INCREASE OF VIRULENCE 137 



For an organism to retain its virulence it is, as a rule, 

 necessary to pass it through a susceptible animal at longer 

 or shorter intervals, and to enhance the virulence re- 

 course must be had to a succession of passages through 

 susceptible and then less susceptible animals. In this 

 way the virulence of organisms has been increased to a 

 point far greater than is ever met with naturally, as in 

 the case of the Streptococcus pyogenes. If an organism 

 retains its virulence even slightly, it is generally possible, 

 by employing large doses, to enhance this by passage 

 through a susceptible animal. Another method may 

 also be adopted, namely, to inject along with it some 

 other pathogenic form, such as the Streptococcus pyogenes ; 

 the combination will kill the animal, and the slightly 

 virulent organism can be recovered and will be found to 

 have increased in virulence. A third method is to inject 

 the organism into a susceptible animal together with a 

 lethal dose of toxin obtained from a virulent form of the 

 same species, or with some substance, such as lactic acid, 

 which lowers the vitality of the tissues. The slightly 

 virulent organism will then be able to grow under the 

 more favourable conditions, and a form which has become 

 completely non -virulent can be made to regain its lost 

 virulence. 



Collodion sacks are frequently used to study the action 

 upon animals of the dialysable products produced by 

 micro-organisms which do not form any appreciable 

 amount of toxin in vitro, for cultivating species which 

 are difficult to grow by ordinary methods, for studying 

 the phenomena of infection when the micro-organisms are 

 protected from the phagocytes, and for other purposes. 

 A glass rod or small test-tube, according to the size 

 desired, is dipped into a beaker containing the ordinary 

 (not flexible) collodion, is then withdrawn and allowed to 

 dry, and the process is repeated two or three times. In 



