CHAPTER XLI 

 FILTERABILITY OF MICRO-ORGANISMS 



S. P. KRAMER 

 Cincinnati, Ohio 



We have come to divide bacteria and viruses into "filterable" and ''non-filter- 

 able," and we have come to think that those organisms, visible or invisible, which are 

 smaller than the pores of our filter are filterable. That size, however, cannot be the 

 sole criterion of filterability we have known from the behavior of certain aniline dyes. 

 Thus Victoria blue B, a basic dye, will not pass a Berkefeld filter, while Congo red, 

 an acid dye, will readily pass through the same filter. It happens that the filters which 

 we use in bacteriological practice — namely, sand, porcelain, diatomaceous earth, and 

 asbestos — are all of some form or compound of silicic acid, so that really when we 

 speak of a filterable organism, dye, or other colloid, we should say "filterable through 

 siliceous filters." 



The selection of siliceous material for filters seems to have been purely empirical. 

 There had been no realization that these materials were other than porous materials 

 which were chemically neutral. Their use goes back into antiquity. Aristotle in his 

 essay De generatione animalia refers to filtration in the following passage: "Flesh is 

 produced therefore through the veins and pores, the nutriment being deduced in the 

 same manner as water through earthen vessels not sufficiently baked, "^ showing that 

 filters of unglazed porcelain (what we now know as the Chamberland type) were used 

 in the days of Aristotle. 



In 1916 I showed before the Research Society of Cincinnati that any filter made of 

 siliceous material, sand, porcelain, powdered glass, colloidal silica, or diatomaceous 

 earth would absorb basic dyes, and that acidic dyes would pass through.^ 



Dr. Stuart Mudd^ described the surface of the pores of a Berkefeld filter as the site 

 of an electrical potential difference, a Helmholtz double layer, in which the wall of the 

 filter carried a negative charge and the liquid a positive charge; and he predicted and 

 found that when suspensions were filtered through such a filter, positively charged 

 particles were absorbed and retained by the filter. 



In Figure i the lines A A' and BB' represent the walls of a capillary or pore of a 

 siliceous filter carrying a negative charge. Minus and plus signs represent particles of 

 matter smaller than the pore of the filter, with their respective charges indicated. The 

 minus particles carrying the same charge as that of the filter pass through the pores. 

 The plus particles carrying a charge of sign opposite to that of the filter are retained 

 by the filter. 



' Quoted by Bolton: Chem. News, 41, 55. 1880. 



'Kramer, S. P.: /. General Physiol., 9, No. 6, 811-12. July, 1926; /. Infect. Dis., 40, No. 2, 

 343-47. Feb., 1927. 



3 Mudd, S.: Am. J. Physiol., 63, 429. 1922. 



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