Sterility testing is a GMP microbiology testing requirement used to confirm sterile products do not contain viable microorganisms before release and patient administration. Sterility testing methods must be as accurate as possible, due to their importance for medical devices, pharmaceutical products, and formulations, tissue materials, and other products that claim to be sterile or free from viable microorganisms.
Sterility testing procedures are applied to products in many industries, including food and beverage manufacturers, but the main industries are the pharmaceutical and medical sectors where the sterility testing of the products remains a vital and routine task for microbiologists.
Compendial methods for the sterility testing of pharmaceutical products requires samples to be cultured in two separate media. Two different types of culture media are used in sterility testing to promote the growth of residual anaerobes, as well as aerobes and fungi. Fluid thioglycolate medium (FTM) is typically used to culture anaerobic and some aerobic bacteria, while soybean casein digest medium (SCDM) is typically used to culture fungi and aerobic bacteria. Samples are incubated for 14 days at 32.5°C and 22.5°C respectively, prior to examination. Any turbidity in the culture media may indicate growth and must be investigated. There are two recommended methods of sterility testing for pharmaceuticals: membrane filtration and direct inoculation.
Membrane filtration sterility testing is the regulatory method of choice for filterable pharmaceutical products, as cited in the USP <71>, EU Pharmacopoeia < 2.6.1>, and JP Pharmacopoeia <4.06>. Samples are passed through a 0.45 µm membrane filter in a filtration canister and culture medium is added for incubation. This method of sterility testing can provide increased sensitivity in comparison to other methods as the whole sample, or a composite sample is passed through a single filter. Filtration also provides an opportunity to rinse away components in the sample that may cause turbidity or inhibit growth, such as antibiotics or preservatives.
In direct inoculation, a small volume of sample is removed aseptically from the sample unit and inoculated directly into a suitable volume of growth medium before incubation. While simple, this method of testing can have some significant limitations. Only small volumes of product can be inoculated into the culture medium, limiting the test sensitivity. If the sample appears cloudy or turbid after inoculation, it can be challenging to detect turbidity from microbial growth at the end of the incubation period. Additionally, if the product has antimicrobial properties, the sample must be neutralized so that microbial growth is not inhibited.
Direct transfer sterility testing is recommended for the sterility testing of medical devices. The device to be tested is in direct contact with the test media throughout the incubation period, during which any microorganism in or on the device will grow and proliferate. Furthermore, product flush sterility testing is preferred for products with hollow tubes, such as transfusion and infusion assemblies, where the fluid pathway is labeled as sterile. The product lumen is flushed with a rinsing fluid, the elute is membrane filtered and is placed in a suitable media for incubation.