THE STABILITY OF BACTERIAL SUSPENSIONS. 



HI. Agglutination in the Presence of Proteins, Normal 

 Serum, and Immune Serum. 



By JOHN H. NORTHROP and PAUL H. De KRUIF. 



{From the Laboratories of The Rockefeller Institute for Medical Research) 



(Received for publication, May 24, 1922.) 



It has frequently been noted that the addition of a small amount of 

 certain substances, especially proteins, markedly affects the behavior 

 of suspensions. It was found by Whitney and Blake^ for instance 

 that the sign of the charge of gold particles in the presence of gelatin 

 could be reversed by acids, a result which did not occur without 

 gelatin. The same effect has been noted by Loeb^ in the case of col- 

 lodion membranes treated with different proteins. The membrane 

 always acquires the isoelectric point of the protein used. It was found 

 by one of the writers that peptone markedly affects the acid agglutina- 

 tion of the bacillus of rabbit septicemia. It has been shown in the 

 preceding paper^ that the isoelectric point was also displaced. This 

 result had been noted by Putter.^ The present paper contains the 

 results of experiments on the effect of proteins and sera on the pro- 

 perties of suspensions of bacteria. 



Fig. 1 shows the effect of various concentrations of egg albumin on 

 the agglutination and charge of the bacillus of rabbit septicemia 

 CType D strain). The method of plotting is the same as in the pre- 

 ceding paper. Increasing the amount of egg albumin gradually shifts 

 the curve to the alkaline side so that the isoelectric point is moved to 

 pH 5.0 which is approximately that of egg albumin. In other 



1 Whitney, W. R., and Blake, J. C, /. Am. Chcm. Soc, 1904, xxvi, 1339. 



2Locb, J., /. Gen. Physiol, 1919-20, ii. 659; 1921-22, iv, 213. 



3 Northrop, J. H., and De Kruif, P. H., J. Gen. Physiol., 1921-22, iv, 639. 



* Putter, E., Z. Immimitdtsforsch., Orig., 1921, xxxii, 538. The same observa- 

 tion had been made independently by one of the writers De Kruif, P. H., J , 

 Gen. Physiol, 1921-22, iv, 345. 



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