290 AGEICULTUEAL AND INDUSTEIAL BACTERIOLOGY 



many years. It has now been shown definitely, however, 

 that the blood serum which is capable of causing flocculation 

 of a particular kind of bacterium does this by sensitizing 

 the bacteria to the action of certain salts. For example, 

 common salt, that is, sodium chloride, in dilute solution does 

 not cause flocculation or agglutination of typhoid bacteria. 

 A dilute solution of serum from a typhoid patient with 

 typhoid bacilli in distilled water will not cause agglutina- 

 tion. "When, however, typhoid serum, bacteria and common 

 salt are all present, flocculation or agglutination does 

 occur. It can be readily shown that the agglutinin of the 

 serum combines with the bacteria and makes these bacteria 

 sensitive to the action of salt. Apparently the same explan- 

 ation holds for precipitation as with proteins when a 

 specific antiserum is used. 



Agglutination. — Agglutinins are usually highly specific, 

 that is, an agglutinin capable of causing clumping of one 

 kind of microorganism will not cause clumping of other 

 species. There are same exceptions to this rule, however. 

 In some cases closely related bacteria will cause antibodies to 

 develop which will cause some agglutination of the other 

 species. In many instances, however, the reaction is so 

 specific that it can be used readily in the diagnosis of 

 disease. For example, the most common method of recog- 

 nizing typhoid fever in the earlier stages of the disease is 

 by the so-called Widal test (named after the man who first 

 described typhoid agglutination). The test may be either 

 microscopic or macroscopic. If the former, some of the 

 diluted serum from the patient suspected of having typhoid 

 is mixed with a drop of typhoid broth culture and examined 

 under the microscope. If the serum contains agglutinin, 

 that is, if the patient has typhoid, the bacteria vrill at first 

 be observed to move about rapidly, finally their motion 

 decreases, they begin to clump together and in a few min- 

 utes to half an hour comparatively few, if any motile bae- 



