APPLICATIONS OF BACTERIOLOGY 253 



throughout the liquid ; the suspension is " homogeneous." If 

 a trace of serum is added from an animal prepared by injections 

 of typhoid bacilli or from a patient suffering from typhoid 

 fever, the bacilli lose their 

 motility and collect into 

 masses : they are said to 

 become agglutinated by the 

 serum. If the serum is 

 added to a broth culture 

 floccules can be seen with 

 the naked eye forming and 

 sinking to the bottom of 

 the tube ; agglutination is 

 also a sedimentation. Nor- 

 mal serum never possesses 



this property, certainly FlG< 7I ._ Ag giutination of the typhoid 

 never to the same degree. bacillus by the serum of a typhoid 



The agglutinating power 

 may be measured by try- 

 ing the effect on a suspension of the bacilli of various dilutions 

 of the serum : one may say that such and such a serum 

 agglutinates at i in 50, i in 100, i in 1000. . . . 



The reaction is capable of two applications. With a bacillus 

 definitely known as a genuine B. typhosus, one can say that 

 the serum which agglutinates it is an antityphoid serum. If, 

 on the other hand, with such a definitely known serum we 

 find a bacillus agglutinated, we may say that it is a true 

 B. typhosus. Agglutination may be used to diagnose now the 

 bacillus, now the disease. The agglutinating power depends 

 on an antibody named the agglutinin. This substance keeps 

 much longer than the time required for its carriage to long 

 distances for examination, when this is necessary. Dead 

 bacteria also agglutinate, so that the method may be employed 

 even without living cultures. Seroagglutination is therefore 

 the simplest and most convenient of the biological methods. 



In performing the test it is necessary to avoid certain 

 sources of error, among others the existence of bacterial strains 



