Research Overview
A study conducted by Taicheng An's team at Guangdong University of Technology, published in Biocontaminant on December 8, 2025 (DOI: 10.48130/biocontam-0025-0017), indicates that environmental stressors, specifically those from incomplete water disinfection, can increase the efficiency with which surviving bacteria acquire antibiotic resistance genes (ARGs). This mechanism potentially influences the spread of antibiotic-resistant bacteria in aquatic environments.
Context of Antibiotic Resistance in Aquatic Systems
Antibiotic resistance genes and antibiotic-resistant bacteria are identified as environmental contaminants present in rivers, lakes, wastewater, and oceans. Aquatic systems provide conditions for ARGs to persist, interact, and disseminate among microorganisms. Bacteria engage in horizontal gene transfer, including transformation, where cells 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 been a subject of ongoing investigation. Modern water treatment systems employ advanced oxidation and light-based technologies. However, fluctuations in treatment efficiency can result in bacteria being stressed but not entirely inactivated. Understanding how these sub-lethal conditions impact ARG transfer is relevant for public health considerations.
Experimental Design and Findings
The study employed a sub-lethal photocatalysis (sub-PC) system to simulate conditions of incomplete water disinfection. This system was used to evaluate the influence of oxidative stress on the transformation of ARGs.
- Two antibiotic-sensitive Escherichia coli recipient strains, DH5α and HB101, were exposed to sub-PC conditions.
- Bacterial inactivation, physiological stress responses, and ARG uptake (using a plasmid containing the ampicillin resistance gene, amp) were assessed.
- Under sub-PC exposure, bacterial abundances decreased by approximately 2 log over 120 minutes, with nearly 10% of cells remaining viable. This viable population was available for horizontal gene transfer via transformation.
- Intracellular reactive oxygen species (ROS) levels increased three- to four-fold above baseline during the initial 0–60 minutes, indicating oxidative stress activation. Antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD) were also significantly induced.
- As treatment progressed, ROS, CAT, and SOD levels declined, consistent with cellular damage and lysis.
- Following plasmid uptake, ampicillin-resistant transformants exhibited enhanced persistence under sub-PC, showing only a ~1 log reduction in abundance. This suggests that ARG acquisition can improve stress tolerance.
- Optimization experiments determined that transformation was most efficient at 37 °C and required high recipient densities, with maximal transformant yields occurring at 10⁸–10⁹ CFU·mL⁻¹. A density of 10⁸ CFU·mL⁻¹ was selected for robust quantification.
- Under these conditions, transformation frequencies increased three- to four-and-a-half-fold, peaking between 50 and 60 minutes before decreasing as cellular damage accumulated.
- Mechanistic analyses confirmed ROS as a key driver; ROS scavengers significantly attenuated, but did not eliminate, the enhancement effect.
- Sub-PC exposure also increased bacterial membrane permeability, elevated intracellular Ca²⁺ nearly four-fold, and depleted ATP, which contributed to Ca²⁺ accumulation by limiting its efflux.
- Gene expression profiling supported these observations, showing early upregulation of genes related to stress response, antioxidant defenses, membrane transport, and DNA uptake, alongside repression of energy metabolism pathways.
Conclusion
The findings indicate that partially effective disinfection in water treatment systems can enhance the spread of antibiotic resistance. Sub-lethal stress not only allows bacteria to survive but also increases their capacity to acquire resistance genes from their environment. This mechanism may contribute to the persistence and amplification of antibiotic resistance in wastewater effluents, surface waters, and downstream ecosystems.