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.
Subject
Antibiotic Resistant BacteriaOxytetracycline
Sorption
Two-site Non-equilibrium Transport Model
Citation
Munoz Urriola, Melisabel Del Carmen (2017). Oxytetracycline in Soil and Conditions that Select for Antibiotic Resistant Bacteria. Doctoral dissertation, Texas A & M University. Available electronically from https : / /hdl .handle .net /1969 .1 /173031.