Redistribution of intracellular and extracellular free & adsorbed antibiotic resistance genes through a wastewater treatment plant by an enhanced extracellular DNA extraction method with magnetic beads

Due to the limitations of current extraction methods, extracellular DNA (eDNA) is rarely discerned from intracellular DNA (iDNA) despite having unique contributions to antibiotic resistance genes (ARGs) propagation. Furthermore, eDNA may be free (f-eDNA) or adsorbed to or suspended solids, including cells (a-eDNA), which affects ARG persistence and transmissivity. We developed a novel method using magnetic beads to separate iDNA, a-eDNA, and f-eDNA to assess how these physical states of ARGs change across a wastewater treatment plant. This method efficiently extracted eDNA (>85.3%) with higher recovery than current methods such as alcohol precipitation, CTAB-based extraction, and DNA extraction kits (<10%). Biological treatment and UV disinfection decreased the concentration of intracellular ARGs (iARGs) and adsorbed extracellular ARGs (a-eARGs), causing an increase of released free extracellular ARGs (f-eARGs). More ARGs were discharged through the wasted biosolids than in the effluent; iARGs and a-eARGs are prevalent in wasted biosolids ((73.9 ± 22.5) % and (23.4 ± 15.3) % of total ARGs respectively), while f-eARGs were prevalent in the effluent ((90.3 ± 16.5) %). Bacterial community analysis showed significant correlations between specific genera and ARGs (e.g., Aeromonas, Pseudomonas and Acinetobacter were strongly correlated with multidrug-resistance gene bla). This treatment system decreased the discharge of iARGs to receiving environments, however, increased eARG concentrations were present in the effluent, which may contribute to the environmental resistome.