FOR 566: Veterinary Medicines in Soils: Basic Research for Risk Analysis
Geosciences
Medicine
Final Report Abstract
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 (mainly 4-OH-SDZ, N-acetyl-SDZ, and sarafloxacin). During manure storage and shortly after application to soil, SDZ concentrations increased due to a back-transformation of the acetyl-metabolite. In soil, SDZ fate was controlled by temperature and governed by rapid equilibrium sorption, followed by reversible kinetic redistribution into soil pores (residual fraction) that were not bio-accessible and formation of organic-matter associated non-extractable residues. 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. The effects of the antibiotics in soil lagged behind their maximum bio-accessible concentrations, because microbial growth and related effects were regulated by available C and N sources in manure and rhizosphere. Both antibiotics changed the structural diversity of the soil microbial community, depending on their specific composition in microhabitats. However, ecological effects, such as effects on N transformation, were limited due to functional redundancies within the soil microbial community. Modeled effective concentrations (EC50) for the individual transformation processes were in the low μg kg-1 soil range. Nevertheless, the treatment of animals with sulfadiazine and its presence in manure 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 (e.g., SDZ, sulfamerazine, oxytetracycline, ciprofloxacin, ampicillin, chloramphenicol, amikacin, streptomycin, trimethoprim) and disinfectants, 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. Effects of the antibiotics on microbial communities and resistance gene abundance and transfer rates were only significant when manure was present. It is essential that ecotoxicological assays are carried out with an adequate supply of carbon sources and nutrients, ideally in the form of manure. The establishment of dose-response relationships remains challenging due to the heterogeneous distribution of antibiotics and microbes in soil, and interactions with other pollutants and stressors. This calls for minimizing use of veterinary antibiotics, heavy metals, and disinfectants by optimizing animal husbandry conditions. Decreasing levels of SDZ, DIF, and resistance genes with prolonged experiment duration after manure application indicated that well-known good agricultural practices could help minimize environmental and health risks of veterinary antibiotics.
