Cytotoxicity, biochemical, histological and gut microbiota changes in Eisenia fetida earthworms after exposure to different sulfamethoxazole and oxytetracycline concentrations

Sulfamethoxazole and oxytetracycline are antibiotics widely used to control or prevent bacterial diseases. They are frequently detected in soil, especially in soils fertilized with animal waste, and may cause toxic effects in non-target organisms. The aim of this study was to evaluate the effect of the antibiotics sulfamethoxazole (SMX) and oxytetracycline (OTC) on the earthworm Eisenia fetida, at concentrations of 0 (control), 0.1, 1.0 and 10.0 mg/kg, after 14 days of exposure. Neither antibiotic affected survival, growth or caused oxidative stress in E. fetida, however, they promoted changes in the cells of the system, decreasing cell viability and riboflavin content. Furthermore, the higher concentration of SMX affected the integrity of the epidermis, reducing the mucopolysaccharides present in the integument. Antibiotics negatively affected the intestinal microbiota of E. fetida, promoting an increase in bacterial phyla that harbor antibiotic resistance genes (ARGs), such as Firmicutes after exposure of earthworms to the two antibiotics and Proteobacteria that increased at the highest concentration of SMX. Furthermore, after 7 and 14 days of exposure, the concentration of antibiotics decreased in the soil, with greater degradation occurring for SMX than OTC, and, after 14 days of exposure, the earthworms bioaccumulated more SMX. Under the conditions studied, SMX proved to be more toxic than OTC, however, both antibiotics changed the intestinal microbiota of E. fetida, promoting the increase in phyla/genera that harbor ARGs. The effects observed at different levels of biological complexity are worrying, given that the concentrations tested are environmentally relevant.