Date of Award

Spring 2017

Degree Type

Honors Project

School

College of Liberal Arts

First Advisor

Presley Martin

Abstract

Naturally-occurring antibiotic resistance genes in soil bacteria represent a potentially important reservoir of genes that could contribute to antibiotic resistance of human pathogens. It has been reported that over 40 genes in bacterial genomes are controlled by concentrations of ferric iron. We examined the effect of soil metal content on the level of resistance to two antibiotics, ampicillin (Amp) and tetracycline (Tet), and the presence of multiple genes that code for efflux pump-mediated resistance. These pumps act to export toxins (e.g. heavy metals and antibiotics, perhaps). Because of this, growth in heavy metal-contaminated soils might select for antibiotic resistance. Ninety-six soil samples were collected over the course of two summers from areas of Minnesota with known high and low ferric iron, as reported by the US Dept. of Interior. Samples were plated on LB plates with either 10 mg/500mL Tet or 50 mg/500mL Amp. Tet resistance was the same in high and low iron soils (p = 0.63, sd = 0.02). Amp resistance was higher in samples from high iron soils only in 2015’s data (2015 p =0.002; 2016 p = 0.75, sd = 38.1). Distribution of resistance was, however, significant for Tet between iron concentrations (p < 0.001). Additionally, total DNA was extracted and PCRs with gel electrophoresis was used to determine the prevalence of 14 different efflux genes (acrB,D,F; emrB,E; mdfA; tehA; yhiV; mexF,Y; tetC,H,B,D) common to soil bacteria. In 2015, five of the eight genes studied were seen in high iron soil, while only one gene was detected in low iron soil. In 2016, four of six genes were found in samples from both soils. Ferric iron levels in the soils tested were not significantly correlated with Tet or Amp resistance levels in soil bacteria in 2016, but were correlated in 2015’s data (Amp only). It is possible that other heavy metals play a more important role in selecting for antibiotic resistance than iron.

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