Antibiotic resistance stands among the most pressing public health threats of the 21st century, with the World Health Organization warning that it could undermine decades of medical progress. While much attention has focused on clinical overuse of antibiotics, the environmental dimension of this crisis is equally alarming. Water systems—rivers, lakes, groundwater, and treated wastewater—serve as major reservoirs and transmission pathways for antibiotic resistance genes (ARGs). These genetic elements can persist through conventional water treatment and spread among bacterial communities, accelerating the emergence of multidrug-resistant pathogens. Against this backdrop, ozone (O3) treatment has emerged as a powerful, oxidant-based approach capable not merely of inactivating bacteria but of directly degrading the DNA molecules that encode resistance. This expanded analysis explores the science behind ARGs, the mechanisms of ozone action, current research evidence, practical implementation challenges, and the promise of ozone-based strategies for safeguarding water quality and public health.

Understanding Antibiotic Resistance Genes in the Water Environment

Antibiotic resistance genes are segments of DNA that confer upon bacteria the ability to survive exposure to antibiotics. They encode proteins that deactivate drugs, pump them out of the cell, alter the drug target, or bypass the antibiotic’s effect. ARGs can reside on the bacterial chromosome or on mobile genetic elements such as plasmids, transposons, and integrons. This mobility is the critical factor: horizontal gene transfer allows ARGs to spread across different bacterial species and genera, even between environmental bacteria and human pathogens. In water systems, high bacterial densities, nutrient availability, and sub‑inhibitory concentrations of antibiotics from agricultural runoff or pharmaceutical waste create ideal conditions for this transfer.

Common ARGs found in water include tet (tetracycline resistance), sul (sulfonamide resistance), bla (beta‑lactam resistance, including extended‑spectrum beta‑lactamases), and qnr (fluoroquinolone resistance). These genes can persist in biofilms, suspended solids, and even in free extracellular DNA after cell lysis. Conventional water treatment processes—coagulation, sedimentation, filtration, and chlorination—can reduce bacterial loads but often fail to eliminate extracellular ARGs or prevent their uptake by surviving bacteria. Chlorination, in fact, may select for resistant strains and can generate disinfection byproducts that induce stress responses, potentially increasing horizontal gene transfer rates. This inadequacy underscores the need for advanced oxidation processes that target both cellular and extracellular genetic material.

Ozone as an Advanced Oxidant: Mechanisms of Action Against ARGs

Ozone is a triatomic molecule (O3) that acts as one of the strongest commercially available oxidants. Its standard reduction potential (2.07 V) is exceeded only by fluorine and the hydroxyl radical. In water treatment, ozone reacts with organic and inorganic compounds either directly as molecular ozone or indirectly via the formation of hydroxyl radicals (•OH) under alkaline conditions. This dual reactivity is key to its efficacy against ARGs. The mechanisms can be grouped into three interrelated pathways:

Direct Oxidation of DNA and RNA

Ozone reacts rapidly with DNA components, particularly the guanine bases, which are the most susceptible to oxidation. Attack on guanine leads to ring opening, strand breaks, and cross‑linking, ultimately rendering the DNA fragment non‑functional. For ARGs, even partial oxidation of the gene sequence can eliminate its ability to express resistance. Studies using quantitative PCR have shown that ozonation reduces the copy number of specific ARGs by several orders of magnitude after short contact times. Importantly, ozone can degrade both intracellular ARGs (inside bacterial cells) and extracellular ARGs (free DNA in the water matrix).

Disruption of Bacterial Cells and Prevention of Gene Transfer

Ozone damages bacterial cell membranes and walls through oxidation of fatty acids, lipoproteins, and peptidoglycan. This damage leads to leakage of cellular contents, lysis, and cell death. By killing donor and recipient bacteria, ozone interrupts the primary route of horizontal gene transfer. Furthermore, even sub‑lethal ozone exposure can impair the competence of bacteria to take up free DNA (natural transformation) and can degrade the pili or flagella involved in conjugation. This multi‑level attack ensures that both the gene pool and the transfer machinery are compromised.

Degradation of Organic Matter and Reduction of Transfer Vectors

Ozone also oxidizes dissolved organic matter, including humic substances and extracellular polymeric substances that form biofilms. These organic matrices often facilitate the persistence and transfer of ARGs by providing surfaces for bacterial attachment and protection from disinfection. By breaking down these matrices, ozone reduces the “safe harbor” for ARGs and diminishes the environmental factors that promote gene exchange. Additionally, ozone can degrade co‑selecting agents such as heavy metals and trace antibiotics, further reducing selective pressure for resistance.

Research Evidence: Ozone’s Effectiveness in Reducing ARGs

A growing body of peer‑reviewed research supports ozone’s ability to significantly lower ARG concentrations in diverse water matrices. A comprehensive study on secondary wastewater effluent found that ozone doses of 0.5–1.0 mg O3/mg dissolved organic carbon achieved >90% reduction in common ARGs such as sul1, tetG, and blaOXA. The reduction was dose‑dependent, with higher ozone exposures correlating with lower residual gene copies. Another investigation into hospital wastewater—a hot spot for antibiotic resistance—reported that ozone treatment reduced both intracellular and extracellular ARGs by up to 4‑log10 after a contact time of 10 minutes. The efficacy extended to mobile genetic elements like integron‑integrase genes (intI1), which are proxy markers for horizontal gene transfer potential.

