THE MECHANISM OF AGGLUTINATION. 145 



tance to them, particularly during the phase of the phenomenon 

 in which the bacteria which have been affected by the agglutinin 

 are still scattered and are beginning to collect to form the typical 

 agglutination. 



These two hypotheses, which were formulated in the beginning 

 of studies on agglutination, could not rest on a very firm basis from 

 lack of sufficient data. An experimental fact of importance was 

 furnished by Kraus.* Kraus showed that if the serum of animals 

 vaccinated against the cholera vibrio is mixed with a limpid filtered 

 culture of this organism a precipitate is formed in the fluid. This 

 reaction is specific and does not occur when any other serum is 

 used in place of cholera serum. When this precipitate has been 

 formed it soon collects in small clumps that recall in appearance 

 masses of agglutinated bacteria. Kraus also showed that the same 

 result could be obtained with other bacteria and their correspond- 

 ing antisera. 



These experiments would seem to corroborate, a priori, the second 

 of the two hypotheses that we have mentioned. It would seem, 

 moreover, to invalidate Gruber's hypothesis, which recognizes as 

 the sole cause of agglutination a structural modification of the 

 affected cells. These experiments indeed show that a flaky pre- 

 cipitation resembling true agglutination can be formed by mixing 

 with serum a fluid containing no definite bacteria but simply the 

 materials of bacterial disintegration. This fact of Kraus' would 

 seem, then, to rule out Gruber's theory entirely. 



3. Nicolle's hypothesis.^ — Nicolle has a somewhat different 

 idea. He has confirmed Kraus' results and agrees that the agglu- 

 tinin precipitates the agglutinable (or agglutinated) substance 

 of bacteria.^ He thinks, moreover, that this agglutinable sub- 

 stance, which in old cultures may become diffused into the surround- 

 ing fluid, is present in large amounts in the membrane of the outer 



* Kraus, K. K. Gesellschaft der Aertze in Wien, April 30, 1S97, and Wiener 

 klinische Wochenschrift, August 12, 1S97, No. 32. 



t Annales de l'lnstitut Pasteur, March, 1898. 



t This is more than a simple adaptation of Kraus' experiment; the expression 

 indicates, perhaps rather hastily, that we must consider the substance precipitated 

 as the one which is of importance in the agglutination of bacteria, the substance 

 which, in other words, represents in bacteria that part which is susceptible to the 

 agglutinin. 



