Influence of abiotic and biotic soil characteristics on Listeria monocytogenes survival in the environment

Listeria monocytogenes is a food born pathogen responsible for the potentially fatal disease listeriosis. Initial contamination of plants vegetables and fruits may occur in cropping systems and originate from soil (that may act as a reservoir), water or organic amendments. The aim of this study was to identify major biotic and abiotic soils characteristics which determine L. monocytogenes survival in soil. Survival of L. monocytogenes was evaluated in a set of 100 soil microcosms representative of land use and pedology of French soils. These soils originated from the RMQS network were thoroughly characterized including chemical properties (pH, ionic content), texture (clay, sand and silt content), land use, and spatial localization. A sub-set of 9 Gamma-radiation sterilized soils was used to investigate whether soil microflora may affect L. monocytogenes survival. Microcosms were inoculated with 106 L. monocytogenes per gram of soil. L. monocytogenes populations were monitored after 7, 14 and 84 days of incubation at 20°C by plate-counts on selective PALCAM media. L. monocytogenes was able to persist in most of the tested soils but survival rates were strongly variable. A short term and a long term survival of L. monocytogenes were reported in 71% and 21% of tested soils, respectively. In 8% of soils, L. monocytogenes was either absent or below detection limits as soon as 7 days after inoculation. This suggests that L. monocytogenes survival depends on abiotic and/or biotic factors. Statistical analysis evidenced that land use and spatial localization of soils did not significantly affect L. monocytogenes survival. Variance partitioning demonstrated that 60% of the differences between survival rates at 7 and 14 days were explained by soil chemical properties. Basic cation saturation ratio was the major soil chemical characteristic explaining short-term survival. Cationic exchange capacity and exchangeable calcium further explained survival of L. monocytogenes at 7 days and 14 days respectively. Long-term survival was driven mainly by soil texture (11% of the variance), especially clay content. Survival of L. monocytogenes in sterile soils differed markedly. Indeed, 4 sterilised soils supported growth of L. monocytogenes while the rate of decrease was lower in the other sterilised soils than when microflora was active. This result points out the critical role played by both the soil microflora and the soil physico-chemical properties in determining the survival rate of L. monocytogenes.