Sterilization by Filtration
Filtration is an effective and reasonably economical method of sterilization. It is used to sterilize heat-sensitive fluids, and air. It is particularly useful for solutions containing toxins, enzymes, drug, serum and sugars. Sugar solutions used for the cultivation of microorganisms tend to caramelise during autoclaving and so they are best sterilized by filtration. Filtration is also used extensively in beer and wine industries. Filters with known pore sizes which are sufficiently small to hold back bacteria are employed. Recently filters that can remove viruses are also available.
Filtration is an excellent way to remove the microbial population from solution containing heat sensitive material.
There are two types of filters namely (Figure 4.4):
1. Membrane filter (surface filtration) and
2. Depth filter
Membrane filtration is used for preparing heat-labile culture media components. It is also useful in removing bacteria from heat-sensitive pharmaceutical products and biological solutions.
Membrane filters are made up of either cellulose acetate, cellulose nitrate, polycarbonate, polyvinylidene fluoride or other synthetic porous materials. These filters remove microorganisms by screening them out, such as a sieve separates large sand particles from small ones. Membranes with pore size of 0.2μm in diameter are used to remove most vegetative cells but not viruses. These filters are used to sterilize pharmaceutical products, ophthalmic solutions, culture media, oils, antibiotics, and other heat sensitive solutions (Figure 4.5a, b & c).
Depth filters are the oldest type of filters and they consist of overlapping layers of fibrous sheets of paper, asbestos or glass fibers. The overlapping fibers create random paths through the filter that trap many particles.
Depth filter are made up of diatomaceous earth (Berkefeld filters) which are used as water purifiers. Examples of types of depth filters (Figure 4.6) contains unglazed porcelain (Chamberl and filters) and asbestos (Seitz Filter).
Air also can be sterilized by filtration.
The air is freed from infection by passing it through High Efficiency Particle Arrester (HEPA) filter. Laminar air flow biological safety cabinets are one of the most important air filtration systems (Figure 4.7). It employes HEPA filters which remove 99.97% of 0.33μm particles size. Some operation theaters and rooms occupied by burn patients receive filtered air to lower the numbers of airborne microbes. HEPA filters remove almost all microorganisms above 0.3μm in diameter.
Various physical methods of sterlization is summarized in Table 4.2