Microbiological Growth Media Demystified

October 2020 - Vol.17 No. 10 - Page #14
Category: Environmental Monitoring Service Providers

Choosing the right microbiological growth medium for the viable air and surface sampling required in USP General Chapter <797> Pharmaceutical Compounding – Sterile Preparations can be confusing, as there are a variety of options depending on the types of testing that will be performed. Moreover, multiple manufacturers and suppliers claim to have compliant products. Unfortunately, some of these claims are inaccurate.

Many variables can influence the outcomes of environmental monitoring, including the quality of the microbiological growth media (referred to as media throughout the remainder of this article). Although a variety of media are available, not all will meet the needs of sterile compounding operations. When choosing media, the sterile compounding pharmacy must:

  1. Understand the types of media available
  2. Determine the type of testing that will be performed
  3. Identify the media that meet USP requirements
  4. Choose a reputable manufacturer

Media Options

Each type of media suits a specific purpose. Broths are made from water and nutrients, and are commonly used for growing dense populations and screening for low numbers of microorganisms, such as in sterility testing. Agars are a mixture of nutrients, water, and a solidifying agent. Agar provides a solid surface on which microorganisms can grow to permit the quantitation of microorganisms.

Broths and agars are available in different classifications according to the microorganisms they will support. Four common classifications are Nutritive (general), Enrichment, Selective, and Differential. See FIGURE 1 for descriptions of the different classifications.

Determining the Type of Testing to Perform

Before choosing the type of media to use, determine what testing will be performed. For pharmacies conducting low- and medium-risk compounding, the current version of USP Chapter <797> (2008) indicates that sampling only needs to be done with a single general growth medium and incubated at a single temperature (30 to 35°C). Those performing nonsterile to sterile compounding (ie, high-risk compounding) are required to use two media: A general growth medium and a specialty medium that can support fungal growth. The general growth medium is incubated at 30 to 35°C, while the medium that supports fungal growth is incubated at 26 to 30°C. Note that the incubation parameters in the 2008 version of USP <797> are recommended, not required. The chapter also states that incubation is to be done “At a temperature and period conducive to the multiplication of microorganisms.” This statement allows for other appropriate incubation parameters.

The incubation parameters in the current version of the chapter for low- and medium-risk compounding are not suitable for the recovery of the variety of microorganisms found in the compounding environment. To address this concern, the June 2019 version of the chapter provides more robust incubation parameters. This new guidance does not differentiate among compounding risk levels, resulting in procedures that are consistent across organizations while improving the likelihood that microorganisms will grow by using the two-stage incubation. See Table 1 for the incubation parameters for USP <797>.

Each pharmacy must decide which testing will be performed, whether one or two types of media will be used, and how the media will be incubated.


Identifying <797> Compliant Media

Tryptic soy agar (TSA) is the general growth medium used for viable air and surface sampling. TSA can be used for all compounding risk levels, as it does support the growth of yeast and mold (the fungal organisms we are concerned about in sterile compounding environments). If an organization chooses to use TSA or another general growth medium, it must be incubated according to the two-stage incubation (30 to 35°C for at least 48 hours followed by incubation at 20 to 25°C for at least 5 additional days) according to the June 2019 version of USP Chapter <797>.

If the organization chooses to use fungal agar, such as malt extract agar or Sabouraud dextrose agar, it may be used for viable air sampling in addition to the TSA for facilities compounding high-risk compounded sterile preparations (CSPs). There is no current chapter requirement to use a fungal medium for those compounding low- and medium-risk CSPs.

The TSA used for surface sampling must contain lecithin and polysorbate 80, which neutralize cleaning agent residues remaining on surfaces, eliminating the chance of a false-negative finding. In the presence of cleaning agent residues, microorganisms may be alive but their growth is inhibited. By neutralizing these residues, any microorganisms recovered on the agar surfaces will be afforded the opportunity to grow.

