Antibiotic resistance on the farm: Evaluating the risks to soil, crop and human health.

Background: The trend towards large 'in-door' dairy farms in the UK means greater reliance on antibiotics to maintain a healthy herd (Livestock Superfarms, 2012, Postnote No. 404, Parliamentary Office of Science & Technology). Cattle dung, floor washings and mastitic milk are generally stored in slurry tanks before field application (Department of Agriculture & Rural Development, 2008, ISBN:978-1-84807-068-4 and Armstrong pers comm.). UoN slurry contains antibiotics including oxytetracycline and amoxicillin (Barrett pers comm./ pilot data) and analysis of 126 E. coli isolates from the slurry showed 62% contained multiple resistances to up to 13 antibiotics (Hobman & Dodd, pers comm./pilot data). What is of interest here is whether these organisms pose a risk once they leave the slurry tank. An estimated 70 million tonnes of animal manure are applied as fertilizer on UK agricultural land each year (Hutchison et al., 2004, Letters in Applied Microbiology, 39:207-214), resulting in potentially significant inputs of resistance genes and antibiotic residues to land used for food production; approximately 70% of antibiotics are excreted in dung rather than being absorbed by the cow (Chee-Sanford et al., 2009, Journal of Environmental Quality, 38:1086-1108). Whilst antibiotic production and resistance are ubiquitous in soils and many resistance genes are common to both soil and faecal bacteria (Forsberg et al., 2012, Science 337: 1107-1111), selection pressures within the slurry tank are likely to be greater than natural selection pressures faced by soil microbes. Nevertheless, natural selection pressures are little understood and predicting the importance of slurry amendments as a reservoir for antibiotic resistance genes in the environment is important in terms of soil functions, food security and human health (Rosi-Marshall & Kelly, 2015, Environmental Science & Technology, 49:5257-5258; Woolhouse et al., 2015, Philosophical Transactions of the Royal Society B, 370: 20140083).