202 BACTERIOLOGY 



terise, both closely related to Bacillus typhosus, no such 

 clumping will occur. 



Again, if we mix with that serum typhoid bacilli derived 

 from any number of cases of typhoid fever the serum will 

 cause clumping of some cultures and not others, even though 

 all cultures came from individuals having the same disease 

 and by the common tests all are alike. 



We note here a high degree of specificity not only a 

 specificity peculiar to certain species but likewise peculiar 

 to certain individual members of the same species. We are 

 justified then in concluding that, from the standpoint of 

 this test, all the cultures of typhoid bacilli that were clumped 

 by the serum used were of the same strain or type as that 

 used in immunizing the animal; while all those that did not 

 clump with the same serum were of different strains or 

 types. If we now immunize an animal from anyone of 

 this latter group of typhoid bacilli we shall find that the 

 serum will cause clumping of the bacilli in the culture used 

 for immunization and will probably react in the same 

 manner with some of the other cultures but not with all, 

 nor with the cultures embraced by our first group. Thus, 

 we will have established at least two groups, or types that 

 have distinct, specific serologic reactions, and so we may 

 go on and perhaps establish additional groups. 



This reaction commonly known as the "agglutination 

 reaction" is invaluable in the efforts to assemble species into 

 groups and subgroups or types specifically different the 

 one from the other insofar as the reaction goes. 



If by this procedure we find that two groups A and B of an 

 infective species can be established and with the members of 

 Type A we render an animal so highly immune that its serum 

 may be expected to possess curative properties for the disease 



