Antibiotic resistance plasmids play important roles in resistance buildup and spread in soil and groundwater. However, very limited work has been reported in the literature to examine the transport behavior of free resistance plasmids in the environment. In this work, the transport of eight resistance plasmid vectors (pUC18, pBlueScript II SK(+), pBR322, pFastBac HT A, pBABE-Puro, pcDNA3.1(+)/myc-His A, pcDNA3.1(+)/Flag-His A+ATM, and pAdEasy-1) in quartz sand was investigated through packed column experiments. At ionic strengths up to 0.05 mol L−1 in NaCl solutions and 0.006 mol L−1 in CaCl2 solutions, adsorption rate coefficients of all the plasmids examined were <0.19 h−1 at a fluid velocity of 6.1 m d−1. This result indicates that under environmentally relevant flow and solution chemistry conditions, free plasmids may transport across significant distances and may have the potential to spread antibiotic resistance to much larger areas and more receptors in the environment. As ionic strength increased, adsorption of plasmids in CaCl2 solutions increased dramatically. Adsorbed plasmids could be partly desorbed at reduced ionic strengths. These observations indicate that the transport behavior of free plasmids is very similar to the transport of colloids commonly examined in the literature (e.g., latex microspheres and bacteria). No noticeable difference in breakthrough concentrations of linear and supercoiled plasmids was observed. At higher ionic strengths (0.02 and 0.05 mol L−1) in CaCl2, the size of the plasmids was found to play a predominant role in governing their transport under identical flow and solution chemistry conditions.

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