Résumé
The reuse of treated wastewater (TWW) for crop irrigation is a solution to reduce the pressure on freshwater resources. Even if this practice is subject to regulations, it still poses a health threat as it may carry antibiotics, antibiotic resistant genes, antibiotic resistant bacteria and pathogens, not considered in regulations. This practice could thus lead to antibiotic resistance (ATBR) dissemination in the environment. Numerous studies have investigated the effect of TWW quality on ATBR dissemination in soils. However, relatively few studies have taken into account the role of plants in the mitigation/accumulation of ATBR or the transfer of ATBR markers within plants.The aim of this study was to measure the effect of three water qualities on the fate of ATBR in the soil, the rhizosphere and within the roots of leeks and lettuce, over two seasons of crop irrigation. Through these experiments, the following questions were addressed (i) What is the effect of water quality on the evolution of ATBR in soils and plants? (ii) What is the effect of the plants on the spatial distribution of ATBR (barrier effect or transfer)? (iii) Does the nature of the plants (leeks vs lettuce) have an effect on the above questions? (iv) How do the ATBR markers evolve between the first and second crop irrigation seasons?The experiments were conducted at the Murviel-Lès-Montpellier wastewater reuse platform, in lysimeters (0.95 x 1.1 x 0.6 m), containing 16 leeks or 4 lettuces/bin and irrigated with either freshwater (FW), raw water (RW) or TWW. They were irrigated via drippers delivering an average water flow of 2 L/h for 3 months per year (june-september). Metabarcoding analysis on the 16S rRNA gene was performed on soil, rhizosphere and root samples. qPCR analyses were achieved on 16S rRNA, ermB (macrolide resistance), sul1 (sulfonamide resistance) and intl1 (class-1 integron-integrase) genes.The results showed that the use of TWW or RW favoured the emergence of particular taxa in soil, which increased over time: Rhodocyclaceae family for RW and Hydrogenophilaceae family, Ignavibacterium and Thiobacillus for TWW. According to the literature, these taxa are associated with high levels of organic matter, can degrade organic pollutants, can be selected in the presence of antibiotics and exert resistance to antibiotics. Along with the evolution of these biomarkers, ermB, sul1 and intl1 abundances increased over time in soils irrigated with either RW or TWW (not observed with FW). This increase was also noticed in roots and rhizosphere after the 1st season of irrigation. But a negative abundance gradient of these genes was noticed within the soil-rhizosphere-root continuum suggesting a buffering or a barrier effect of plants to ATBR dissemination.Finally, it seemed that the nature of plants (leek or lettuce) did not have any effect on ATBR markers, even if they impacted soil microbial communities (increase of Actinomadura genus with lettuce and increase of Acidobacteria subgroup_6 genus with leeks).In conclusion, the use of TWW (and RW) for fields irrigation seems to lead to an increase of the ATBR markers in soils and plants, associated with an increase in the relative abundance of some Firmicutes and Proteobacteria. Ongoing analysis of the results obtained from the second irrigation season will show if these changes are maintained over time (are they perennial or transitory) and enable to assess the adaptation or resilience of these soils after two irrigation seasons. Although the presence of plants seems to act as a barrier to ATBR indicators, further studies are needed to assess whether this effect is due to a physical limitation (limited transfer of antibiotics/resistant bacteria) and/or to endophytic bacteria mitigating the transfer of antibiotic resistant bacteria.