ARNOLD H. EGGERTH AND MARGARET BELLOWS 679 



The experiments described above establish the influence of proteins 

 upon the stability of bacterial suspensions at different H ion concen- 

 trations, and relate this influence to the isoelectric point of the added 

 protein and its concentration. It must be noted that bacteria are 

 not the only suspensions that are effected by proteins in this way. 

 We have tested the effect of proteins on sols of cellulose nitrate, 

 cellulose acetate, and paraffin emulsions, with results that closely 

 parallel our experiments with bacteria. Although Walpole (1913) 

 does not relate his results to the isoelectric point of the protein, it 

 seems clear, from the curve given by him, that gelatin, albumin, and 

 globulin, affect the stability of oil emulsions and mastic and gold sols 

 at different concentrations of H ions in the same manner that these 

 substances affect bacterial suspensions. The same may be said of the 

 action of albumin on Fe(0H)3 sol at different salt concentrations, as 

 reported by Brossa and Freundlich (1914). Of the same import are 

 the observations of Putter (1921) that in the presence of peptone, 

 acids change the sign of the charge on Bacterium coli, and of Coulter 

 (1922) who shows that erythrocytes in contact with specific sensitizer, 

 or even with normal homologous or heterologous sera, agglutinate 

 most promptly when the pH is such that the euglobulins of these 

 sera are isoelectric. 



Loeb (1920) has shown that when collodion membranes are treated 

 with proteins, a combination takes place between the collodion and 

 the protein, and the membrane becomes isoelectric near the H ion 

 concentration at which the protein is isoelectric. Reactions acid 

 to this point now charge the membrane positively. It is obvious 

 that the effect of proteins upon bacteria (and probably upon other 

 suspensions and emulsions) is of the same nature. Where concentra- 

 tions of about 0.25 per cent of protein are used, the bacteria agglutinate 

 at or near the isoelectric point of the protein; increasing the H ion 

 concentration beyond this point causes the original charge upon the 

 bacteria to be reversed, and prevents agglutination. 



SUMMARY. 



1. The effect of adding pure proteins to bacterial suspensions at 

 different H ion concentrations has been studied. 



