Filtration:

In order to sterilize solutions which is heat sensitive, filtration is an excellent way to reduce the microbial population. The filters simply remove the microbes instead of killing them. Depth filters consists of fibrous or granular materials that have been bonded into a thick layer filled with twisting channels of small diameter. The solution is passed through the filter which is sucked through this layer under vacuum and microbial cells are removed. The material used mostly is unglazed porcelain, asbestos or other similar materials.Membrane filters are also used and have replaced depth filters in recent times. These filters are made up of cellulose acetate, cellulose nitrate, polycarbonate, polyvinylidene fluoride, and other synthetic materials. These filters vary in size with pore sizes mostly of 0.2 to 0.5 µm in diameter and used to remove most vegetative cells, but not viruses, from solutions ranging in volume from 1ml to many litres. These filters are mostly used to sterilize pharmaceuticals, ophthalmic solutions, culture media, oils, antibiotics and other heat sensitive solutions.

The other way this method is used is in the laminar flow biological safety cabinets where the air is sterilized by filtration. These cabinets contain high-efficiency particulate air (HEPA) filters, which remove 99.97% of 0.3µm particles. The safety cabinets are most useful as the culturing of any organisms requires contamination free air to reduce the growth of other undesired organisms or for the preparation of media, examining tissue cultures etc (Fig. 10).

Fig. 10 . Biological safety cabinet