486 MICROBIOLOGY 



changes are or are supposed to be caused by one or the other 

 of these species. And (3) mixed or polyvalent vaccines com- 

 posed of the killed organisms of several species. In addition 

 to these, various serums, culture filtrates and tissue extracts 

 such as leucocyte extract are used and sometimes called vac- 

 cines. 



In measuring the dose of dead bacteria to be injected 

 Wright employed the opsonic index. It has been found, how- 

 ever, that reliance can not be placed upon it for that purpose 

 and consequently the dose must be determined experimentally. 

 From 500,000 to 25,000,000 organisms or even more are usually 

 given as an initial dose.* This practical appreciation of bac- 

 teria in therapeutics is in the process of demonstration. At 

 present, the indications are that many of the vaccines (killed 

 bacteria) will be of great assistance in the treatment of nu- 

 merous infections. 



* For a discussion of this subject the student is referred to the 

 small volume by R. W. Allen, 3rd. edition, on vaccine therapy and 

 its theory and practice. Also Wolff-Eisner's Handbuch der Serum- 

 therapie und experimentellen Therapie, 1910. 



