BACTERIAL VACCINES 191 



out with a suspension of dead bacilli, the injections 

 being given every seven days for three weeks. The 

 conferred immunity lasts for about two and one- 

 half years. 



In the employment of bacterial vaccines, one 

 must constantly keep in mind the nature of the 

 bacterium with which one is working, and the kind 

 of immunity one wishes to bring about. Every- 

 thing depends on the way in which the vaccine is 

 prepared. With bacteria making considerable 

 quantities of a toxin, it will be necessary, if we 

 wish to immunize against this toxin, to grow the 

 culture for the requisite length of time and under 

 the proper conditions for producing the toxin. 

 In the case of bacteria possessing certain endo- 

 toxins, it may be necessary to let the cultures 

 autolyze, so as to set these substances free, or the 

 bacteria may be crushed and ground for the same 

 purpose. On the other hand, we may wish to use 

 these bacteria for producing a specific agglutinat- 

 ing serum. In that case we often try to avoid 

 injecting these toxic substances. Our entire pro- 

 cedure might then have to be quite the reverse of 

 what has just been indicated. 



The Vaccines. Wright's method of preparing a 

 staphylococcus, typhoid, streptococcus, or gono- 

 coccus vaccine, is as follows: 



Several streak slant agar cultures are planted and 

 incubated for twenty to twenty-four hours. The 



