174 PRINCIPLES OF BACTERIOLOGY 



shall find that the serum does not completely affect these cultures. 

 As already stated, such a condition of things is met with in inflammations 

 due to streptococci and other bacteria, and is, therefore, of considerable 

 practical importance. It is because of this fact that a serum acts best 

 only in a certain percentage of cases. In order to overcome this difficulty 

 in persons infected with these bacterial species we have no choice but to 

 make sera, not by means of one culture, but by means of a number of 

 different strains of the same species. The result of this will be that, 

 corresponding to the various partial elements in these different cuL- 

 tures, we shall obtain a serum containing a large number of the partial 

 groups. Such a serum will then exert a specific action on a large num- 

 ber of different cultures, but not quite as great an influence on any- 

 one as if only that variety had been injected. 



In other words, the development and the closer analysis of the prob- 

 lem of immunity, especially during the past few years, have shown us 

 that we must make use, more than heretofore, of so-called polyvalent 

 or multipartial sera. In the serum therapy of streptococcus infections, of 

 dysentery, etc. , the production of such multipartial sera is an advantage 

 in practice. Owing to these partial groups also, a serum e. g., anti- 

 typhoid serum can specifically affect a closely allied species of bac- 

 * terium, like bacillus coli, for example. For it is known that closely 

 related species of bacteria, such as typhoid and colon bacilli, possess 

 certain partial groups in common, and a serum is thus produced which 

 to a certain extent acts on both species. This constitutes what is 

 known as the "group reaction." 



