65 



THE FLOCCULATION OF BACTERIA. 



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



Small particles of clay or finely divided chemical precipitates 

 may remain suspended in water for a long time. When, however, 

 certain salts are added to the water, the microscopic particles 

 are seen to settle to the bottom of the liquid in which they were 

 formerly suspended, with greater or less rapidity according to the 

 kind of salt, the amount of salt per volume of liquid, and the 

 temperature. In a previous paper* I have shown that the sus- 

 pension of the particles is caused by the molecular pressure of 

 the individual water molecules upon the surfaces of the suspended 

 solid, with the result that there is, as it were, a hydrate formed. 

 By reason of its superior attraction for water, the saline floccu- 

 lating agent causes the withdrawal of the water molecules which 

 had crowded upon the surfaces of the particles. What was 

 fomierly a surface pressure, now becomes a surface tension, which, 

 being exerted upon all the particles, causes them to run together 

 into little clumps which quickly gravitate to the bottom of the 

 liquid. 



The flocculating action is to be traced chiefly to the metallic 

 portion of the salt, and in a small degree only to the acid radicle. 

 The metals vary in the intensity of their action, some being 

 strong, others weak; as an instance, calcium is about 160 times 

 more powerful than potassium. These agents not only cause the 

 coagulation of particles which are visible with the microscope, 

 but they also induce substances which are in solution to precipi- 

 tate. The latter are in what is called •'pseudo-solution" — that 

 is, the}' consist of molecular aggregates which are just retained in 

 solution and no more. The flocculating agent induces a further 

 coalescence of the molecular aggregates, and these being no longer 

 able to remain in solution separate out as a precipitate. 



If we look upon bacteria growing in a culture fluid as particles 

 in suspension, it seems reasonable to hope that they might be 

 sensitive to the action of flocculating agents precisely like particles 



* Journ. Soc. Chem. Industry, xvi. S72; xvii. 117. 



