In pharmaceutical microbiology, Soybean Casein Digest Medium (SCDM) is the most commonly used growth medium for media fills and sterility-related tests. It is commercially available under different names, such as Trypticase Soy Broth (TSB), and its official composition can be found in all pharmacopoeias (Example: USP <71>). Although other pharmacopeias may show minor variations in the recipe, SCDM remains the preferred and widely accepted medium for validating aseptic processes.
Why SCDM Is Used in Media Fills
Soybean Casein Digest Medium is chosen because of its strong ability to support the growth of a wide range of microorganisms. This ensures that if contamination occurs during aseptic processing, the medium will promote visible microbial growth, allowing the process to be evaluated accurately.
SCDM can be purchased from qualified commercial vendors. However, proper storage, shipping, and handling procedures must be strictly followed to maintain medium integrity and stability. The medium must always be used within its specified shelf life and stored according to label recommendations.
Media Fills for Aseptic Process Qualification
Media fills are essential to validate the aseptic process in a pharmaceutical facility. To initially qualify an aseptic production process:
- Three media fills must be conducted
- On three separate days
- Using the exact process being validated
Media fills should also be repeated whenever any significant change occurs, such as changes in equipment, personnel, or critical components. If sterility failures are observed, media fills must be performed again to requalify the process.
Operators performing media fills must be trained in aseptic techniques like proper gowning, disinfection, and sterile handling. Additionally, for operator qualification, each operator must complete three successful media fills on three separate days and perform at least one media fill every year to remain qualified.
Quality Control Testing of Growth Media
Before using Soybean Casein Digest Medium for media fills, three commercial batches or lots from the same supplier should be tested. These tests ensure that the medium meets expected standards and matches the Certificate of Analysis (CoA). Tests generally include:
- Visible inspection
- pH testing
- Sterility testing
- Growth promotion testing
If three different batches are not available initially, then the first three batches received must be tested individually.
According to USP <71>, SCDM should meet the following criteria:
- Sterility after 7 days of incubation
- pH 7.3 ± 0.2
- Ability to promote growth of aerobic species such as Staphylococcus aureus, Pseudomonas aeruginosa, Bacillus subtilis, Aspergillus brasiliensis, and Candida albicans.
Once a supplier has been consistently reliable, periodic verification of the CoA and growth promotion results is sufficient.
Why Growth Promotion Test Matters
A growth promotion test ensures that the medium used in media fills can support the growth of microorganisms.
Growth promotion test is a crucial control since the desired outcome of a media fill is “no growth.” Only when the medium can support microbial growth can a “no growth” result be trusted.
Growth promotion testing is performed by inoculating the medium with a small number (≤10²) of microorganisms and confirming visible growth.
- For in-house prepared medium, growth promotion testing must be done for every batch.
- Medium must be sterilized using autoclave processing, not filtration.
- For ready-to-use commercial media, growth promotion must confirm suitability unless a qualified vendor is already established.
Once a supplier is proven consistent, a positive control test during the media fill can replace a full growth promotion test.
Positive Control in a Media Fill
A positive control demonstrates that the drug product container filled with the medium can support microbial growth after the filling process. This ensures that the filling process does not alter or weaken the medium.
A positive control must be included for each media fill:
- It can be inoculated during the media fill
- Or after the incubation period by inoculating a sterile container
- Inoculation must always occur outside the critical manufacturing area
When a qualified medium supplier is used, this positive control also serves as the growth promotion confirmation.
Conclusion
Soybean Casein Digest Medium (SCDM) plays a central role in aseptic process validation through media fills. From medium qualification to operator training and ongoing process verification, SCDM ensures that pharmaceutical products meet the highest sterility standards. By following proper procedures for testing, handling, and process qualification, companies can maintain a reliable and compliant aseptic manufacturing environment.
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