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RESEARCH PRODUCT

Incidence and survival of human pathogens in French soils, impact of land use, pedoclimatic and biologic soil factors

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Soil contamination by bacterial pathogens can occur through manure, sewage sludge spreading or irrigation using waste water treatment plants effluents. Agricultural soils may act as reservoirs for these pathogens, play a significant role in their dissemination, leading to the potential contamination of food and water resources. Health risk associated with the occurrence of pathogens in environmental matrices has to be thoroughly evaluated. In this context, the objectives of this work were: i) to determine the prevalence of two pathogenic bacterial species (Listeria monocytogenes and Enterococcus faecalis) in a large collection of French soils originated from a systematic soil survey of the territory, called RMQS (16x16 km grid), ii) to determine major biotic and abiotic parameters driving the survival of bacterial pathogens in soils. Two approaches were used to reach these objectives: i) the prevalence of the two pathogens was monitored in 1200 soils using specific molecular detection tools (real time PCR using TaqMan probe detection system) and ii) the survival of the 2 pathogens, inoculated in soil microcosms, was determined over a 84 days incubation period under laboratory conditions, on a subset of 100 soils from the RMQS survey, using classical microbiological methods. L. monocytogenes was not detected in the set of 1200 soils (with a detection limit estimated to be 104 bacteria per gram of soil) using the molecular detection method, while E. faecalis was detected in approximately 4% of the soils tested (on a smaller set of 150 RMQS soils). The two bacterial pathogens were able to survive in the majority of soil, although L. monocytogenes survived in a greater number of soils. Soils where both pathogens survived represented 44% of soils. The survival of both pathogens is strongly impaired in sandy acidic soils. However, textural and chemical parameters driving survival of the two pathogens differed: L. monocytogenes survival is increased in soils with higher BCSR (basic cation saturation ration) and CEC (cation exchange capacity). L. monocytogenes long-term survival is favored by higher soil clay content. E. faecalis survival is impaired in soils with high total Ca content (calcareous soils). Soil microflora inhibits survival of both pathogens especially in soil with alcaline pH’s. The results of our study will allow implementing survival models for these two pathogens. Such data is invaluable for a better and safer managment of soil manuring using various organic residues.