Filtration is the preferred method of sterilizing heat sensitive liquid and gases without exposure to denaturing heat. Rather than destroying contaminating microorganisms, it simply removes them. It is the method of choice for sterilizing antibiotic solutions, toxic chemicals, radioisotopes, vaccines, and carbohydrates, which are all heat-sensitive.
The liquid or gas is passed through a filter, a device with pores too small for the passage of microorganisms, but large enough to allow the passage of the liquid or gas. These filters are made of different materials;
|Materials||Name of the filter|
|Asbestos pad||Seitz filter|
|Diatomaceous earth||Berkefeld filter|
|Sintered glass disks||Sintered glass filter|
|Borosilicate glass fiber||HEPA filter|
|Clay, mud||Candle filter|
The selection of filters for sterilization must account for the size range of the contaminants to be excluded. The most commonly used filter is composed of nitrocellulose and has a pore size of 0.22μm. The size of the bacteria ranges from 0.3 to 0.5 μm whereas the size of the viruses ranges from 20 nm to 0.36 μm. Thus a filter of 0.22μm retains all bacteria and spores but not all viruses.
Solutions of intravenous use are made pyrogen-free using filtration. Heat sterilization of such solutions may kill the organisms but heat-resistant endotoxins (lipopolysaccharide of the gram-negative bacteria) may still remain and cause fever.
Working Mechanism of Filtration Sterilization
Filters work by physically trapping particles larger than the pore size and by retaining somewhat smaller particles via electrostatic attraction of the particles to the filters. Besides porosity, other factors also influence the efficiency of filtration, they are:
- electric charge of the filter
- electric charge carried by the organisms
- nature of the fluid being filtered
Filtration of liquids is accomplished either by pulling the solution through a cellulose acetate or cellulose nitrate membrane with a vacuum (i.e, by applying negative pressure in the filter paper) or by forcing the solution through filter paper by imposing positive pressure above the fluid.
Filtration of air is accomplished using high-efficiency particulate air (HEPA) filters designed to remove organisms larger than 0.3 μm from isolation rooms, operating rooms, and biological safety cabinets.
A depth filter is a fibrous sheet or mat made from a random array of overlapping paper or borosilicate (glass) fibers. The depth filter traps particles in the network of fibers in the structure.
- Filter sterilization of air in industrial processes
- Forced air heating and cooling systems used in houses contains simple depth filter to trap dust, spores, and allergens
- Use in biosafety applications such as biosafety cabinets. Biological safety cabinets contain a filter known as high-efficiency particulate air (HEPA) filter, which is a type of a depth filter.
A typical HEPA filter is a single sheet of borosilicate glass fiber that has been treated with a water-repellent binder. The filter, pleated to increase the overall surface area, is mounted inside a rigid, supportive frame. HEPA filters come in various shapes and sizes, from several square centimeters for vacuum cleaners to several square meters for biological containment hoods and room air systems.
Control of airborne particulate materials with HEPA filters allows the construction of “clean rooms” and isolation rooms for quarantine, as well as specialized diagnostic/research laboratories. HEPA filters typically remove 0.3 μm test particles with an efficiency of at least 99.97% including most microorganisms, from the airstream.
Membrane filters are the most common type of filters used for liquid sterilization in the microbiology laboratory. Membrane filters are composed of high tensile strength polymers such as cellulose acetate, cellulose nitrate, or polysulfone. Membrane filters are prepared as circular membranes of about 150μm thickness and contain millions of microscopic pores of uniform diameters; the size of which is adjusted based on requirements, during the polymerization process.
Porosities of membrane filters range from 0.1μm to 10μm and the most commonly used membrane filter has the pore size of 0.22μm and 0.45μm.
The membranes are held in special holders and often preceded by depth filters made of glass fibers to remove larger particles that might clog the membrane filter. The solution is pulled or forced through the filter and is collected in previously sterilized containers.
Uses of membrane filter
- Sterilization of fluid materials (pharmaceuticals, ophthalmic solutions, antibiotics, and other heat-sensitive solutions in laboratories and industries
- Identification and enumeration of microorganisms
Find more about membrane filter technique
You may also like: Bacteriological analysis of Water using Membrane Filtration Technique
Advantages of Filtration Sterilization
- Less capital intensive
- Suitable for heat-sensitive liquids (infusions, vaccines, hormones, etc).
- Large volume of liquids can be filtered reasonably fast
Limitations of Filtration Sterilization
- Only liquids and gases can be sterilized by this process
- Filters are expensive to replace, especially nano-filters
- Inherent limitations of materials used in filters affect the efficacy of this process i.e, breakage of glass filters, rupture of the membrane filter and absorption of the filtrate by Sietz filter
- Clogging may occur
References and further readings
- Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
- Willey, Joanne M, Sherwood, Linda M, & Woolverton, Christopher J. (2016). Prescott’s Microbiology (10 edition). McGraw-Hill Education.
- How Does Filtration of Liquids in the Lab Work? | Tuttnauer. Retrieved May 5, 2020.