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Fate and Ecological Effects of Livestock Antibiotics in Soils
Department of Plant and Soil Sciences
Millions of pounds of antibiotics are used each year to prevent disease and promote animal growth in livestock industries, a large fraction of which are excreted in manure, and enter rivers and streams from surface runoff from manure-amended lands. The primary concerns are that contamination will increase the spread of antibiotic resistant bacteria and suppress bacterially-mediated processes in soils (e.g. nutrient cycling). Once antibiotics are deposited to soils, they can be removed from the bioavailable pool through sorption and microbial degradation, depending on temperature, water content, soil mineralogy, solution composition, pH, organic matter, nutrients, and oxygen.
The first objective of this project is to develop an accurate and sensitive technique for determining antibiotic concentrations in manure and soil samples.
The second major objective of this research proposal is to determine the effect of landscape position on changes in antibiotic levels and ARB numbers after application of manure and antibiotics to agricultural fields.
2010 Project Description
Mentored two graduate students who conducted research to investigate
1) the potential for horizontal gene transfer of antibiotic genes from soil bacteria to a pathogen (Enterococcus faecium) in poultry litter and
2) the effect of antibiotics on nitrogen cycling and microbial community composition in soils. Mentored three high school students on research experiments that investigated the effects of livestock antibiotics on nitrogen cycling by soil microorganisms.
As a result of the work performed in my lab, the students won several awards at local, regional, and state science fairs, and qualified to compete in the international science fair.
This project resulted in new knowledge about the effects of livestock antibiotic dissemination to the soil environment. Specifically, it was discovered that high concentrations of antibiotics (bacitracin, roxarsone, virginiamycin) significantly affected soil bacterial population composition, growth, and nitrification, but not denitrification. Levels of antibiotics that affected microorganisms, however, were manifold times higher than would be expected in litter or litter-amended soils. Therefore, results from this study indicate that it is unlikely that the antibiotics tested in this study would significantly affect microbial community composition or activities in these soils.
Two possible explanations, sorption and antibiotic resistance, were further evaluated to explain these results. As expected, antibiotics were rapidly and strongly sorbed to soil surfaces, which reduced antibiotic concentration in the bioavailable pool and protected microorganisms from toxicity affects. Another even more surprising finding, however, is that native soil microorganisms were very tolerant to high levels of antibiotics, even when chemicals were present in the bioavailable pool. This was surprising because the microorganisms have not been previously exposed to antibiotics, except maybe by those produced by microbes in the soil. This is clear evidence that native populations are intrinsically resistant to antibiotics.
One concern with the presence of antibiotic resistant bacteria in the environment is the potential for genes responsible for resistance to be transferred to other microorganisms, such as pathogens in animal manure that are spread on soils. This would accelerate the development of antibiotic resistant pathogens that could affect human and animal health and treatment.
Fortunately, results from our work did not reveal extensive horizontal transfer of antibiotic genes between soil microorganisms and pathogens, such as E. faecium that is prevalent process in animal manure. This result is probably attributed to natural barriers to horizontal gene transfer. Considering that antibiotic resistant bacteria are prevalent in soils, it is clear that further work is needed to investigate environmental factors that affect HGT and the development of antibiotic resistant pathogens in the environment.
D'Angelo, Elisa M. and J. Grove. 2007. Health Risks Associated with Land Application of Arsenic and Bacterial Pathogens in Broiler Litter. Feature article in the Environmental and Natural Resource Issues Newsletter. College of Agriculture, University of Kentucky.
Elisa D'Angelo. 2009. Impacts of Poultry Litter on Human and Environmental Health. Environmental & Natural Resource Issues Newsletter. College of Agriculture, University of Kentucky.
Elisa D'Angelo. 2009. Understanding Antibiotics in the Soil. The Magazine. College of Agriculture, University of Kentucky.
Sagarika Banerjee. 2010. Effects of livestock antibiotics on nitrification, denitrification, and microbial community composition in soils along a topographic gradient. University of Kentucky Masters Thesis.