Fate and transport of antibiotic resistance genes in saturated soil columns

Interest in the fate of microorganisms (genetically modified or not) added to soil for a range of applications (e.g., biological fertilizer, bioremediation) has led to the examination of the transport and dispersion of bacteria in soil and ground water environments. The fate of added microorganisms can also be related to the fate of their DNA or any DNA found within soil or groundwater. However, studies have not examined the movement and fate of plasmids themselves in water-saturated soils. Continuous-flow water-saturated soil columns were used to examine the environmental fate and movement of antibiotic resistance genes. The genes were located on a plasmid pLEP01, which was either introduced into the soil columns directly as a mixture of supercoiled and open circular forms or after linearization by restriction enzymes. Both quantitative and qualitative measurements were done throughout the study. Quantitative measurements included the total DNA and conservative tracer concentration in the column effluent and the transformation frequencies for E. coli DH 10B and Acinetobacter sp. BD 413 in the presence of column effluent. Qualitative measurements included relative degradation of the introduced DNA by gel electrophoresis and the potential of effluent DNA to transform bacteria leading to the production of antibiotic resistant bacteria. The extent of DNA degradation was found to be proportional to its residence time in the soil column. In addition, tests on the biological activity (bacterial transformation) of the DNA demonstrated that this activity was roughly proportional to the remaining DNA after its degradation. Results suggest a potential for biologically active DNA to be transported over considerable distances in water-saturated soil and ground water.