Sublethal water disinfection may inadvertently promote the spread of antibiotic resistance

Sublethal water disinfection may inadvertently promote the spread of antibiotic resistance

The study shows that environmental stressors don't just kill bacteria; They can also prime surviving cells to take up resistance genes more efficiently, raising concerns about the spread of antibiotic-resistant bacteria in waterways.

Antibiotic resistance genes and antibiotic-resistant bacteria are now recognized as emerging environmental contaminants, commonly detected in rivers, lakes, sewage and even oceans. Aquatic systems provide ideal conditions for the survival, interaction and spread of resistance genes among microorganisms. Bacteria exchange genetic material through horizontal gene transfer, including transformation, a process in which cells absorb free DNA directly from their environment. Although transformation is known to contribute to the spread of resistance, its behavior under realistic environmental stresses - such as incomplete disinfection - is poorly understood. Modern water treatment increasingly relies on advanced oxidation and light-based technologies. However, fluctuations in processing efficiency can result in bacteria remaining alive but stressed and not completely inactivated. Understanding how these sublethal conditions influence ARG transmission is critical to protecting public health.

A study (DOI:10.48130/biocontam-0025-0017) published inBiocontaminanton December 8, 2025 by Taicheng An's team, Guangdong University of Technology, shows that sublethal water disinfection can inadvertently accelerate the spread of antibiotic resistance by promoting stress-induced uptake of resistance genes in surviving bacteria.

Using a sublethal photocatalysis (Sub-PC) system to simulate incomplete water disinfection, this study systematically investigated how oxidative stress influences the conversion of ARGs. Two antibiotic-susceptible recipient strains,Escherichia coliDH5α andE.coliHB101 were exposed to sub-PC conditions and assayed for bacterial inactivation, physiological stress responses, and ARG uptake using a plasmid carrying the ampicillin resistance gene (amp). With identical sub-PC exposure, bacterial abundance gradually decreased by about 2 logarithms after 120 min, yet almost 10% of cells remained viable, representing a sufficient pool for horizontal gene transfer by transformation. Accordingly, intracellular reactive oxygen species (ROS) levels increased significantly in the early phase (0–60 minutes), reaching three to four times the baseline value, while the antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD) were strongly induced, indicating activation of defenses against oxidative stress. As treatment progressed, excessive damage led to a decrease in ROS, CAT, and SOD levels, which was associated with cell lysis and leakage. After plasmid uptake, ampicillin-resistant transformants showed increased persistence under sub-PC and only showed an approximately 1 log reduction in abundance, supporting the notion that ARG acquisition improves stress tolerance. Optimization experiments revealed that transformation was most efficient at 37 °C and required high recipient densities; The maximum transformant yield was 10⁸–10⁹ CFU·ml⁻¹, with 10⁸ CFU·ml⁻¹ chosen for robust quantification. Under these optimal conditions, transformation frequencies increased three- to four-and-a-half-fold, peaking at 50–60 minutes before decreasing as cell damage increased. Mechanistic analyzes showed that ROS scavengers significantly attenuated the amplification effect but did not abolish it, confirming that ROS is a key driver. Sub-PC also increased membrane permeability, increased intracellular Ca²⁺ almost fourfold, and decreased ATP, thereby limiting Ca²⁺ efflux and enhancing its accumulation. Gene expression profiling confirmed these trends, showing early upregulation of stress response, antioxidant, membrane transport and DNA uptake genes, as well as suppression of energy metabolic pathways.

The results highlight a critical but underestimated risk in water treatment systems: partially effective disinfection can promote the spread of antibiotic resistance instead of preventing it. Sublethal stress not only allows bacteria to survive, but also actively increases their ability to acquire resistance genes from the environment. This mechanism could contribute to the persistence and amplification of antibiotic resistance in wastewater, surface water and downstream ecosystems.

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