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THE MECHANISM OF AGGLUTINATION. 

 By R. Greig Smith, M.Sc, Macleay Bacteriologist. 



Pfeiffer and his pupils about 1894 discovered that when an 

 animal is repeatedly inoculated with certain organisms, its serum 

 has the power of causing the organisms in a bouillon culture to 

 become altered and to cohere or agglutinate into microscopical 

 masses or clumps. The serum only reacts in this manner or is 

 active with the bacteria with which the animal has been inoculated, 

 and this fact caused the reaction to be used as a diagnostic for 

 that particular organism. Widal inverted the reaction and used 

 a culture of typhoid bacteria to discover whether or not a serum 

 was active, and especially in human practice to determine if a 

 patient had typhoid fever. To apply the test a drop or loop of the 

 blood serum which has separated from the clot is added to about 

 30 drops or loops of bouillon containing typhoid bacteria. Should 

 the bacteria collect into clumps in half-an-hour the reaction is 

 positive, and is by some considered as a proof of typhoid fever, 

 by others (i) as a symptom of that disease. The test has been 

 extended to other diseases. 



The phenomenon of agglutination forcibly recalls that of 

 fiocculation or coagulation of inorganic particles, where instead 

 of adding an indefinite active serum there is added a definite 

 chemical substance, and a natural assumption would be that they 

 are brought about by the same causes. One difference, however, 

 between the two phenomena is that the bacteria are living and 

 sensitive, while inorganic particles are insensitive. Before the act 

 of agglutination they are actively motile; the agglutinine in the 

 serum causes them to lose their motility in great part or entirely; 

 they become immobilised. After a variable time, it may be hours 

 or days, the bacteria regain their motility and the clumps break 



