Growth promotion testing is a requirement for all media used in a testing laboratory to prove the media supplied is fit for purpose prior to use in the testing facility.
Microbiological reference materials are now readily available from multiple suppliers in all major locations. They are available in many different forms, including qualitative and quantitative formats. Quantitative reference materials contain a defined number of viable microorganisms and are normally a freeze-dried or gel suspension supplied with a Certificate of Analysis (COA) specifying the number of viable microorganisms that should be recoverable.
Prior to the availability of high-quality reference materials, growth promotion testing was usually performed by plating a serial diluted microorganism suspension on both a new and a previously released media batch to compare recoveries. This method proved difficult in obtaining accurate results (5). In addition, this approach is potentially flawed in that the inoculum does not come with a COA and a gradual decline in viability might not be readily detected.
Testing with a reference material provides an independent and precise external calibration point. Every batch of ready-to-use reference material should come from an ISO 17034:2016 accredited manufacturer and offer quantitative data specific to the batch on the COA (6). The COA should report a mean colony forming unit (cfu) count and the standard deviation for each batch.
Used for media that is used for enumeration testing i.e. Tryptone soya agar, Place Count agar etc. Recovery specification applied is 50-200% recovery when the new batch (Test) is compared to a previously certified batch (Control).
Used for selective media plates such as MacConkey agar, Mannitol salt agar etc. Recovery specification applied is the growth observed on the new batch (Test) must be characteristic of the growth observed on a previously certified batch (Control).
Used for selective media plates such as MacConkey broth, Tryptone soya broth etc. Recovery specification applied is the Turbidity or Colour Change observed on the new batch (Test) must be characteristic of the turbidity or colour change observed on a previously certified batch (Control).