Many factors determine the productivity of a medium. The composition must be correct, the ingredients must be good quality, the correct sterilization temperature must have been used, and the pH must be correct. If one of these factors is not in line with requirements, the enumeration may be much lower than expected. Whether or not a supplier is accredited can also make a difference.
When testing for Legionella, a water company laboratory recorded 1,300 colony-forming units on the medium from Supplier A and 7,600 on a comparable medium from Supplier B. This demonstrates the scope of the potential discrepancies and, therefore, the importance of knowing for certain that the medium is performing as expected. A medium comprises chemical components and extracts from natural materials, such as meat, with the greatest variation often resulting from these natural components.
Testing for greater reliability
Microbiological reference material can be used to determine whether the right medium is being used, whether the intended microorganisms are growing well and whether the laboratory technicians are preparing the media according to the correct procedures (if applicable). A medium must often meet a requirement that at least 50% of the microorganisms being investigated must be found on the medium used, compared to growth on a non-selective reference medium. This means, for example, that if 100 microorganisms are expressed on the reference medium, at least 50 colonies should be found on the selective medium.
The ISO 11133:2014 standard describes the procedures for testing different media. These procedures are comprehensive and must be followed meticulously. Performing these tests offers extra assurance about the quality of the media used and increases the reliability of the results of microbiological testing carried out in laboratories.
Consequences for production
Laboratories that do not test their media cannot be certain they are of sufficient quality. This can result in inaccurate microbiological results. For example, a poorly functioning medium can mean pathogens are missed, resulting in a false negative report. It is also possible to use a medium for years that is not productive enough concerning a particular microorganism. In production, this leads to the incorrect assumption that products are of good quality, while the analysis method has not shown sufficient microorganisms to be certain that this is the case. If high-quality media is then used and limits are shown to have been exceeded, production methods may need to be adjusted, or products may need to be downgraded. This is all the more reason to be certain that the media used in the laboratory meets the requirements.