Enrichment with enumeration. Surveillance enumeration of Listeria monocytogenes levels in contaminated food is now required. Detection may be done first and if contamination is detected, a reserve sample portion can be enumerated. This is probably the preferable method as, generally, only a few percent of samples can be expected to be positive and then most often at a level of only about 1 cfu/25g. However, the option of combining regulatory detection and enumeration is provided in Enumeration.