The reuse of treated wastewater (TWW) for irrigation is widely applied to alleviate pressure on freshwater resources. However, TWW contains antibiotics that once in soils, can exert selective pressure, promoting the emergence and spread of antimicrobial resistance (AMR) in the environment. Current environmental risk assessments for antibiotic residues rely on indicators such as Predicted No Effect Concentrations (PNECs), usually determined in liquid media. These PNECs aim to predict antibiotic concentrations that may promote resistance in the environment. Given the complexity of soil matrices, few studies have established PNEC values for soil, which likely differ significantly from aquatic environments.To address this gap, we developed a simplified experimental model using soil microcosms irrigated with TWW and the antibiotic sulfamethoxazole (SMX) to estimate threshold concentrations favouring resistance transfer or/and emergence within the soil microbiome. We identified SMX concentrations between 0.01 and 0.1 mg/kg_{dry soil} that likely increased the abundance of sulfonamide resistance genes in soil. A time window of 1 to 7 days post-exposure showed a temporary rise in sul1 and intl1 gene abundance (over 1 log/soil 16S rDNA), the appearance of SMX transformation products, and an increase in some Rhodocyclaceae. After 1.5 months of incubation and complete SMX transformation, the relative abundance of sul1 and intl1 remained about 0.5 log higher than in SMX-free controls and soils with SMX levels below 0.1 mg/kg _{dry soil}. A persistent transformation product, 4-N-glucuronide-SMX, was also observed.Here, the estimated PNEC for SMX in soil, between 0.01 and 0.1 mg/kg, exceeds typical SMX concentrations found in soils exposed to TWW. This may suggest low impact on resistance selection for this compound in the context of TWW exposure. However further studies on other soils, water, and antibiotics need to be conducted to expand our knowledge on soil PNECs.

中文翻译：

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暴露于磺胺甲噁唑浓度增加的土壤微生物群落和抗生素耐药性标志物的时间动态


处理后的废水 （TWW） 再利用用于灌溉，被广泛用于缓解淡水资源的压力。然而，TWW 含有抗生素，一旦进入土壤，就会施加选择压力，促进抗菌素耐药性 （AMR） 在环境中的出现和传播。目前抗生素残留的环境风险评估依赖于预测无影响浓度 （PNEC） 等指标，通常在液体介质中确定。这些 PNEC 旨在预测可能促进环境中耐药性的抗生素浓度。鉴于土壤基质的复杂性，很少有研究确定土壤的 PNEC 值，这可能与水生环境有很大不同。为了解决这一差距，我们开发了一个简化的实验模型，使用用 TWW 和抗生素磺胺甲噁唑 （SMX） 灌溉的土壤微观世界来估计有利于土壤微生物组内抗性转移或/和出现的阈值浓度。我们确定了 0.01 至 0.1 mg/kg_干燥土壤之间的 SMX 浓度，这可能增加了土壤中磺酰胺抗性基因的丰度。暴露后 1 至 7 天的时间窗显示 sul1 和 intl1 基因丰度暂时增加（超过 1 log/土壤 16S rDNA），出现 SMX 转化产物，以及一些红环菌科的增加。孵育 1.5 个月并完成 SMX 转化后，sul1 和 intl1 的相对丰度仍比无 SMX 的对照和 SMX 水平低于 0.1 mg/kg _干土的土壤高约 0.5 log。还观察到一种持久的转化产物 4-N-葡萄糖醛酸苷-SMX。这里，土壤中 SMX 的估计 PNEC 在 0.01 和 0 之间。1 mg/kg，超过了暴露于 TWW 的土壤中的典型 SMX 浓度。这可能表明在 TWW 暴露的情况下对该化合物的抗性选择影响较小。然而，需要对其他土壤、水和抗生素进行进一步研究，以扩大我们对土壤 PNEC 的了解。