AGGLUTININS AND AGGLUTINATION 151 



ti nation and precipitation of the bacteria occurred at once. 

 This observation brought the phenomenon of bacterial agglu- 

 tination into close relation with the precipitation of colloids by 

 electrolytes, Bordet comparing it to the precipitation of par- 

 ticles of inorganic matter suspended in the fresh water of rivers 

 that occurs when the fresh water meets the salt water of the 

 ocean. He found that the agglutinin combined with the bac- 

 teria in the absence of the salts, and the resulting compound was 

 precipitated by the addition of minute amounts of electrolytes, 

 which did not precipitate or agglutinate the bacteria or the 

 serum alone. This indicates that the agglutinins cause a change 

 in the bacteria which brings them under the same physical laws 

 as the inorganic colloidal suspensions, which are characterized 

 by being precipitated by the addition of traces of electrolytes. 1 

 This precipitation is undoubtedly due to changes in solution 

 tension and surface tension (see " Precipitation of Colloids," 

 introductory chapter). Before the agglutinin combines with 

 the bacteria they behave like the colloidal solutions of organic 

 colloids, being only precipitated by the salts of heavy metals, 

 alcohol, formalin, etc., or by great concentrations of neutral 

 salts. 



According to Bechhold 2 normal bacteria behave like inorganic 

 suspensions that have each particle protected by an albumin-like 

 membrane, which prevents them from being thrown out of 

 suspension by solutions of alkali salts, etc. After being acted 

 on by agglutinin they are so altered that they behave like the 

 unprotected inorganic suspensions, and are precipitated by salts 

 and other electrolytes. This suggests the possibility that the 

 agglutinin makes the bacteria permeable for these electrolytes. 

 Agglutination obeys the same laws as other similar physical 

 phenomena ; the rate of agglutination depends upon the con- 

 centration of the suspension and of the electrolytes, and varies 

 with the valence of the cations. Although bacteria in an elec- 

 tric stream move toward the anode like all suspensions, after 

 being acted on by agglutinin they are agglutinated by the 

 current between the poles ; this indicates the importance of the 

 electrical charges of the bacterial surfaces in their agglutination 

 reactions. 



In all respects the behavior of bacteria and agglutinin 

 resembles the behavior of colloidal mixtures in suspension 



1 Arrhenius (Zeit. physikal. Chem., 1903 (46), 415) has attempted to show 

 that the gas laws are applicable to the partition of agglutinin between the 

 bacteria and the medium, which he compares to the partition of iodin between 

 water and carbon disulphid. This idea is not accepted by Craw (loc. cit.). 



2 Zeit. f. physikal. Chem., 1904 (48), 385. 



