Choosing Media

USP <797> requires the use of microbiological media for gloved fingertip sampling, viable air sampling, and surface sampling. While the chapter includes certain requirements, it falls short in providing details on media. Unless you have a microbiology background or have worked in aseptic manufacturing, finding the right media can feel overwhelming! In this blog we’ll fill in the knowledge gaps, reviewing everything you need to know about choosing microbiological growth media.

Here are the chapter requirements…

A general microbiological growth media that supports the growth of bacteria and fungi must be used.

• This would be trypticase soy agar (TSA).
• The use of a fungal media is not required but can be used if the organization chooses to do so.

Certificates of analysis (COAs) from the manufacturer must verify that the media meets the expected growth promotion and sterilization requirements.

• Review the COA for growth promotion and confirmation of sterility.
• Keep the COA on file for future reference.

Surface sampling devices must contain general microbial growth media supplemented with neutralizing additives to neutralize the effects of any residual disinfecting agents.

• This would be TSA containing lecithin and polysorbate 80.
• If a fungal media is used for surface sampling, it would also need to include neutralizers.

Surface sampling devices must have a raised convex surface.

• Plates, slides, or paddles can be used.

That’s all the chapter gives us, but we can’t really blame USP. It can’t give us ALL the details. This is where industry guidance and accepted best practices come into play. Here are four things to consider when choosing media that you won’t find in the chapter.

Irradiated

CETA Application Guide (CAG)-009 recommends using terminally sterilized media, because it is designed to be used in controlled, sterile environments. Media comes two ways: aseptically filled and terminally sterilized, through irradiation. Think of it in the same way as the CSPs you prepare. One is definitely sterile, while one may or may not be sterile. When it comes to your viable sampling, why would you want to start with something that may not be sterile and risk bringing contaminated media into your controlled environment?

Now, you’re probably saying to yourself, “I check the plates before I use them, they look fine.” This could be okay if the media is required to be stored at room temperature. In this case, if there was contamination, you would see it on the plates before use. However, much of the aseptically filled media requires refrigeration, slowing microbial growth. Unless you are preincubating the media at 20 to 25°C before use, contamination won’t be evident until the test samples are incubated, making it difficult to know if the growth was a true result or contamination.

There is a definite price difference, with the aseptically filled plates being significantly cheaper. It is up to you to decide if you’d rather save money up front on the cost of plates or save time and money by not having to investigate false positive results.

Double or Triple Bagged for Cleanroom Use

CAG-009 also recommends using media that is double or triple bagged. This makes transferring media into the cleanroom suite and primary engineering control (PEC) super simple.

For double bagged media, wipe the exterior package with the facility-designated agent. If using a pass-through system, place the media into the pass-through. If using a dirty to clean cart system place the media on the clean cart. Once in the buffer room or perimeter of the segregated compounding area (SCA), remove the outer bag and place the media in the PEC. There is no need to wipe with sterile 70% IPA because the inner packaging is sterile. For triple bagged media, simply shed a layer at each transfer.

Storage Conditions

Your sampling frequency and the number of personnel will determine how much media you will need at any given time. You need somewhere to store all the media. Some media requires refrigeration, while other media can be stored between 2 and 25 °C. Be sure that you have sufficient space to store the media at the proper temperature and that the storage temperature is monitored.

Multiple Uses

You are definitely going to need media for gloved fingertip and surface sampling, which would be TSA with lecithin and polysorbate 80. However, if you choose to do your own viable air sampling, you’ll need media for this sample collection as well. Most viable air samplers take one of two types of plates: settle plates or contact plates. If you choose an air sampler that takes contact plates, you only need to stock one type of plate for all three types of sampling. And yes, it’s okay for viable air sampling media to contain neutralizers. If you choose an air sampler that takes settle plates, you have two media options. You can use TSA settle plates with no neutralizers for the air and contact plates for the surface and gloved fingertip sampling or you can use TSA with lecithin and polysorbate 80 settle plates for the air and gloved fingertip sampling and contact plates for surface sampling.

Summary

When it comes to choosing media for gloved fingertip, viable air, and surface sampling, USP <797> does not provide enough information to ensure success. This is a situation where you need to go beyond the chapter and look to other industry guidance.

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