Impact of ciprofloxacin and tetracycline on soil enzyme activities and antibiotic-resistant genes and their removal by earthworms

Antibiotics with a wide range of effectiveness, including ciprofloxacin (CIP) and tetracycline (TET), administered to livestock and poultry for the treatment of bacterial diseases, are poorly absorbed and excreted into the environment as parent compounds or as active metabolites. The residual antibiotics strongly bind to soil components, facilitating the spread of bacteria and genes resistant to antibiotics in the surrounding ecosystem. Consequently, this process gradually alters the rhizobacterial population dynamics due to antibiotic pollution-induced selective pressure. Biodegradation constitutes a primary mechanism for antibiotic removal. Earthworms remediate organic contaminants by enhancing microbial and enzymatic activities in the gastrointestinal tract and contaminant-bearing soil substrates. Previous research has demonstrated that earthworms can accelerate soil organic compound deterioration. Nonetheless, their efficacy in eliminating antibiotic contamination by altering the antibiotic resistance genes (ARG) abundance remains an enigmatic scientific terrain. We have employed the chromatographic approaches, genomics, enzyme assays, and qRT-PCR to decipher the intricate mechanisms of the earthworm-mediated alteration of the ARGs abundance in antibiotic-amended soil. Ciprofloxacin (CIP) and tetracycline (TET) removal was significantly higher in antibiotic-treated soil inoculated with earthworms than in soil without. Earthworms induced a paradigm shift in the soil bacterial composition, with an increased relative abundance of Bacteroidetes and decreased Firmicutes, Proteobacteria, and Actinobacteria. The soil enzyme activities were significantly reduced in CIP/TET amended soil, which was successfully balanced when administered with earthworms. Earthworms alleviated the abundances of qnrA, qnrS, tet(M), tet(O), tet(A), and tet(Q) genes in CIP and TET amended soil, respectively. Furthermore, the application of earthworm treatments led to a reduction in integron 1 and 2, potentially diminishing the risk associated with the horizontal transmission of ARGs. Therefore, it is speculated that earthworms might mitigate the deleterious effects of the antibiotic contamination in soil by removing CIP/TET and ARGs. This may ultimately recuperate rhizomicrobiome activity by changing the bacterial community structure.