Introduction:
Definition: Sterilization is defined as complete removal of microorganisms from an object, surface or a product.
Sterilization by filtration is the preferred method of sterilizing thermolabile liquid and gases.
It simply removes microbes instead of killing them.
It is the method of choice for sterilizing antibiotic solutions, toxic chemicals, radioisotopes, vaccines, and carbohydrates, which are all heat-sensitive.
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
Biological indicator:
Micromonospora diminuta.
Procedure:
The solutions to be sterilized is passed through the filter and collected in the sterile receiver by the application of positive pressure to the nonsterile compartment or negative pressure to the sterile side.
Precautions to be taken:
Avoid excessive positive or negative pressure.
Prolonged filtration must be avoided to prevent the growth of a contaminant through the filter media and entry into the sterile solution.
Filter elements of porcelain or of sintered materials those are used repeatedly should be tested for cracks or leaks prior to each use.
Fiber shedding filters such as asbestos filters should be avoided. If, however, the use of this type of filters is unavoidable, a fiber-retaining filter should be used downstream of the asbestos filter.
The effective pore size of the sterilizing filters should be confirmed before use and the continued integrity of the filters should be confirmed after use.
The process of sterilization must be validated by monitoring the microbial load in the solution to be filtered.
Advantages of sterilization by filtration:
Thermolabile solutions can be sterilized.
It removes all the living microorganisms.
Disadvantages of sterilization by filtration:
Filters may break down suddenly or gradually on use.
Sterility testing is compulsory on the filtered solution.
Filter media may be absorbed on the filter surface.
Viruses are not removed by filtration.
MEMBRANE FILTERS
They are made of cellulose-derivative (acetate or nitrate).
They are 150μm thick.
They are very fine and are fixed in some suitable holders.
Nominal pore size is 0. 22 plus, minus 0. 02μm or less is required.
The membranes are brittle when dry, in this condition they can be stored for years together.
They become very tough when dipped in water.
They are sterilized by autoclaving or by ethylene oxide gas.
They cannot be sterilized by dry heat as they decompose above 120°C.
They are suitable for sterilizing aqueous and oily solutions.
Not suitable for organic solvents such as alcohol, chloroform etc.
Membrane filters are generally blocked by dirt particles and organisms.
Pre-filtration (through glass-fibre paper prefilter) reduces the risks of blockage of the final filter.
Examples of membrane filters:
MF-Millipore – it is a mixture of cellulose esters
Sartorius Regular ¬¬– it is made of cellulose nitrate
Gelmen Triacetate Metricel – cellulose triacetate
5) Suspensions and oils cannot be sterilized by this method due to their heavy load of particulate matters and viscosity.
SINTERED (or FRITTED) GLASS FILTERS:
Also called “Fritted Glass Filter” or “Morton filters”.
Borosilicate glass is finely powdered in a ball-mill and the particles of required size are separated.
This is packed into disc mounted and heated till the particles get fused.
The discs thus made have a pore size of 2 micrometer and are used for filtration.
They are cleaned with the help of sulfuric acid.
Low absorptive properties hence good.
Brittle and very expensive.
Seitz Filter (Asbestos Filter):
Disposable, single use discs made up of Asbestos (Magnesium trisilicate).
Pore size ranges from 0.01 to 5 microns.
It is supported on a perforated metal disc inside a metal funnel.
After single use the filter is discarded and then the entire assembly is sterilized.
Filter Candles:
Made up of porous porcelain or kieselguhr.
Usually available as cylindrical candles with thick walls.
They were fitted to the filter assembly and placed in a mantle.
The liquid to be filtered is then poured into the mantle.
The vacuum pushes the liquid through the filter.
Filtrate is collected after removal of candles.
Gets easily clogged, requires pressure for filtration.
HEPA filter:
High Efficiency Particulate Air Filter.
A typical HEPA filter is a single sheet of borosilicate glass fiber that has been treated with a water-repellent binder.
The filter, folded 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.
HEPA filters typically remove 0.3 μm test particles with an efficiency of at least 99.97% including most microorganisms, from the airstream.
Used to maintain aseptic areas and clean rooms.
