Division 2.2 Enumeration of Listeria monocytogenes

The enumeration of Listeria monocytogenes shall be performed according to EN ISO 11290-2:1998 ‘Microbiology of food and animal feeding stuffs — Horizontal method for the detection and enumeration of Listeria monocytogenes — Part 2: Enumeration method’ and its modification EN ISO 11290-2:1998/Amd 1:2004 ‘Modification of the enumeration medium’.

If the sample is found to be contaminated, it is assumed that the majority of products would contain low contamination levels of Listeria monocytogenes. To enable the estimation of low numbers in samples (between 10 and 100 cfu/g), 1 ml of the primary dilution shall be tested in duplicate as indicated in EN ISO 11290-2:1998/Amd 1:2004:

1. (a)

   spread onto the surface of three 90-mm diameter plates, or

2. (b)

   spread onto the surface of one 140-mm diameter plate.

Because of the possibility of higher contamination levels of Listeria monocytogenes, 0,1 ml of the primary dilution must be spread onto the surface of one plate to allow the enumeration of up to 1,5 × 10⁴ cfu/g. This plating must be performed in single as provided in ISO 7218:2007 ‘Microbiology of food and animal feeding stuffs — General requirements and guidance for microbiological examinations’.