Many liquids such as solutions of antibiotics or certain components of culture media become chemically altered at high temperatures, so the use of any of the heat regimes described above is not appropriate. Rather than killing the microorganisms, an alternative approach is simply to isolate them. This can be done for liquids and gases by passing them through filters of an appropriate pore size. Filters used to be made from materials such as asbestos and sintered glass, but have been largely replaced by membrane filters, commonly made of nitrocellulose or polycarbonate (Figure 13.3). These can be purchased ready-sterilised and the liquid passed through by means of pressure or suction. Supplies of air or other gases can also be filter-sterilised in this way. A pore size of 0.22 /j,m is commonly used; this will remove bacteria plus, of course, anything bigger, such as yeasts; however, mycoplasma and viruses are able to pass through pores of this size. With a pore size 10 times smaller than this, only the smallest of viruses can pass through, so it is important that an appropriate pore size is chosen for any given task. A drawback with all filters, but especially those of a small pore size, is that they can become clogged easily. Filters in general are relatively expensive, and are not the preferred choice if alternative methods are available.
High efficiency particulate air (HEPA) filters create clean atmospheres in areas such as operating theatres and laboratory laminar-flow hoods.
Figure 13.3 Membrane filtration. Membrane filters are used to sterilise heat-labile substances. They are available in a variety of pore sizes, according to the specific application
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