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Listeria monocytogenes in Food – Challenge Testing

Repeatedly, public recalls of food occur by the qualitative detection of Listeria monocytogenes. The legal basis for these recalls is Regulation (EC) No. 2073/2005 on microbiological criteria for foodstuffs.

Chapter 1 lays down food safety criteria for ready-to-eat foods which may favour the reproduction of Listeria monocytogenes and which are not intended for infants or special medical purposes.


A distinction is made between two cases:

1. the food business operator may demonstrate to the satisfaction of the competent authority that the content of Listeria monocytogenes in that product does not exceed 100 CFU/g throughout the storage life of the product. Listeria monocytogenes, for example, is subject to a limit of 100 CFU/g throughout its shelf-life.

2. the food business operator cannot demonstrate that the content does not exceed 100 cfu/g throughout the storage life. Then Listeria monocytogenes must not be detected in 5 subsamples of a batch tested, each containing 25 g. This limit value applies before the food has left the direct control of the producing food business operator.


In practice, however, products which have already been on the market and for which Listeria monocytogenes has been qualitatively detected in 25 g are often recalled if the food business operator cannot ensure that the content of Listeria monocytogenes remains below 100 KbE/g until the date of minimum durability is reached.


Challenge tests for the detection of Listeria monocytogenes


In order to provide this proof to the satisfaction of the authority, challenge tests are usually necessary to document the growth behaviour of Listeria monocytogenes in a product.

This is done by inoculating the product with Listeria monocytogenes and storing it until the best-before date. The Listeria monocytogenes content is determined immediately after inoculation and at the MHD. This allows the growth potential of Listeria monocytogenes to be determined in the food tested.

In challenge experiments, all factors that influence the growth behaviour of Listeria monocytogenes in food must be taken into account. These include in particular the storage temperature (considering temperature fluctuations during transport and the usual storage temperatures in the consumer’s refrigerator) and the packaging of the product. In order to meet the different growth characteristics of the different Listeria strains, a mixture of several strains must always be used for the inoculation.

The European Reference Laboratory for Listeria monocytogenes has issued a technical guideline for the implementation of these complex tests. We offer you the performance of challenge testing according to this guideline – also in prior consultation with your competent authority.


Please do not hesitate to contact us.


Tobias Stapper
bilacon GmbH, Head of Microbiology
P +49 30 206 038 370

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