Enantioselectivity of Antibiotic Resistance Genes in the Gut of Earthworms Exposed to Metalaxyl

Abstract

Chiral pesticides often exhibit an enantioselective environmental behavior and ecological effects. Using a soil-earthworm microcosm, we investigated the dissipation and bioaccumulation of metalaxyl enantiomers and their effects on gut microbiota and antibiotic resistance genes (ARGs). R-metalaxyl dissipated rapidly (half-life 7–9 d), whereas S-metalaxyl persisted much longer (about 90 d), imposing prolonged selective pressure. Earthworms preferentially accumulated R-metalaxyl, with a maximum bioaccumulation factor of 0.42. High-concentration S-metalaxyl increased the total ARG abundance in earthworm guts by approximately 35% relative to the control. Antioxidant enzyme activities showed an enantioselective pattern of initial stimulation followed by decline over time. Metalaxyl increased Streptococcus abundance by 1–3% and reduced sensitive genera, while R-metalaxyl selectively enriched rifamycin-, multidrug-, and glycopeptide-resistance genes. Over 60% of ARGs co-occurred with plasmids, indicating an elevated horizontal gene transfer risk. These results confirm significant enantioselective effects of metalaxyl on dissipation, enrichment, microbiome structure, and antibiotic resistance transmission in earthworm guts.