LYSINS, AGGLUTININS, PRECIPITINS, ETC. 229 



noticed that if the respective immune sera were added to bouillon cul- 

 tures of these two species, the cultures would lose their turbidity and 

 flake-like clumps would sink to the bottom of the tube, the supernatant 

 fluid becoming clear. Gruber, at the same time, called attention to the 

 fact that immune sera would affect in this way not only the microor- 

 ganism used in their production, but, to a less energetic extent, other 

 closely related bacteria as well. 



Widal, very soon after Gruber and Durham's announcement, ap- 

 plied the agglutination reaction to the practical diagnosis of typhoid 

 fever, finding that the serum of patients afflicted with this disease 

 showed agglutinating power over the typhoid bacillus at early stages 

 in the course of the fever. The reaction, thus practically applied to 

 clinical diagnosis, was soon shown to be of great importance in its 



Fig. 59. — Microscopic Agglutination Reaction. 



bearing on bacteriological species differentiation. Since animals im- 

 munized against a definite species of bacteria acquire in their sera 

 specific agglutinating powers for these bacteria and at best only slight 

 agglutinating powers for other species, immune sera can be used ex- 

 tensively in differentiating between bacterial varieties. 



Agglutination may be observed microscopically or macroscopically. 

 Bacteria brought into contact with agglutinating serum in the 

 hanging drop rapidly lose their motility, if motile, as in the case of 

 typhoid bacilli, and gather together in small clumps or masses. The 

 microscopic picture is striking and easily recognized and the reaction 

 takes place with varying speed and completeness, according to the 

 strength of the agglutinating serum. 



As the reaction approaches completeness, the clumps grow larger, 



