Filtration: Types of Filters

Several types of filters are available for use in the microbiology laboratory. Inorganic filters are typified by the Seitz filter, which consists of a pad of porcelain or ground glass mounted in a filter flask.

Organic filters:

Organic filters are advantageous because the organic molecules of the filter attract organic components in microorganisms. They are given below:

1) Berkefeld filter:

One example, The Berkefeld filter, utilizes as substance called diatomaceous earth. This material contains the remains of marine algae known as diatoms. Diatoms are unicellular algae that abound in oceans and provide important foundations for the world's food chains. Their remains accumulate on the shoreline and are gathered for use in swimming pool aquarium filters, as well as for microbiological filters used in laboratories.

2) Membrane Filter:

The membrane filter is at third type of filter that has received broad acceptance. It consists of a pad of organic compounds such as cellulose acetate (cellulose esters) or polycarbonate (plastic polymers), mounted in a holding device. These filters are only 0.1 mm thick. The pores of membrane filters include, for example, 0.22μm and 0.45μm sizes, which are intended for bacteria. This filter is particularly valuable because bacteria multiply and for colonies on the filter pad when the pad is place on a plate of culture medium. Microbiologists can then count the colonies to determine the number of bacteria originally present. For example, if a 100-ml sample of liquid were filtered and 59 colonies appeared on the pad after incubation, it could be assumed that 59 bacteria were in the sample. However, Some very flexible bacteria, such as spirochetes, or the wall less mycoplasma, will sometimes pass through such filters.

Membrane filters used to trap bacteria form air and water samples can be transferred directly to agar plates, and the quantity of bacteria in the sample can be determined. Alternatively, the filters can be transferred from one medium to another, so organisms with different nutrient requirements can be detected. Filtration is also used to remove microorganisms and other small particles from public water supplies and in sewage treatment facilities. This technique, however, cannot sterilize; it merely reduces contamination.

3) HEPA filters:

Air can also be filtered to remove microorganisms. The filter generally used is a high-efficiency particulate air (HEPA) filter. This apparatus can remove over 99 percent of all particles, including microorganisms with a diameter larger than 0.3 μm. The air entering surgical units and specialized treatment facilities, such as burn units, is filtered to exclude microorganisms. In some hospital wards, such as for respiratory diseases, and in certain pharmaceutical filling rooms, the air is recirculated through HEPA filters to ensure its purity. Used filter are soaked in formalin before they are disposed of.

Industrial Fluid Filtration:

Two examples of filters used in conjunction with fluids. (a) A filter of woven mesh Dacron (arrow) is used to trap clumps of unwanted blood cells that might otherwise enter the recipient's circulation during a transfusion. (b) A cartridge filter removes contaminants from fluids to be used for intravenous injections or for other medical purposes.

The Membrane Filter Technique:

a) The membrane filter consists of a pad of cellulose acetate, or similar material, mounted in a holding device. (b) The holding device is secured by a clamp, and a measured amount of fluid is filtered by pouring it into the cup. The solution runs through to a flask beneath, and bacteria are trapped in the filter material. (c) The filter pad is place onto a plate of nutritious medium, and the plate is incubated. (d) After incubation, colonies appear on the surface of the filter pad. The colony count reflects the original number of bacteria in the fluid sample.

Suitable Selection of Filters --- By pore size

In the manufacture of vaccines that require the presence of live viruses, it is important to select a filter pore size that will allow viruses to pass but prevent bacteria from doing so. By selecting a filter with a proper pore size, scientists can separate polioviruses from the fluid and debris in tissue cultures in which they were grown. This procedure simplifies the manufacture of polio vaccine. Cellulose acetate filters with extremely tiny pores are now available and are capable of removing many viruses (although not the very smallest) from liquids. However, these filters are expensive and clog easily.