Persistence of Listeria monocytogenes in three sorts of soil

Life time of Listeria monocytogenes (strain PS 10401, serovar 4b) was studied in three sorts of soil: a chalky soil, poor in organic matters (pH 8.3) a peaty soil rich in organic matters (pH 5.5) a mixture of a chalky and peaty soil (pH 7.9). About 10(5) colony forming units (c.f.u.) per g of dry material were inoculated to each sort of soil which was incubated at 4 degrees C or at 20 degrees C. Periodically samples were taken and a numeration of germs was made on Tryptose Soy Agar. When direct numeration was negative, an enrichment was made at low temperature. In peaty soil, L. monocytogenes disappeared after incubation of 162 days at 4 degrees C and 156 days at 20 degrees C. In chalky soil, L. monocytogenes disappeared after 394 days of incubation at 20 degrees C, whereas at 4 degrees C, 10(4) c.f.u./g of dry matter were found 1500 days later. In mixture of chalky and peaty soil, 10(4) c.f.u. were found 1500 days after inoculation at 4 degrees C whereas 10(3) c.f.u. were found at 20 degrees C. Acid soils do not allow the persistence of L. monocytogenes for more than 160 days. Low temperature are significantly favourable to the persistence of L. monocytogenes.