This is a method of making liquids or gases sterile by permitting them to filter through the appropriate material.
1. Air Filters:
In the laboratory most of out test tubes and flasks are stoppered with a plug of control, Air and other gases pass freely in or out of the tubes through the cotton, but micro-organisms in the dusty air are caught in the meshes of the cotton and cannot get through into the tube (unless the cotton is wet). Therefore, the cotton plugs serve as efficient filters.
2. Filtration of Liquids:
Filters for making liquids free of any visible or cultivable microbes have very important uses in microbiology. There are several types of laboratory filters; those most commonly employed are the Berkefeld and Mandler filters.
They are made of diatomaceous earth pressed into the shape of a hollow candle. The filter candle and a suction flask to receive the filtrate are sterilized, usually in the autoclave, and then carefully fitted together.
The liquid to be filtered is poured into the glass “mantle” about the filter candle suction is applied, and the liquid is drawn slowly through the porous filter and falls into the sterile flask below.
As the fluid passes through the filter candle, all the ordinary micro-organisms in it are held back, and the filtrate is bacteriologically sterile, that is, free of any microbe that can be cultivated on lifeless media.
Filters are used in the laboratory to sterilize blood-serum, ascetic fluid, sugar solutions or other liquids which cannot be sterilized by heat without altering them in some undesired way. Also exotoxins, enzymes, and other products of the growth of bacteria are separated from the organisms which produce them by use of these filters. Finally, the filterable viruses are isolated from the body tissues or fluids which contain them, and separated from ordinary microbes, by means of these filters.
3. Filtration of Drinking Water:
In most communities, filtration through sand and gravel is one of the most important measured used for the purification of the public water supply. In this kind of filtration there is more than a mechanical straining.
On the surface of the sand filter a scum develops which consists largely of putrefactive micro-organisms, and as the water passes slowly through the sand, most of the contained bacteria are held back in this surface scum, and there destroyed in competition with the hardy putrefactive forms. Properly filtered water is nearly free of all bacteria and quite free of dangerous germs.