC MICROORGANISMS 



are most rapidly destroyed by twenty per cent alcohol, other and 

 stronger disinfectants being less efficient, probably because of slighter 

 powers of diffusion. 



Virulence and Pathogenicity. — The virulence of pneumococci is 

 subject to much variation, depending largely upon the length of time 

 during which the microorganism has been cultivated under artificial 

 conditions. It has been mentioned above that under certain conditions 

 — such as those prevailing in dried sputum or blood ^ — the virulence of 

 pneumococci may be preserved for several weeks. Ordinarily, however, 

 the virulence diminishes gradually as the cocci adapt themselves more 

 saprophytically to life upon artificial media. Upon media containing 

 animal albumin, such as ascitic fluid or blood agar, this attenuation is 

 less rapid than upon the simple meat-infusion preparations. 



In the blood of rabbits dead of a pneumococcus infection, taken 

 directly into sterilized tubes, sealed and kept in the dark, Foa^ has been 

 able to preserve the virulence of pneumococci for as long as forty-five 

 days. Whether or not the virulence of pneumococci is attenuated by 

 sojourn within the human body during disease is a question much dis- 

 cussed but hardly settled. It is a matter of fact, however, that many 

 pneumococci obtained by blood culture from more or less chronic cases 

 of pneumococcus septicemia fail to kill susceptible test animals, even 

 when injected in considerable doses. The attenuation of virulent 

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

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



Freshly isolated from the human saliva or pneumonic lesions, the 

 differences in virulence between various strains of pneumococci are not 

 very marked, almost all such strains showing considerable pathogenic 

 powers toward the usual test animals. 



The virulence of attenuated cultures may be rapidly enhanced by 

 the passage of the organisms through the bodies of susceptible animals. 

 The extreme virulence of some of these pneumococcus strains may be 

 illustrated by citing the experiments of Eyre and Washburn^ who 

 possessed cultures of which one miUionth of a loopf ul would kill a mouse 

 within four days. 



Among the domestic animals those most susceptible to pneumococcus 

 infection are white mice and rabbits. Guinea-pigs, dogs, rats, and cats 



' Gtmrnieri, loc. cit. 



■'Foa, Zeit. f. Hyg., iv, 1888. 



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



■• Eyre and Washburn, Jour, of Path, and Bac, v. 



