Fate and Effects of Veterinary Antibiotics in Soil.

There are increasing concerns that veterinary antibiotics used in animal husbandry and the presence of antibiotic residues in manure select antibiotic resistance determinants and disturb microbial communities and functioning in soils. The research unit FOR566 identified and quantified key processes that control both the fate and the related ecotoxicological effects of the bacteriostatic agent sulfadiazine (SDZ) and the bactericide difloxacin (DIF) in soil, using laboratory, greenhouse, field, and modeling studies. The compounds were administered to pigs and, when needed, labeled with radioactive and/or stable isotopes.

The animals excreted the antibiotics almost completely as parent compounds and specific metabolites of which the acetyl-metabolite was back-transformed in soil to the parent antibtiotic. In soil, SDZ fate was controlled by temperature and governed by rapid equilibrium sorption, followed by reversible kinetic redistribution into soil pores. These mechanisms extended the half-life of antibiotics to several months and beyond. The slow release of SDZ from sequestered forms maintained low bio-accessible concentrations of residues in soils over months. In contrast, fluoroquinolone binding was always so strong that little if any of the antibiotic was detected in bio-accessible forms.

Effects of the antibiotics on microbial communities and resistance gene abundance and transfer rates were only significant when manure was present. The effects of the antibiotics in soil lagged behind their maximum bio-accessible concentrations. Both antibiotics changed the structural diversity of the soil microbial community, but ecological effects, such as effects on N transformation were limited due to functional redundancies within the soil microbial community. Modeled effective concentrations were in the low µg kg^-1 soil range. Nevertheless, the treatment with sulfadiazine increased the concentrations and the transfer of sul resistance genes in manure and soils, for which LowGC-type and IncP-1ɛ plasmids played a major role. The captured plasmids often conferred multiple resistances to antibiotics and heavy metals, indicating potential co-selection of resistance. Indeed, DIF in manure did not affect the concentrations of qnr fluoroquinolone resistance genes in soil, but did increase concentrations of sul genes. In the vicinity of plant roots, a faster antibiotic dissipation potentially counterbalanced higher gene-transfer rates.