456 PATHOGENIC MICROORGANISMS 



longer than eight hours. Apparently cultures grown for as long 

 as twenty-four hours diminish in virulence much more rapidly. To 

 some extent this observation is in keeping with Chesney's 59 state- 

 ment that growth is best obtained when the transfers are made from 

 broth at the period of maximum growth rate. 



Swift has recently succeeded in keeping organisms of the pneu- 

 mococcus and streptococcus varieties alive and virulent for a long 

 time by centrifugating broth cultures, taking off the supernatant 

 fluid, and drying the residue in a frozen condition in vacuo. This 

 material can be kept for a very long time without death of the 

 bacteria, and without appreciable loss of virulence. 



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

 directly into sterilized tubes, sealed and kept in the dark, Foa 60 has 

 been able to preserve the virulence of pneumococci for as long as 

 forty-five days. Preservation in the spleen of animals dead of pneu- 

 mococcus infection, as practiced by Neufeld, has been mentioned 

 above. Whether or not the virulence of pneumococci is attenuated 

 or enhanced by sojourn within the human body during disease is 

 uncertain. The attenuation of virulent pneumococci on artificial 

 media may be hastened, according to Frankel, 61 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 

 susceptible. Guinea-pigs, dogs, rats, and cats are much more resis- 

 tant. Guinea-pigs can be given astonishingly large doses of pneu- 

 mococci without injury. Birds are practically immune. Kyes who 

 has studied pneumococcus infection in birds particularly has shown 

 that the fixed tissue cells of the liver, spleen and lungs destroy the 

 organisms by prompt and effective phagocytosis. His attempts at 

 treating pneumonia with immune chicken serum, suggested by these 

 observations, will be referred, to below. 



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 



m Che*ney, Jour, of Exper. Med., 24, 3916, 387. 

 "Foa, Ztschr. f. Hyg. iv, 1888. 

 "Franlcel, Deut. med. Woch., 13, 1886. 



