A treatment designed to make contaminated water safer may, under certain conditions, help antibiotic resistance genes move more efficiently through microbial communities. That is the warning from a study by Zhou, Pan, Ma and colleagues, which examines an unexpected consequence of peroxymonosulfate-based water treatment. The researchers report that humic acid—a complex mixture of natural organic compounds found widely in rivers, lakes, groundwater and wastewater—can protect genetic material from chemical destruction. Rather than simply reducing the effectiveness of the treatment, this sheltering effect may create conditions in which antibiotic resistance genes remain intact long enough to be acquired by other microorganisms. The finding highlights a difficult paradox in environmental engineering: a process that removes chemical pollutants may simultaneously influence the biological spread of resistance.
Peroxymonosulfate is an increasingly important advanced oxidation reagent. When activated by heat, transition metals, carbon-based catalysts, ultraviolet light or other methods, it produces highly reactive chemical species, including sulfate radicals, hydroxyl radicals and, depending on the reaction environment, singlet oxygen and other oxidants. These species attack persistent contaminants by breaking chemical bonds and converting complex molecules into smaller, less harmful products. Because peroxymonosulfate can operate across a range of water-treatment conditions and is effective against many difficult pollutants, it has attracted attention as an alternative or complement to conventional treatment technologies. Yet the same reactive chemistry that destroys organic contaminants may not affect extracellular DNA, intracellular DNA or microbial cells in a uniform way.
Antibiotic resistance genes are fragments of genetic information that enable bacteria to survive exposure to antibiotics. They may be located on bacterial chromosomes, but many are carried by mobile genetic elements such as plasmids, integrons and transposons. These elements can move between cells through horizontal gene transfer, allowing resistance traits to spread without requiring generations of bacterial reproduction. Transformation occurs when cells take up free DNA from their surroundings; conjugation involves direct cell-to-cell transfer, often through plasmids; and transduction uses bacteriophages as genetic vehicles. Water-treatment systems can contain all of these potential routes in some form, particularly when wastewater, hospital discharges, agricultural runoff or sewage-derived organic matter enters the treatment stream.
The new work focuses on what happens when humic acid is present during peroxymonosulfate treatment. Humic acid is not a single molecule but a chemically diverse collection of aromatic, oxygen-rich and sometimes highly reactive structures produced as plants and microorganisms decompose. It can absorb light, bind metals, interact with pollutants and alter the formation and lifetime of reactive oxygen and sulfur species. In water, humic acid may also associate with cells, membrane fragments, colloids and DNA, creating microscale environments that differ substantially from the surrounding solution. These interactions are important because the fate of genetic material is determined not only by the bulk concentration of an oxidant, but also by whether the DNA is exposed, adsorbed, encapsulated or physically shielded.
According to the study, humic acid’s role is paradoxical. It can consume part of the oxidative capacity generated during peroxymonosulfate activation, potentially reducing the direct attack on resistance genes. At the same time, its large, chemically heterogeneous molecules may surround or bind genetic material and microbial components. This protection can reduce oxidative damage to DNA and preserve the structural integrity required for gene uptake or transfer. In other words, humic acid may act as an antioxidant at one scale and a molecular shelter at another. The result is not necessarily a simple increase in the total amount of DNA, but an increase in the fraction that remains biologically functional after treatment.
That distinction is central to the public-health implications of the research. Standard monitoring often measures the concentration of antibiotic resistance genes using molecular techniques such as quantitative polymerase chain reaction. These methods can detect DNA sequences even when the genes are damaged and no longer capable of functioning. A lower signal after oxidation may therefore be interpreted as successful removal, while residual fragments could still matter if they remain sufficiently intact to be taken up by competent bacteria. Conversely, a treatment could leave a measurable DNA signal without preserving its biological activity. The study’s significance lies in connecting chemical persistence with the possibility of dissemination, emphasizing that the critical question is not only whether resistance genes can be detected, but whether they remain transferable.
The researchers’ findings suggest that treatment chemistry may reshape the risk landscape rather than eliminate it outright. Peroxymonosulfate can damage bacterial cells and degrade free-floating DNA, but humic acid may alter the balance between destruction and protection. If resistance genes survive within organic coatings, aggregates or cell-associated structures, they could persist through treatment and enter receiving waters, sediments or microbial communities downstream. Even a small surviving pool may become important if it encounters bacteria carrying compatible plasmids or other mobile elements. Environmental conditions such as pH, dissolved oxygen, ionic strength, metal content, organic-matter concentration and the type of catalyst used to activate peroxymonosulfate could all influence the outcome.
The work also exposes why antibiotic resistance cannot be treated solely as a clinical or pharmaceutical problem. Resistance genes circulate through interconnected human, animal and environmental systems. Wastewater treatment plants can remove many pathogens and contaminants, yet they also serve as meeting points where bacteria, extracellular DNA, antibiotics, disinfectants and organic matter are concentrated. Chemical treatments may reduce some hazards while changing the biological availability of others. Humic substances are especially relevant because they are common in natural waters and may be introduced or concentrated through sludge processing, agricultural drainage and the breakdown of plant material. A treatment strategy tested in clean laboratory water may therefore behave differently in real waters rich in natural organic matter.
The findings do not mean that peroxymonosulfate treatment should be abandoned. Instead, they point to the need for more precise design and monitoring. Engineers may need to measure not only the disappearance of resistance-gene sequences, but also their integrity, cellular location and ability to support transformation or conjugative transfer. Treatment conditions could be adjusted to account for dissolved organic matter, and combinations of oxidation with membrane filtration, adsorption, biological treatment or carefully controlled disinfection could reduce the chance that protected genetic material escapes. Catalysts and operating conditions might also be selected to maximize destruction of DNA while minimizing the formation of protective organic microenvironments. Such decisions will require experiments that reproduce the complexity of actual wastewater rather than relying exclusively on simplified solutions.
The broader message is that advanced water treatment can have unexpected biological consequences when chemistry and microbial ecology intersect. Humic acid is often regarded as an interfering background substance, something that consumes oxidants and makes contaminant removal more difficult. Zhou, Pan, Ma and colleagues show why that view may be incomplete: the material can also determine whether antibiotic resistance genes remain chemically intact and biologically mobile. As water scarcity drives greater reuse and as treatment systems increasingly rely on powerful oxidation technologies, understanding these hidden pathways will become more urgent. The study turns a familiar ingredient of natural waters into a warning signal for environmental health: destroying pollutants is not enough if the treatment leaves behind genetic instructions that bacteria can still read, copy and share.
Subject of Research: The effect of humic acid on the dissemination of antibiotic resistance genes during peroxymonosulfate-based water treatment.
Article Title: Humic acid’s sheltering effect paradoxically promotes antibiotic resistance gene dissemination during peroxymonosulfate water treatment.
Article References: Zhou, F., Pan, S., Ma, C. et al. Humic acid’s sheltering effect paradoxically promotes antibiotic resistance gene dissemination during peroxymonosulfate water treatment. Nature Communications (2026). https://doi.org/10.1038/s41467-026-76727-6
Image Credits: AI Generated
DOI: 10.1038/s41467-026-76727-6
Keywords: Antibiotic resistance genes, humic acid, peroxymonosulfate, water treatment, advanced oxidation processes, horizontal gene transfer, environmental microbiology, wastewater treatment.
