Tylosin is an antibiotic commonly used in swine industry at subtherapeutic levels to improve growth rates and efficiency of feed utilization. When manure is applied to subsurface drained agricultural fields, antibiotic residues in manure provide selective pressure for the development of microbial resistance. Transport of these microorganisms through soils into tile drainage lines and ultimately into surface waters is a serious threat for public health. This study was performed to investigate the occurrence and transport of tylosin-resistant enterococci from tile drained agricultural fields receiving semi-annual swine waste applications. The field study was conducted at the Iowa State University Northeast research farm at Nashua, Iowa in April and November of 2009. Liquid swine slurry from an operation feeding tylosin at subtherapeutic levels was injected into no-till field plots. Samples collected from field were assayed for total enterococci concentration and enterococci exhibiting resistance to tylosin at an MIC of 35 mg/L. All the enterococci in manure samples were found to be resistant to tylosin. Concentrations of total and tylosin resistant enterococci in soil samples was 6.84x105 cfu/100ml and 5.25x105 cfu/100ml, respectively. There was a statistically significant difference between the total and tylosin-resistant enterococci concentrations in water samples (p=0.05). Total enterococci concentrations in water samples ranged from 1.3x101 to 5.0x103 cfu/100mL while tylosin resistant enterococci concentrations ranged from 1.3x101 to 1.2x103 cfu/100mL. The percent of tylosin resistant enterococci were highest in manure samples and lowest in drainage water samples, suggesting that enterococci lose resistance as selective pressure from antimicrobial residues decrease.
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