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The 3M Petrifilm Environmental Listeria plates sounds like a great product. It is a sample-ready culture medium containing selective agents, nutrients, a cold-water-soluble gelling agent, and a chromogenic indicator that facilitates Listeria colony detection. 3M claims the plates were designed to analyze environmental samples and to help increase the efficiency of monitoring plant sanitation. The presence of indicator Listeria such as Listeria innocua provides evidence that environmental conditions are suitable for the occurrence of Listeria monocytogenes. As the two usually co-exist together.

Here is the interesting remark, according to 3M the Petrifilm Environmental Listeria plate detects the majority of environmental Listeria, consisting of Listeria monocytogenes, Listeria innocua, and Listeria welshimeri.*. Here is the sub-clause about the other strains of listeria. “* For further information on the prevalence of Listeria species, please contact the official 3M Microbiology representative nearest you. L. ivanovii, L. grayi/murrayiand L. seeligeri grow but do not form typical colonies.”

That’s right, although all listeria colonies will grow, not all strains will form typical colonies and therefore as a selective medium this is deceptive marketing. Interestingly, the first interpretation guides did not mention the strains in question. That is Listeria ivanovii, Listeria grayi/murrayi and Listeria seeligeri.

Normally a good microbiologist will pick this up, however there are some managers or technical personnel who do not have a clue about this product and will be persuaded by one of the 3M sales representative.

Although Listeria monocytogenes is the main organisms of interest, some regulatory bodies will want all listeria species to be detected in their product. Therefore if this is the case then the 3M Petrfilm Environmental Listeria Plates is not a good option for environmental monitoring and I would stick to the normal ELISA based methods. If on the otherhand, you are not concern with other species of listeria, then this product is a great product.

My point is 3M would be better to develop two different plates:
1. a plate that detects Listeria monocytogenes / innocua only
2. a plate that detects all listeria strains (including Listeria ivanovii, Listeria grayi/murrayi and Listeria seeligeri.)

Finally any pathogen testing must be conducted by qualified personnel’s in an accredited laboratory otherwise the risk of further contamination is raised.

Would I buy this product?
No as some listeria strains do not form typical colonies.

Listeria: A Practical Approach to the Organism and Its Control in Foods No Synopsis Available