Typically, the media used is in the format of plates for both viable air and surface sampling. Surface sampling plates, also known as contact plates, are approximately 55 mm in diameter. The size of the plate used for viable air sampling varies depending on the volumetric air sampler used; some air samplers accommodate 100 mm diameter petri dishes, while others use a contact plate. If your air sampler uses a 55 mm plate, then you can purchase one SKU to simplify purchasing and inventory management. The presence of lecithin and polysorbate 80 required for surface sampling will not interfere with air sampling, so pharmacies may be able to purchase the same plates for both viable air and surface sampling.

Choosing a Reputable Manufacturer

The sterile compounding organization is ultimately responsible for the media used in testing, and it is the pharmacy’s responsibility to ensure the media meets its needs and is sourced from a reputable manufacturer. Even if the organization subcontracts the work to an outside vendor, the pharmacy is still responsible for verifying the media is appropriate for use. This verification must occur before sampling is performed.

Media used for viable air and surface testing should be prepared by a known manufacturer with effective quality systems in place to ensure the media is appropriate for use. Purchase media from reputable companies that have an ISO Certificate of Registration (such as an ISO 9001 or ISO 13485 facility) and are registered with the United States Food and Drug Administration, which provides confidence in their quality. Many manufacturers have a Quality Self-Assessment document that describes their company, the accreditations they hold, and their quality systems. Before choosing a media manufacturer, request a copy of their self-assessment and review it. Contact them about any concerns regarding production or quality assurance.

Although not required, the sterile compounding organization can vet the manufacturer further; most will accommodate an onsite audit. This activity is well beyond the needs of a 503A compounding facility, but would be within the scope of a 503B outsourcing facility’s quality assurance needs.

The formulation of the media, which includes the ingredients and how the media is made, cannot be overlooked. USP Chapter <797> references additional USP chapters for specific requirements for media formulations. When choosing media from a manufacturer, review the ingredient formulation, which can sometimes be found on the Certification of Analysis (CoA) or the Certificate of Quality (CoQ). Confirm that the ingredients closely match the appropriate USP formulation. For example, the formulation for TSA, also known as soybean casein digest medium, is found in USP <61> Microbiological Examination of Nonsterile Products: Microbial Enumeration Tests.1 Performing this comparison ensures that the media is appropriate for the type of testing for which it will be used.

Each lot of media received must have a CoA or CoQ that documents the results of the quality control testing performed by the manufacturer to ensure the lot is suitable for release. Examine the CoA or CoQ for each lot upon receipt for accuracy and completeness and retain it on paper or electronically. Quality control testing data to be reviewed includes:

  • A verification of sterility
  • The ability of the medium to promote growth
  • pH
  • Visual characteristics, such as color and clarity of the agar

Also documented on the CoA or CoQ will be the procedure by which the media was produced (either aseptic filling or terminal sterilization). Although media produced by aseptic filling is generally less expensive, we strongly recommend the use of terminally sterilized media. Terminally sterilized media guarantees the media will be sterile when used for sampling. There is a risk that aseptically filled media could be contaminated and if you choose to use aseptically filled media, it should be preincubated at 20 to 25°C for at least 48 hours to allow for any possible contamination to grow out.

Some manufacturers indicate on their CoA or CoQ that media was manufactured according to USP Chapter <797> requirements. However, note that the chapter does not specify any media manufacturing requirements. Thus, such a claim should be questioned.


Performing viable air and surface sampling according to USP Chapter <797> requirements is essential to collecting the data necessary to evaluate the achievement and ongoing maintenance of a state of control. Each sterile compounding organization is responsible for ensuring that the media used for testing meets USP requirements and will perform as expected. Thus, be sure to take the time to research the media manufacturers and suppliers to ensure that your organization selects and uses correct, high-quality products in its microbiological testing.


  1. USP <61> Microbiological Examination of Nonsterile Products: Microbial Enumeration Tests. United States Pharmacopeial Convention, Inc. USP43-NF-38.

Abby Roth, CMQ/QE, QP503A qualified, is the senior director of business operations at CriticalPoint, LLC. In her current role she develops curriculum for CriticalPoint’s e-learning modules and live training classes. She is a faculty member for the Sterile Compounding Boot Camp Live Training Series.


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