Bacterial Filters

We have come to divide bacteria and viruses into filterable and nonfilterable, 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 we have known from the behavior of certain aniline dyes.* Thus Victoria blue, a basic dye, will not pass a Berkefeld filter while Congo red, an acid dye, will readily pass through the same filter. Now it happens that the filters which we use in bacteriologic practice, namely, sand, porcelain, and diatomaceous earth, are all of them some form or compound of silicic acid, so that really when one speaks of a filterable organism, dye or other colloid, one ought to say filterable through siliceous filters. One may speak of a filter when it is in action as a suspension of the material of which the filter is composed, in the fluid which is being filtered. Now silica has a definite negative charge and it may be that if one constructs a filter of a material of charge opposite to that of silica, one might find that bacteria or colloids which are filterable through silica filters are non-filterable through such other filters, and vice versa. Accordingly, filters were made of piaster of Paris and experiments