Sterility Testing

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 as follows

• provides a means of sterility testing for materials that cannot be easily filtered
• 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 and Direct Transfer

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). An advantage of the membrane filtration method is that

• can accommodate large volume samples (up to 500 ml)
• if the test article contains substances that inhibit the growth of microorganisms, rinsing the filter membrane with a suitable rinse agent can remove all or most of these inhibitory substances

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

MDS 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.

Bacteriostasis/Fungistasis (B&F) contamination

In this test, low numbers of microorganisms are added to a media volume containing the test material to confirm that the test material does not interfere with the organismsEgrowth.