Incomplete water disinfection may accelerate the spread of antibiotic resistance genes

Incomplete water disinfection may unintentionally accelerate the spread of antibiotic resistance genes in aquatic environments, according to a new study published in Biocontaminant.

Researchers found that sub lethal photocatalysis, a condition that can occur when water treatment does not fully inactivate bacteria, significantly increases the likelihood that bacteria will acquire antibiotic resistance genes from their surroundings. The findings raise important questions about how water disinfection practices may influence the global rise of antibiotic resistant bacteria.

Antibiotic resistance genes are widely present in rivers, lakes, wastewater, and other aquatic systems. These genes can move between bacteria through a process known as horizontal gene transfer, allowing even previously harmless bacteria to become resistant to antibiotics. One such pathway is transformation, in which bacteria take up free DNA from their environment.

While transformation has been studied under normal conditions, little was known about how environmental stress affects this process. To address this gap, the research team simulated incomplete disinfection by exposing bacteria to sub lethal photocatalysis, a light based treatment commonly explored for water purification.

“Our results show that when bacteria are exposed to stress levels that are not strong enough to kill them, they may actually become more likely to acquire antibiotic resistance genes,” said corresponding author Taicheng An. “This is an unintended consequence that deserves serious attention.”

The team studied two commonly used antibiotic sensitive strains of Escherichia coli and exposed them to photocatalytic treatment strong enough to damage cells but not eliminate them. They then introduced a plasmid carrying a gene that confers resistance to ampicillin, a widely used antibiotic.

The results were striking. Under sub lethal photocatalytic stress, the frequency of gene transformation increased by three to four and a half times compared with untreated bacteria. Nearly ten percent of the bacteria survived the treatment in a weakened but still active state, providing an opportunity for antibiotic resistance genes to be taken up and expressed.

Further experiments revealed why this happens. Sub lethal stress triggered a surge in reactive oxygen species inside bacterial cells, creating oxidative stress. In response, bacteria activated their antioxidant defense systems and altered the permeability of their cell membranes. These changes made it easier for foreign DNA to enter the cell.

The researchers also observed a sharp rise in intracellular calcium levels and a decrease in cellular energy stores, both of which are known to influence stress responses and gene uptake. Together, these physiological changes created favorable conditions for transformation to occur.

“Stress does not just damage bacteria,” said An. “It also activates survival mechanisms that can unintentionally promote the spread of resistance.”

Importantly, once bacteria acquired the resistance gene, they became more resilient to further photocatalytic stress than non resistant bacteria. This suggests that incomplete disinfection may not only facilitate the spread of resistance genes but also select for bacteria that are better able to survive future treatment.

The findings have direct implications for water treatment systems, particularly wastewater treatment plants where fluctuating conditions can lead to incomplete disinfection. While advanced technologies like photocatalysis hold promise for sustainable water treatment, the study highlights the importance of ensuring that such methods achieve fully lethal conditions.

“Antibiotic resistance is a global public health threat, and water environments play a critical role in its spread,” said An. “Our study emphasizes that disinfection strategies must be carefully designed and managed to avoid creating conditions that favor gene transfer.”

The researchers hope their work will inform the development of safer and more effective water treatment practices, helping to reduce the environmental dissemination of antibiotic resistance genes before they reach human and ecological systems.

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Journal reference: Sun T, Ji H, Cai Y, Li G, Wong PK, et al. 2025. Enhanced transformation mechanisms of antibiotic resistance genes in water under the stress of sub-lethal photocatalysis. Biocontaminant 1: e017  

https://www.maxapress.com/article/doi/10.48130/biocontam-0025-0017 

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About Biocontaminant:
Biocontaminant is a multidisciplinary platform dedicated to advancing fundamental and applied research on biological contaminants across diverse environments and systems. The journal serves as an innovative, efficient, and professional forum for global researchers to disseminate findings in this rapidly evolving field.

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