Several factors have been identified to play a role in enhancing extracellular DNA (eDNA) transformation by bacteria. These factors include chemical compounds such as disinfection by products (DBP) and pharmaceutical drugs; moreover, environmental stressors such as solar irradiance has been shown to play also a role in the increase of eDNA transformation. According to these studies, factors that can cause mutagenicity or alter cell permeability would increase transformation frequency. This is due to the initiating of DNA damage repair mechanism in the case of mutagenic effect, or the increase of eDNA translocation inside the cell as cell membrane gets altered. However, all these factors have been studied individually which do not represent the true environmental scenario in which factors co-exist simultaneously. In this study, we identified the effect of several factors (DBP, solar irradiance and carbamazepine) on cell mutagenicity and permeability using Salmonella enterica Typhimurium TA100 and Acinetobacter baylyi respectively. Furthermore, we employed a reporter system for transformation in A. baylyi to define both the individual and combined effect of studied factors on eDNA transformation. Our data suggests that the interaction between different stressors can result in a different outcome than the individual effect, where positive factors for transformation do not always have to result in an additive interaction. Instead, different combinations resulted in antagonistic, additive or synergistic interactions. Moreover, mutagenicity does not necessarily always cause an increase in eDNA transformation because the direct interaction between a stressor and eDNA plays also a role in this phenomenon. The current study highlights the potential hazard of eDNA existence in treated water that can enrich for the presence of ARG, which may be difficult to predict and truly assess the extent of hazard risk. Hence, this study reiterates the need to improve the removal of eDNA from treated water.
