54 PATHOGENIC BACTERIA 



shown to take place between the serum of animals experimen- 

 tally immunized against certain diseases and cultures of the spe- 

 cific bacilli producing such, could be obtained during the period 

 of infection, and in this way serve as a means of clinical diagno- 

 sis. If thte blood serum of an animal which has been rendered 

 immune against, e. g., the typhoid bacillus be added in a tost 

 tube to a living broth culture of the same microbe, the bacilli in 

 the culture rapidly cohere and subside to the bottom of the tube. 

 This is known as " clumping" or "agglutination," and is taken 

 to indicate the presence of an agglutinating substance or " agglu- 

 tinin" which has formed in the blood in consequence of the 

 presence of the bacilli experimentally introduced into the ani- 

 mal. The blood serum acquires this property, however, before 

 immunization is established, i. e., during the progress of infec- 

 tion itself, and the property may be utilized as a means of diag- 

 nosis, for the important reason that the serum of a typhoid 

 patient, whilst it will agglutinate typhoid bacilli, will not agglu- 

 tinate those of other kinds. What is true in the case of typhoid 

 is true also of other bacterial diseases. The phenomenon admits 

 of two applications; a disease may be diagnosed by the action of 

 the blood serum upon a known bacillus; or a bacillus may be 

 diagnosed by the action of a known "immunized serum." It 

 is to the former that importance attaches in practical medicine 

 and surgery. 



After this brief explanation of the rationale of Widal's re- 

 action, the method of carrying it out may be described. 



The broth culture of typhoid bacillus used must not be 

 more than of twenty-four hours' growth, and must be grown in 

 the incubator at 37 C, preferably in a broth that is not alka- 

 line, but amphoteric (giving simultaneously an acid and an alka- 



