Background/Question/Methods The rapid evolution of antibiotic resistance in bacteria is one of the most pressing public health problems in the nation. Research on antibiotic resistance is primarily focused on clinical settings. Antibiotic resistant bacteria are, however, found in the environment as well. Intense use of antibiotics for livestock and other agricultural practices contributes to this pool of resistance, but not all cases of resistance are caused by direct use of antibiotics. Many metals, including mercury, lead, cadmium, cobalt, nickel, copper, and vanadium, are often correlated with antibiotic resistance. Naturally occurring resistance also stems from interactions between microbes in the environment. Recent research highlights the need to understand patterns of antibiotic resistance in the environment, as well as in clinical settings. Our research is one of the first to examine the large-scale distribution of antibiotic resistance in enteric bacteria inhabiting soil. We determine the relationship of antibiotic resistance in Enterobacter aerogenes in Lancaster County, PA to land use and concentration of heavy metals.

Results/Conclusions From soil samples collected from 84 randomly selected sites, we found mean proportions of isolates resistant to ampicillin (0.50), chloramphenicol (0.49), trimethroprim-sulfamethoxazole (0.42), kanamycin (0.05), tetracycline (0.03), and multiple drugs (0.23). Levels of resistance were typically lower in forest versus residential, pasture, and crop land uses but resistance among land uses was only significantly different for ampicillin and kanamycin. Analyses of heavy metal concentrations did not fully explain antibiotic resistance patterns. Lead was significantly related to ampicillin resistance, copper was significantly related to kanamycin and chloramphenicol resistance, and nickel was significantly related to kanamycin resistance. Our research documents current levels of antibiotic resistance in the environment and provides an initial assessment of variables affecting that resistance. We suggest a greater effort be made to understand the relationship between land use, heavy metals, and antibiotic resistance in the environment and the implications of these relationships on public health.