PASSAGE THROUGH ANIMALS, 91 



virulence when they are cultivated for any length 

 of time on the ordinary culture media. 



A common device for the maintenance or increase 

 of virulence is that of passage through some suit- 

 able animal. The process consists of the inocula- 

 tion of the animal with a pure culture, permitting 

 the infection to run its course or to proceed for a 

 number of days, and at this time recovering the 

 micro-organism on culture media from the tissues 

 of the animal. Eoux and Metchnikoff modified this 

 technic by placing a culture of the organism in a 

 sealed collodion sac, which is then imbedded in the 

 peritoneal cavity of a living animal. After a suffi- 

 cient length of time the sac is removed and the 

 process repeated. By repeating the passage at suit- 

 able intervals virulence may be maintained at a 

 rather constant high point (cholera vibrio, strepto- 

 cocci,, etc.). An effective substitute for passage is 

 sometimes found in the use of a culture medium, 

 which in its constitution approximates that of the 

 tissues of an animal. Thus the virulence of pneu- 

 mococci and streptococci is retained by cultivation 

 on blood agar or in rabbit or human serum or in 

 ascitic fluid. 



Animal passage increases virulence in the most 

 marked manner for the species of animal which is 

 used in the experiment, although some increase is 

 usually manifested toward other susceptible species. 

 In a number of instances passage through one .ani- 

 mal results in a decrease of virulence for one 

 species and an increase for another. Thus Pasteur 

 found that passage of the virus of swine erysipelas 

 (Schweimrotlauf) through pigeons increased 

 virulence for the swine, but it was decreased for 

 this animal when passed from rabbit to rabbit. 



