NATURE OF AGGLUTINATION 227 



by the organisms and second the agglutination of the 

 organisms charged with agglutinin. These two stages 

 can be shown by treating the bacteria with agglutinating 

 serum at C. ; no agglutination takes place at this 

 temperature, but the organisms absorb agglutinin, for 

 if they be centrifuged, washed, and suspended in saline 

 at 20-37 C., agglutination then occurs. A certain 

 amount of sodium chloride or of some other salt must 

 also be present for agglutination to take place. 



Various hypotheses have been formulated in the past 

 to account for agglutination, such as vital paralysis of 

 the bacteria, the development of adhesiveness by the 

 bacteria, or a precipitating action in the medium carrying 

 down the bacteria, but none of these is satisfactory. 

 Bordet first recognised that agglutination is very much 

 akin with the flocculation of colloidal solutions by satura- 

 tion with salts. If a \oung broth culture of, say, the 

 typhoid bacillus be examined in a hanging-drop, the 

 actively motile bacilli will be noticed to collide with one 

 another continuously, yet they do not aggregate into 

 masses. Some force of repulsion must exist, otherwise 

 the organisms would certainly aggregate, for, owing to the 

 force of surface tension present in the liquid, there is a 

 tendency for fine particles to aggregate into masses, as is 

 seen in the flocculation of the particles of protein in a 

 heated or salted solution of egg-white. The force of 

 repulsion is an electrical one, the bacteria may be con- 

 sidered as being electrically charged particles, and par- 

 ticles bearing the same electric charge mutually repel 

 one another. We may, therefore, regard the similar 

 electric charge as constituting the repulsive force and 

 surface tension as being the attractive force, and if the 

 former be sufficiently powerful to overcome the latter, 

 the particles, i.e. the bacteria, will remain apart. The 

 absorption of agglutinin by the bacteria reduces the 



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