VIRULENCE OF ORGANISMS 9 



other animals. The importance of proper selection of the animal 

 species for increasing bacterial virulence is emphasized by the work 

 of Hussy, who found that the passage of streptococci through 

 mammals and fish increased the virulence, but their passage through 

 birds decreased the virulence. In the increase of virulence by means 

 of animal passage, the organism apparently may develop a mechan- 

 ism of resistance against the protective activity of the animal body, 

 as has been discussed above. Another method of increasing virulence 

 is to place the organism in collodion sacks. These are planted in 

 the peritoneal cavity of an animal and apparently the slow diffu- 

 sion of the animal fluids into the sack permits the organism to 

 acquire resistance to the antagonistic substances of the animal and 

 thus increases its virulence. A third method of increasing virulence 

 is to grow organisms upon media which contain blood serum or 

 other animal fluids. By several transfers upon such media the 

 organisms may acquire resistance similar to that obtained in the 

 other methods. A fourth method has been applied, depending upon 

 the separation of the more virulent individuals in a culture from 

 the less virulent at the height of phagocytosis. A culture is in- 

 oculated into the peritoneal cavity of an animal, such as the guinea- 

 pig, and by removing small quantities at regular intervals the time 

 of greatest phagocytosis by the peritoneal cells is determined. The 

 entire exudate is then withdrawn and slowly centrifuged, so as to 

 throw down the cells, leaving the unphagocyted bacteria in the 

 supernatant fluid. The organisms in the supernatant fluids are 

 cultivated, and if they are not sufficiently virulent the process may 

 be repeated until a satisfactory culture is obtained. 



Decrease of Virulence. The virulence of pathogenic organisms 

 may be decreased by removing them from the favorable environ- 

 ment of the animal host and growing them upon artificial culture 

 media. As they become accustomed to this type of existence they 

 usually lose considerably in virulence. As has been indicated above, 

 there are instances where animal passage may decrease the virulence 

 of certain infective agents. Whereas the virus of rabies increases 

 up to a standard maximum on passage through rabbits, similar pas- 

 sage through monkeys will decrease its virulence. It is probable, 

 also, that the natural passage from dog to dog decreases virulence. 

 It is now generally accepted that cowpox is the same disease as 

 smallpox, yet the inoculation of cowpox into man produces a very 

 mild form of disease. Therefore, it is to be presumed that the pas- 

 sage of smallpox virus through the calf reduces the virulence. A 

 similar example is found in the work of Hussy on the streptococcus 

 quoted above. Decrease of virulence by animal passage is not 

 clearly understood. It may be due to the same factors that influ- 

 ence virulence in artificial culture media, whereby the organisms in 

 an unfavorable environment lose their ability to combat the resist- 

 ance of the animal host, or it may be due to a direct lowering of 

 virulence as the result of more or less successful attacks of the pro- 



