Research Accomplishment Reports 2007

Fate and Ecological Effects of Livestock Antibiotics in Soils

E. D'Angelo
Department of Plant and Soil Sciences

 

Project Description

Poultry litter is considered a valuable fertilizer source when applied to agricultural soils, however, repeated and intense applications of litter can also contaminate surface and groundwater with livestock antibiotics and pathogenic bacteria, which pose health risks to people living in areas where poultry manure is used as a soil amendment. Unfortunately, little is known about the factors that regulate the occurrence and effects of these compounds and pathogenic bacteria in the environment (e.g. bacterial diversity, development of antibiotic resistant bacteria, carbon and nutrient cycling, etc.).

During the past year, field experiments were conducted to determine (i) the prevalence of different antibiotic resistance genes in soils along topohydrosequences, (ii) the identity of bacteria that contain antibiotic resistant genes in soils, and (iii) whether the antibiotic resistance genes from soil bacteria could be transferred to pathogenic bacteria that are commonly found in biosolid wastes and deposited to soil environments. It was hypothesized that antibiotic resistant genes from soil bacteria could be transferred to pathogenic bacteria in manure wastes, and that the extent of gene transfer would be greater in certain landscape positions than others due to differences in microbial growth conditions in the soils. Results from the experiment showed that all soils along the topohydrosequences contained the vat(D) gene, which codes for an enzyme that nullifies streptogramin antibiotics that are widely used in animal and human medicine.

The identity of soil bacteria that were highly resistant to streptogramin antibiotics and contained the vat(D) gene included members of the genera Pseudomonas, Acinetobacter, and Klebsiella. It was discovered that Enterococcus faecium, a common pathogen found in biosolid wastes, rapidly (<1 day) became resistant to streptogramin antibiotics after being added to the soil, presumably by acquiring the vat(D) gene from the streptogramin-resistant soil bacteria. As expected, Enterococcus faecium acquired streptogramin-resistance to a greater extent in certain landscape positions than others due to differences in soil properties including moisture content and temperature.

Impact

Kentucky is one of the largest poultry producers in the US, and hence large amounts of poultry waste are generated and applied to soils in the state. Since this waste contains considerable amouts of nutrients, antibiotics, and pathogenic bacteria, it is critical to understand how these agents impact environmental quality and animal and human health once they are applied to different soil environments.

In this project, the effect of environmental factors on the development of antibiotic resistant pathogens were elucidated. It is expected that results from this project will help regulators and land managers make improved decisions about when and where to apply manure in order to maximize the benefits and minimize the ecological/health risks associated with the land application of manure wastes. Thus, the proposed research has a direct impact on both production agriculture and natural resource management in KY and the nation.