Preservation of the virulence of Listeria monocytogenes in different sorts of soil.

Listeria monocytogenes (strain PS 10401, serovar 4b) whose virulence was well known, has been placed for 600 days in a sterile chalky soil (pH 8.3) and in a mixture of chalky and peaty soil (pH 7.9) and incubated at 4 degrees C and 20 degrees C. Groups of six OF1 mice were challenged by subcutaneous injection in the rear left hind foodpad with each sort of germs. Three days after inoculation, mice were killed by cervical disruption and numeration of L. monocytogenes was made in the spleen. Chemical composition of soil does not change virulence of germs. But low temperatures significantly increase virulence.