Chemical and physical stresses faced by bacteria during wastewater treatment can affect gene transfer between them. But researchers at the University of Science and Technology of Cartagena found that while certain combinations of stressors significantly increased the rate of gene transfer, other combinations decreased it. This finding could inform best practice design and management of wastewater treatment and reuse.
Researchers at King Abdullah University of Science and Technology have found that a combination of stressors in wastewater treatment affects bacterial gene transfer rates. Their results showed that microfiltration membranes were more effective than sand filtration in reducing bacterial and eDNA concentrations, thereby minimizing gene transfer. Image source: 2023KAUST; HenoHwang
Globally, many regions consider treated wastewater as a potentially valuable freshwater resource. "As part of the Saudi Vision 2030, there is a need to increase water reuse and treatment rates," said Bothayna Al-Gashgari, a doctoral student in Hong Peiying's group who led the research. "Promoting safe disposal and reuse is vital."
Bacteria can naturally absorb extracellular DNA (eDNA) from their surrounding environment and integrate functional genes into their own genomes. Treated wastewater may contain relatively high concentrations of bacteria and eDNA. It also exposes bacteria to stressors known to enhance eDNA uptake and integration, including ultraviolet light, disinfection chemical by-products, and drugs.
"Several studies have highlighted the potential impact of individual stressors on bacterial horizontal gene transfer in chlorinated wastewater," Al-Gashgari said. "But in real wastewater environments, multiple stressors exist simultaneously. Our aim was to understand the combined impact of these factors."
Treating wastewater for safe reuse provides valuable freshwater resources. Image source: 2023KAUST; HenoHwang
The researchers hypothesized that multiple stressors would have additive effects on gene transfer rates. But surprisingly, the situation is much more complicated. Depending on their mode of action, some combinations synergistically significantly increase gene transfer rates, some have a neutral effect, and others reduce gene transfer rates.
"For example, when a stressor that increases bacterial cell wall permeability (such as the drug carbamazepine) is combined in sequence with a stressor that causes DNA damage (such as sun exposure), the two stressors can have a synergistic effect," Al-Gashgari said. "We also found that if a stressor (such as chloroform) interacts directly with eDNA in a deleterious manner, it blocks the integration of the DNA into the bacterial genome, creating an antagonistic effect."
This complexity makes the combined effects of multiple stressors difficult to predict, complicating the ability to assess whether there may be unintended consequences in the downstream reuse environment. However, the study results have clear conclusions for wastewater treatment.
A key goal in wastewater treatment processes should be to maintain low concentrations of bacteria and eDNA in wastewater, thereby minimizing gene transfer.
"We believe that wastewater treatment facilities should be equipped with microfiltration membranes rather than sand filtration because microfiltration membranes can remove bacteria and extracellular DNA to levels that are detrimental to natural transformation," Hong said. "Microfiltration membranes are more expensive to install and operate than sand filtration, but we urge utilities to take this precaution."