Importantly, studies have also examined the regrowth potential post‑ozonation. While some bacteria can repair oxidative damage, many researchers found that when sufficient ozone residual is maintained or followed by a secondary treatment (e.g., chloramine), regrowth is minimal and ARG levels remain suppressed over days. A World Health Organization fact sheet on antimicrobial resistance acknowledges that environmental interventions like improved water treatment are critical components of a “One Health” approach.

Comparative studies place ozone among the most effective technologies. UV irradiation, while effective against bacteria, requires high doses to damage DNA and is less effective against extracellular ARGs because of UV shielding by particles. Chlorination can reduce cellular ARGs but may increase the abundance of certain resistance genes due to selection. Advanced oxidation processes (e.g., O3/H2O2, UV/H2O2) often show synergistic improvements, but ozone alone already outperforms many conventional methods on a per‑dose basis. For a detailed review of ARG removal across treatments, the study by Czekalski et al. (2020) provides a quantitative meta‑analysis.

Challenges and Practical Considerations for Ozone Implementation

Despite its strong performance, deploying ozone for ARG reduction at scale presents several challenges. First, ozone generation requires significant energy input—corona discharge or electrolytic methods consume electricity, and the cost can be a barrier for smaller treatment plants. Second, ozone is a hazardous gas: proper containment, monitoring, and destruction systems (catalytic or thermal) are mandatory to protect operators and the surrounding community. Third, ozone’s effectiveness is matrix‑dependent. High levels of suspended solids, alkalinity, and organic carbon can increase ozone demand and may require pre‑treatment to achieve cost‑efficient ARG reduction. Nitrite and bromide, when present, can be oxidized to undesirable byproducts such as bromate (EPA regulatory limit for bromate is 10 µg/L in drinking water). Operators must carefully control ozone dose and pH to minimize byproduct formation while maximizing ARG degradation.

Another practical issue is the need for adequate contact time and mass transfer. Ozone is only sparingly soluble in water; therefore, efficient dissolution via bubble columns, venturi injectors, or membrane contactors is essential. The disinfection kinetics of ARGs often follow a lag phase followed by rapid decay, meaning that short‑circuiting or poor mixing can leave pockets of untreated water. Real‑time monitoring of ozone residual and surrogate parameters (absorbance at 254 nm, dissolved organic carbon) can help optimize dosing. Furthermore, the capital investment for ozone equipment—including oxygen feed systems, generators, contactors, and off‑gas destruction units—can be 2–5 times higher than for chlorination systems, though operating costs may be competitive when considering reduced chemical purchases and sludge handling.

Future Directions: Synergistic Technologies and System Integration

Recent research points toward combining ozone with other treatments to overcome its limitations and enhance ARG removal. Ozone followed by biological activated carbon (BAC) filtration, for example, can remove oxidation byproducts while further degrading residual organic matter that may harbor ARGs. The combination of ozone with hydrogen peroxide (the peroxone process) generates hydroxyl radicals more rapidly, improving the degradation of recalcitrant genes at lower ozone doses. Similarly, ozone followed by UV irradiation can provide a multi‑barrier approach: ozone attacks DNA chemically, while UV causes photochemical damage. These sequential processes are being evaluated at pilot scale in several European water reclamation facilities.

From a policy perspective, incorporating ARG reduction targets into water quality guidelines could accelerate adoption of advanced treatments. Currently, few regulatory frameworks specify limits for antibiotic resistance genes, but the European Union’s Water Framework Directive and the U.S. EPA’s Contaminant Candidate List are starting to consider resistance markers as indicators of treatment performance. Ozone’s ability to simultaneously inactivate pathogens, reduce micro‑pollutants, and degrade ARGs positions it as a versatile technology for future water reuse and drinking water safety plans. Continued investment in ozone generation efficiency, byproduct control, and real‑time monitoring will be essential to make these solutions accessible to utilities worldwide.

Conclusion

Antibiotic resistance genes in water systems represent a hidden but formidable threat to public health, enabling the spread of resistant infections and undermining the efficacy of last‑line antibiotics. Ozone treatment offers a scientifically validated, dual‑action approach: it directly oxidizes and destroys ARGs in both intracellular and extracellular forms, while simultaneously killing the bacterial hosts that transfer them. Research consistently demonstrates that ozone reduces the abundance of clinically relevant ARGs by orders of magnitude across diverse water matrices, often outperforming chlorine, UV, and other conventional treatments. Yet, challenges remain—cost, safety, matrix variability, and byproduct formation must be addressed through careful engineering and optimization. By integrating ozone into comprehensive water treatment trains, and by coupling it with adjunct processes such as biological filtration or advanced oxidation, utilities can achieve robust removal of resistance genes while meeting other water quality objectives. The path forward requires sustained research, investment in infrastructure, and regulatory support to realize the full potential of ozone as a tool against the silent pandemic of antibiotic resistance.