Researchers from KAUST have found that the combination of various stressors in wastewater treatment can affect the rates at which bacteria transfer genes among themselves. According to their research, microfiltration membranes are superior to sand filtration in limiting both bacterial and extracellular DNA (eDNA) concentrations, thereby reducing the likelihood of gene transfer. Credit: © 2023 KAUST; Heno Hwang
Complex factors govern the dissemination of antibiotic-resistant genes during the treatment and recycling of wastewater.
The interaction of chemical and physical stressors affecting bacteria throughout the wastewater treatment process can either accelerate or decelerate the rate of gene exchange between bacterial cells. A recent study by KAUST researchers reveals that while some stressor combinations notably augment the gene-transfer rate, others serve to decrease it. These insights could guide the optimal design and operation of wastewater treatment facilities, particularly for water reuse applications.
In a global context where treated wastewater is increasingly viewed as a precious source of freshwater, Bothayna Al-Gashgari, a Ph.D. student in Peiying Hong’s research group, stated that aligning with the objectives of Saudi Vision 2030 requires enhancement of water reuse and treatment. Ensuring the safety of these processes is of utmost importance, she emphasized.
Natural Assimilation of Extracellular DNA by Bacterial Cells
Bacteria have the innate ability to absorb eDNA from their environment and incorporate the functional genes contained within it into their genetic structure. Wastewater that has undergone treatment can have relatively high concentrations of both bacteria and eDNA. Additionally, such water exposes bacteria to stressors that may facilitate eDNA uptake and integration. These include ultraviolet light, byproducts of disinfection chemicals, and pharmaceutical substances.
Bothayna Al-Gashgari elaborates that while previous studies have examined the influence of individual stressors in chlorinated wastewater on bacterial gene transfer, the current research aims to understand the cumulative impact of these stressors.
Unanticipated Outcomes of Combined Stressors
Originally, the researchers hypothesized that the effect of multiple stressors on gene-transfer rates would be cumulative. However, the actual findings were more intricate. Depending on their respective modes of action, some combinations resulted in a synergistic increase in gene-transfer rates, some had a neutral effect, and others led to a decrease in the rate.
For instance, when a pharmaceutical known to increase bacterial cell wall permeability, such as carbamazepine, was combined with a stressor causing DNA damage, like solar irradiation, the result was a synergistic effect. Conversely, if one stressor adversely interacts with eDNA—like chloroform—the result can hinder DNA integration into the bacterial genome, exhibiting an antagonistic effect.
This multi-layered influence of combined stressors complicates predictions and risk assessments regarding unintended consequences in wastewater treatment and downstream reuse, notes Peiying Hong.
Guidelines for Optimizing Wastewater Treatment
The primary objective of wastewater treatment should focus on minimizing bacterial and eDNA concentrations to limit gene transfer. Hong argues that wastewater treatment plants should consider adopting microfiltration membranes instead of sand filtration. Although this option may incur higher operational costs, it would effectively mitigate the risks associated with gene transfer.
Reference: “Impact of chemicals and physical stressors on horizontal gene transfer via natural transformation” by Bothayna Al-Gashgari, David Mantilla-Calderon, Tiannyu Wang, Maria de los Angeles Gomez, Fras Baasher, Daniele Daffonchio, Taous-Meriem Laleg-Kirati, and Pei-Ying Hong, published on 13 July 2023, in Nature Water.
DOI: 10.1038/s44221-023-00110-8
