Hypovirulent Listeria monocytogenes strains are less frequently recovered than virulent strains on PALCAM and Rapid' L. mono media.

Several selective media have been developed to detect Listeria monocytogenes contaminated foodstuffs. Polymyxin-acriflavine-LiCl-ceftazidime-aesculin-mannitol (PALCAM) and Oxford media, required for the EN ISO method 11 290-1, are used for the detection of Listeria spp. in 2 days based on the expression of esculinase activity. Selective agar media such as Rapid' L. mono and Agar Listeria according to Ottaviani and Agosti (ALOA), based on the activity of phosphatidylinositol phospholipase C (PI-PLC) that allows the specific detection of L. monocytogenes in 2 days, are also used. However, no medium can assess the level of virulence of L. monocytogenes strains. Using a plaque-forming assay followed by subcutaneous footpad inoculation in mice, 15 virulent, 8 hypovirulent and 17 avirulent strains were discriminated among L. monocytogenes strains mainly originating from food (36/40). Their growth was tested on the four selective media. After 2 days, the number of colony forming units (cfu) of all the virulent strains was significantly superior to the number obtained with avirulent strains on all the four media tested, and superior to the number obtained with hypovirulent strains on PALCAM and Oxford media. These results showed a relationship between the level of virulence of L. monocytogenes strains and their growth on the selective agar media tested. Moreover, 1 out of 8 hypovirulent and 5 out of 17 avirulent strains did not grow on Rapid' L. mono medium, and 1 hypovirulent and 8 avirulent strains grew but did not express PI-PLC activity during the 7 days of incubation. The lack of detection of PI-PLC activity on Rapid' L. mono was not related to a gene mutation since these strains expressed enzymatic activity on ALOA medium, which detected up to 92% of the hypo- and avirulent strains. In contrast, some of these strains without growth or enzymatic activity expression would not be detected with PALCAM and Rapid' L. mono in foodstuffs on the second day.