Dynamics of Antibiotic Resistance Genes in Crop Production Systems Following the Application of ‘Fecal' Fertilizers.

Animal manures and biosolids are an important source of nutrients for crop production. Antibiotic residues and antibiotic resistant bacteria carried in these materials could potentially increase the reservoir of antibiotic resistance genes in amended soils, and increase human or animal exposure to antibiotic resistance genes if they are transferred to crops. We have been evaluating under field conditions over multiple seasons the dynamics of some gene targets associated with horizontal transfer and resistance to various antibiotics following the application of biosolids, swine, or dairy manure. Gene targets in amendments, in soils and on vegetable crops at harvest are detected by PCR and quantified by qPCR. Molecular data is complemented by viable plate counts of antibiotic resistant coliforms, indicator and pathogenic bacteria. This presentation will consider how gene dynamics in soil and transfer to crops varies with material applied (human, swine, dairy), pre-treatment (eg. anaerobic digestion, dewatering, composting), seasonal climate conditions, crop harvest delay, and vegetable crop type (eg. lettuce, radish, carrot). Molecular endpoints will be contrasted with conventional bacteriology typically used in sanitary surveys of crop quality.