Oxytetracycline in Soil and Conditions that Select for Antibiotic Resistant Bacteria

Abstract

The increased occurrence of antibiotic resistant bacteria (ARBs) in the environment is posing significant and increasing pressure on human health care in the U.S. and globally. The misuse, overuse and partial metabolism of antibiotics in humans and the animalproducing industry over the years has been accompanied by unintentional environmental antibiotic contamination. With the increased incidence of ARBs, attention has been paid to the environmental fate of antibiotics, including Oxytetracycline (OTC).

OTC is one of the most commonly administered antibiotics to livestock and has been categorized by the World Health Organization (WHO) as “critically important ”because it is used as an alternative treatment of serious infections in humans and to treat diseases caused by bacteria that may be transmitted to humans from non-human sources. Even though OTC has a low potential for mobility due to its high sorption partition coefficient (Kd range between 115 to 269,097 L/Kg), OTC has been detected in surface and groundwater. OTC is strongly retained by soil component (alumninosilicates, organic matter, metal oxides) through multiple mechanisms, yet is still bioavailable to microorganisms suggesting a potential scenario for toxicity and/or emergence of antibiotic resistance.

This work presents an inventory of reported OTC concentrations distributed in aqueous and solid media and an evaluation of the fate of OTC in the environment. OTC concentrations were compared to threshold limits that delineate selective windows of resistance to assess the selective potential for resistant bacteria. A model to predict OTC partition coefficients based on soil properties was developed considering the importance of sorption on the fate, mobility and bioavailability of OTC, the variability of reported sorption and the complex interaction of OTC with soil components.

The fate and transport of OTC in soil was addressed by using a Two-site, One-rate Non-equilibrium model. Simulation results were compared to antibiotic resistant selection regions to evaluate which scenarios resulted in the potential for antibiotic resistant harboring. The model predicted soil-bound OTC concentration levels that were within the antibiotic resistance selection ranges. Therefore, surface application of slurry and diluted slurry with manure-associated OTC concentrations on the order of 101 to 102 g=Kg could potentially select for antibiotic resistant bacteria (ARB), particularly in cases where no incorporation of manure is considered. Predicted concentrations resulted in potential selection of ARBs throughout the entire simulation period (120 days) for slurry application without incorporation and up to 50 days for slurry application with homogeneous incorporation of manure to a depth of 10 cm. For the cases of diluted slurry application without incorporation, predicted concentrations resulted in potential selection of ARBs throughout the entire simulation period (120 days) and up to 30 days for diluted slurry application with homogeneous incorporation of manure to a depth of 10 cm.

These results suggest that OTC concentrations in swine manure together with current waste management practices of land application of manure as fertilizer present the potential for selection of antibiotic resistance, in particular when no incorporation of manure is practiced. The incorporation of manure into soil effectively reduces OTC concentrations in soil and also in the aqueous phase to levels below antibiotic resistance selectivity. Incorporation of manure into soil and other manure management practices to reduce manureassociate OTC, such as stockpiling and composting, can be effective in minimizing the potential selection for antibiotic resistance. Additional research is needed to assess the microbial activity of soil-bound OTC to compare OTC concentration levels in soil with antibiotic selectivity ranges.