Dormancy induced by oxidative damage during disinfection facilitates conjugation of ARGs through enhancing efflux and oxidative stress: A lagging response

Disinfection is known to greatly alter bacterial characteristics in water, and high horizontal gene transfer (HGT) frequency occurs in eutrophic conditions. Interestingly, these two seemingly irrelevant phenomena were closely linked by a lagging response of the increased conjugation frequency probably via daily water disinfection in this study. Three disinfection methods (UV, chlorine, and UV/chlorine) were selected to investigate the increased frequency of conjugation of ARGs during the stage of continuing culture after disinfection. The results showed that the conjugative transfer frequency was inhibited for all disinfection treatments after 24 h of co-incubation. Unexpectedly, after 3-7 days of co-cultivation, the HGT frequencies were increased by 2.71-5.61-fold and 5.46-13.96-fold in chlorine (30 min) and UV/chlorine (1 min) groups compared to the control, but not in UV-irradiated groups. A neglected lagging response was found for the first time, i.e., oxidative disinfection-induced dormancy promotes conjugative transfer of ARGs. Furthermore, mechanistic insights were gained from (1) membrane permeability, (2) conjugation-regulated system, (3) efflux pump system, and (4) oxidative stress system, suggesting the critical role of enhancing efflux and oxidative stress in the propagation of ARGs. Finally, the known instantaneous effect of oxidation disinfection was compared to address the controversial debate in this research field, proposing that the dormancy level of donor bacteria is the key to evaluating whether it can promote the HGT process. This study has important environmental implications for elucidating the transmission of ARGs after oxidation disinfection.

Response of FANIR system to starvation stress: "Dormancy"

Anaerobic ammonium oxidation (Anammox) process has been successfully applied in the nitrogen removal from high-strength wastewaters. However, little information is available for its treatment of low-strength wastewaters. In this study, a Famine Anammox NItrogen Removal (FANIR) system was developed to investigate the effect of long-term substrate starvation at the low nitrogen concentration (the influent total nitrogen was set at ∼1 mg/L). The results showed that the response of FANIR system to the starvation stress took on two phases: the functional decline phase (0-54 day) and the functional stabilization phase (62-116 day). Over the two phases, the Nitrogen Removal Rate (NRR) of anammox reactor firstly dropped sharply; and then came to a constant level. The activity and settleability of Anammox Granular Sludge (AnGS) firstly deteriorated seriously, and then stayed in a stable range. The relative abundance of Anaerobic Ammonium Oxidation Bacteria (AnAOB) firstly decreased markedly, and then approached a steady state with the change of dominant genus from Candidatus Brocadia to Candidatus Kuenenia. The abundance of 16S rRNA gene and hzs gene of AnAOB and their transcription level firstly declined largely as well, and then became stable with the 16S rRNA gene, hzs gene, 16S rRNA and hzs-mRNA of AnAOB at 23.9%, 9.1%, 1.2% and 1.0% of the initial value, respectively. To our delight, the behavior of FANIR system in the functional stabilization phase was proved indeed consistent with the feature for AnAOB to enter the dormancy state. These findings are helpful to understand the physiology of AnAOB over the starvation stress and to promote the extension of anammox process to the treatment of low-strength wastewaters.