BY DEAD CULTURES OF BACTERIA 557 



cultures in small, that is non-lethal, doses. In subsequent 

 inoculations the doses may be increased in amount. For 

 example^ immunity may thus be obtained in rabbits against the 

 bacillus^ pyocyaneus. Such a method, however, is difficult to 

 carry out, and it has been found more convenient to commence 

 the process of immunisation with dead or attenuated cultures, 

 and then to continue with virulent cultures. 



Exaltation of the Virulence. — The converse process to attenua- 

 tion, i.e., the exaltation of the virulence, is obtained chiefly by 

 the method of cultivating the organism from animal to animaf — 

 the method of' passage discovered by Pasteur (first, we believe, 

 in the case of an organism obtained from the saliva in hydro- 

 phobia, though having no causal relationship to that disease). 

 This is most conveniently done by intraperitoneal injections, as 

 there is less risk of contamination. The organisms in the 

 peritoneal fluid may be used for the subsequent injection, or a 

 culture may be made between each inoculation. The virulence 

 of a great number of organisms can be increased in this way, 

 the animals most frequently used being rabbits and guinea-pigs. 

 This method can be applied to the organisms of typhoid, cholera, 

 pneumonia, to streptococci and staphylococci, and in fact to 

 those organisms generally which invade tissues. 



2. Immunity by Dead Cultures of Bacteria. — In some cases 

 a high degree of immunity against infection by a given microbe 

 may be developed by repeated and gradually increasing doses 

 of the dead cultures, the cultures being killed sometimes by 

 heat, sometimes by exposure to the vapour of chloroform. In 

 this method the so-called endotoxins will be injected along with 

 the other substances in the bacterial protoplasm, but the result- 

 ing immunity is chiefly directed against the vital activity of the 

 organisms — is antibacterial rather than antitoxic (vide infra). 

 The cultures when dead produce, of course, less effect than when 

 living, and this method may be conveniently used in the initial 

 stages of active immunisation, — to be afterwards followed by 

 injections of the living cultures. The method is extensively 

 used for experimental purposes, and is that adopted in anti-plague 

 and anti-typhoid inoculations, and in the treatment of infections 

 by means of vaccines. 



Combination of Methods. — The above methods may be com- 

 bined in various ways. By repeated injections of cultures at 

 first in the dead condition, then living and attenuated and 

 afterwards more virulent, and by increasing the doses, a high 

 degree of immunity may be obtained. 



3. Immunity by Sensitised Dead Cultures. — In this method, 



