Accelerated dissipation, soil microbial toxicity and dispersal of antimicrobial resistance in soils repeatedly exposed to tiamulin, tilmicosin and sulfamethoxazole.

The application of manures leads to the contamination of agricultural soils with veterinary antibiotics (VAs). These might exert toxicity on the soil microbiota and threaten environmental quality, and public health. We obtained mechanistic insights about the impact of three VAs, namely, sulfamethoxazole (SMX), tiamulin (TIA) and tilmicosin (TLM), on the abundance of key soil microbial groups, antibiotic resistance genes (ARGs) and class I integron integrases (intl1). In a microcosm study, we repeatedly treated two soils (differing in pH and VA dissipation capacity) with the studied VAs, either directly or via fortified manure. This application scheme resulted in accelerated dissipation of TIA, but not of SMX, and accumulation of TLM. Potential nitrification rates (PNR), and the abundance of ammonia-oxidizing microorganism (AOM) were reduced by SMX and TIA, but not by TLM. VAs strongly impacted the total prokaryotic and AOM communities, whereas manure addition was the main determinant of the fungal and protist communities. SMX stimulated sulfonamide resistance, while manure stimulated ARGs and horizontal gene transfer. Correlations identified opportunistic pathogens like Clostridia, Burkholderia-Caballeronia-Paraburkholderia, and Nocardioides as potential ARG reservoirs in soil. Our results provide unprecedented evidence about the effects of understudied VAs on soil microbiota and highlight risks posed by VA-contaminated manures. ENVIRONMENTAL IMPLICATION: The dispersal of veterinary antibiotics (VAs) through soil manuring enhances antimicrobial resistance (AMR) development and poses a threat to the environment and the public health. We provide insights about the impact of selected VAs on their: (i) microbially-mediated dissipation in soil; (ii) ecotoxicity on the soil microbial communities; (iii) capacity to stimulate AMR. Our results (i) demonstrate the effects of VAs and their application-mode on the bacterial, fungal, and protistan communities, and on the soil ammonia oxidizers; (ii) describe natural attenuation processes against VA dispersal, (iii) depict potential soil microbial AMR reservoirs, essential for the development of risk assessment strategies.