Effects of Veterinary Medicines Introduced via Manure into Soil on Microbial Communities

Multiple antibiotic resistant bacterial pathogens pose a major threat for the successful therapeutic use of antibiotics in human and veterinary medicine. Veterinary antibiotics used in animal husbandry are assumed to contribute to an increased antibiotic resistance among bacteria in manure. Bacteria carrying transferable antibiotic resistances, antibiotics, metabolites and nutrients are inevitably brought into soil when manure is used as fertilizer. Some antibiotics such as sulfadiazine (SDZ) are almost completely excreted and persist during manure storage. Here we summarize the results of a study on 15 field-scale piggery manures by means of cultivation-dependent and -independent methods that clearly revealed that transferable antibiotic resistances are routinely introduced into soil via manure. In soil microcosms we showed that both manure spreading and SDZ spiking synergistically increased the abundance of SDZ resistance genes and their transferability in soil. Self-transferable plasmids carrying multiple antibiotic resistances could be easily captured into plasmid-free E. coli recipient strains and transfer frequencies were enhanced in soils treated with manure with SDZ added. Molecular characterization of the plasmids captured provided insights into the extant diversity of the soil mobilome.