What is sterility test in Microbiology?

Sterility testing is a microbiology laboratory test that assesses if a product is free of viable microorganisms, or sterile. It’s performed on manufactured biological materials, such as blood products, pharmaceutical products, and sterile medical devices.

The industry standard for sterility testing is USP <71>, which involves two procedures: direct inoculation of the culture medium and membrane filtration. The type of procedure used depends on the product being examined.

Direct Inoculation
The product or medical device comes into direct contact with the test media during the incubation period. This method requires less sample volume and is useful for samples that cannot be filtered.

Direct inoculation is a sterility testing method that involves adding a test sample directly to media, typically in tubes or bottles, and then incubating it for 14 days.

The sample should be less than 10% of the media’s total volume. The direct inoculation method is used for products that are not filterable or deformable, such as surgical sutures, gauze, and dressings. It also consumes less product volume than other methods.

Membrane Filtration
Samples are cultured in two separate media to promote the growth of residual anaerobes, as well as aerobes and fungi.

Membrane filtration sterility testing is a regulatory method for pharmaceutical products that use a 0.45 μm pore size filter to retain microorganisms. It’s the most popular method for sterility testing and is suitable for samples containing compounds that inhibit microbial growth, such as preservatives, bacteriostatic, or fungistatic compounds.

The Process Involves
▪️Sterilizing and assembling the filter unit

▪️Transferring the sample contents to the filter

▪️Passing the sample through the filter in a filtration canister
Adding culture medium for incubation

▪️Rinsing out inhibiting compounds with a rinse solution

Sterility tests are performed immediately after manufacture, and again after stability testing for a period of time, such as six months or a year. Sterilization validations are performed to confirm that sterilization processes are working, and that a certain threshold of microbial death occurs consistently.

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