Sub-lethal water disinfection can unintentionally boost the spread of antibiotic resistance

The study reveals that environmental stressors do not merely kill bacteria; they can also prime surviving cells to take up resistance genes more efficiently, raising concerns about how antibiotic-resistant bacteria may spread in aquatic environments.

Antibiotic resistance genes and antibiotic-resistant bacteria are now recognized as emerging environmental contaminants, widely detected in rivers, lakes, wastewater, and even oceans. Aquatic systems provide ideal conditions for resistance genes to persist, interact, and spread among microorganisms. Bacteria exchange genetic material through horizontal gene transfer, including transformation, a process in which cells directly absorb free DNA from their surroundings. While transformation is known to contribute to resistance dissemination, its behavior under realistic environmental stress-such as incomplete disinfection-has remained poorly understood. Modern water treatment increasingly relies on advanced oxidation and light-based technologies, yet fluctuations in treatment efficiency can leave bacteria alive but stressed rather than fully inactivated. Understanding how these sub-lethal conditions influence ARG transfer is critical for public health protection.

A study (DOI:10.48130/biocontam-0025-0017) published in Biocontaminant on 08 December 2025 by Taicheng An's team, Guangdong University of Technology, reveals that sub-lethal water disinfection can unintentionally accelerate the spread of antibiotic resistance by promoting stress-induced uptake of resistance genes in surviving bacteria.

Using a sub-lethal photocatalysis (sub-PC) system to simulate incomplete water disinfection, this study systematically evaluated how oxidative stress influences the transformation of ARGs. Two antibiotic-sensitive recipient strains, Escherichia coli DH5α and E. coli HB101, were exposed to sub-PC conditions and assessed for bacterial inactivation, physiological stress responses, and ARG uptake using a plasmid carrying the ampicillin resistance gene (amp). Under identical sub-PC exposure, bacterial abundances declined gradually by approximately 2 log after 120 min, yet nearly 10% of cells remained viable, providing a sufficient pool for horizontal gene transfer via transformation. Correspondingly, intracellular reactive oxygen species (ROS) levels increased markedly during the early phase (0–60 min), reaching three- to fourfold higher than baseline, while antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD) were strongly induced, indicating activation of oxidative stress defenses. As treatment progressed, excessive damage led to declining ROS, CAT, and SOD levels, consistent with cell lysis and leakage. Following plasmid uptake, ampicillin-resistant transformants exhibited enhanced persistence under sub-PC, showing only a ~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; maximal transformant yields occurred at 10⁸–10⁹ CFU·mL⁻¹, with 10⁸ CFU·mL⁻¹ selected for robust quantification. Under these optimal conditions, transformation frequencies increased three- to four-and-a-half-fold, peaking at 50–60 min before declining as cellular damage accumulated. Mechanistic analyses showed that ROS scavengers significantly weakened, but did not abolish, the enhancement effect, confirming ROS as a key driver. Sub-PC also increased membrane permeability, elevated intracellular Ca²⁺ nearly fourfold, and depleted ATP, limiting Ca²⁺ efflux and reinforcing its accumulation. Gene expression profiling corroborated these trends, showing early upregulation of stress response, antioxidant, membrane transport, and DNA uptake genes, alongside repression of energy metabolism pathways.

The findings highlight a critical but underappreciated risk in water treatment systems: partially effective disinfection may promote, rather than prevent, the spread of antibiotic resistance. Sub-lethal stress not only allows bacteria to survive but actively enhances their capacity to acquire resistance genes from the environment. This mechanism could contribute to the persistence and amplification of antibiotic resistance in wastewater effluents, surface waters, and downstream ecosystems.