Quantifying Attachment and Antibiotic Resistance of Escherichia coli from Conventional and Organic Swine Manure

Broad-spectrum antibiotics are often administered to swine, contributing to the occurrence of antibiotic-resistant bacteria in their manure. During land application, the bacteria in swine manure preferentially attach to particles in the soil, affecting their transport in overland flow. However, a quantitative understanding of these attachment mechanisms is lacking, and their relationship to antibiotic resistance is unknown. The objective of this study is to examine the relationships between antibiotic resistance and attachment to very fine silica sand inEscherichia coli collected from swine manure. A total of 556 isolates were collected from six farms, two organic and four conventional (antibiotics fed prophylactically). Antibiotic resistance was quantified using 13 antibiotics at three minimum inhibitory concentrations: resistant, intermediate, and susceptible. Of the 556 isolates used in the antibiotic resistance assays, 491 were subjected to an attachment assay. Results show that E. coli isolates from conventional systems were significantly more resistant to amoxicillin, ampicillin, chlortetracycline, erythromycin, kanamycin, neomycin, streptomycin, tetracycline, and tylosin (P \u3c 0.001). Results also indicate that E. coli isolated from conventional systems attached to very fine silica sand at significantly higher levels than those from organic systems (P \u3c 0.001). Statistical analysis showed that a significant relationship did not exist between antibiotic resistance levels and attachment in E. coli from conventional systems but did for organic systems (P \u3c 0.001). Better quantification of these relationships is critical to understanding the behavior of E. coli in the environment and preventing exposure of human populations to antibiotic-resistant bacteria