Back to Sterility Testing

Three principal methods for sterility testing are used:

• Direct inoculation (DI)
• Membrane filtration (MF)
• Direct transfer

Direct Inoculation

In the direct inoculation method, the test articles are inoculated directly into tubes or bottles that contain an appropriate medium and are incubated for a period of days. The advantages of the direct inoculation method are

• it provides a means of sterility testing for materials that cannot be easily filtered
• it consumes less product volume during the conduct of the bacteriostasis and fungistasis (B&F) testing

The disadvantage of this method is volume constraint. Also, inherently bacteriostatic or fungistatic components cannot be removed or adequately neutralized.

Membrane Filtration

In the membrane filtration method, the test article is passed through a membrane filter, which is designed to retain microbial contaminants while permitting the passage of liquid test articles and inhibitors out of the test system. After the test article passes through the filter, the membrane is rinsed with an appropriate sterile rinse fluid. The membrane filters would capture the microorganisms, if present. The filter units are then inoculated with the appropriate sterile and generally are incubated for the same time as in the direct inoculation method (i.e., 14 days). Advantages of the membrane filtration method include:

• Accommodation of large volume samples (up to 500 ml)
• Removal of inhibitory substances that inhibit the growth of microorganisms by rinsing the filter membrane with a suitable agent

The disadvantage of this method is that it can consume a large volume of final vialed product during B&F testing, especially in the case of final container testing.

Direct Transfer

BioReliance offers custom protocols for direct transfer method. Typically, the direct transfer method is used for solid dose forms, medical devices, ointments, and creams. For test articles produced by nonaseptic manufacturing processes, a bioburden (or microbial limits) assay should be performed. Often, it is necessary to evaluate a nonsterile test article for the presence of objectionable organisms, depending on the intended use of the material. A microbial limit approach is recommended for the evaluation for objectionable organisms.