The war against resistant bacteria in wastewater

Multi-mechanism strategies to remove pathogens and antibiotic-related genes

The growing spread of multidrug-resistant (MDR) bacteria and antibiotic resistance genes (ARGs) has emerged as a critical threat to both public health and the environment. These elements, often the result of overuse and misuse of antibiotics, persist in various ecological matrices. They can spread rapidly through water systems, particularly those receiving effluents from hospitals, pharmaceutical industries, and densely populated urban areas.

Wastewater treatment systems, though effective at removing conventional pollutants, are increasingly challenged by the biological complexity of resistant microorganisms and their genetic material. Standard processes often fail to neutralize these agents. As a result, multidrug-resistant (MDR) bacteria and antibiotic resistance genes (ARGs) can survive treatment stages and re-enter natural ecosystems. This creates the potential for them to circulate back to human populations. In addition, commonly used disinfection methods such as chlorination, UV irradiation, and ozonation can generate harmful by-products. More over, they require high energy inputs, and show variable performance depending on water turbidity or sunlight availability.

Why targeting bacteria isn’t enough 

While the threat of antibiotic-resistant bacteria is well known, the persistence of their resistance genes (ARGs) poses a more insidious challenge for conventional treatment technologies. These genetic elements encode traits that allow bacteria to withstand antibiotics and can be horizontally transferred across species, facilitating the spread of resistance even in the absence of the original host organisms.

In wastewater, ARGs are initially present within living or partially inactivated bacterial cells, referred to as intracellular ARGs (iARGs). However, during treatment processes or natural cell lysis, these genes can be released into the surrounding environment, forming extracellular ARGs (eARGs). These free-floating genetic fragments may remain suspended in water or adsorbed onto organic and inorganic particles. Because most disinfection strategies are designed to inactivate whole microbial cells, they often fail to target these extracellular forms. As a result, eARGs can persist in treated effluents and retain the potential to re-enter microbial communities through horizontal gene transfer.

This calls for treatment methods that not only eliminate microbial cells but also disrupt the genetic material responsible for resistance propagation. Technologies capable of producing highly reactive oxygen species (ROS), such as advanced oxidation processes and photocatalytic systems, are among the most promising. They can induce oxidative damage directly to DNA. This effectively breaks down ARGs before they can spread.

ROS-producing nanomaterials 

Photocatalysis is a light-driven process in which a catalyst, typically a semiconductor, absorbs photons and triggers chemical reactions that produce reactive oxygen species (ROS), such as hydroxyl radicals and superoxide anions. These ROS are highly reactive molecules capable of attacking a wide range of biological targets. They damage cellular membranes, denature proteins, and break down nucleic acids, effectively inactivating pathogens and disrupting the genetic material responsible for antibiotic resistance.

Nanomaterials are increasingly used as photocatalysts due to their unique properties at the nanoscale. Their large surface area offers more active sites for chemical reactions, while their enhanced light absorption and chemical stability make them ideal for sustained photocatalytic activity. These advantages translate into higher ROS generation and more efficient pollutant degradation, even under visible light. Materials such as titanium dioxide (TiO₂) and zinc oxide (ZnO) are among the most studied photocatalytic nanomaterials for these reasons, and are already employed in environmental remediation efforts.

The role of hybrid systems 

However, conventional photocatalysis faces limitations, such as low efficiency under visible light and the recombination of photogenerated charges, which can reduce ROS yield. To address this, hybrid disinfection systems often integrate complementary technologies, each contributing synergistically to improved microbial and genetic pollutant removal.

Blue LEDs emit light that aligns with the band gap energy of many photocatalytic nanomaterials. This spectral match boosts the excitation of electrons and facilitates the formation of reactive oxygen species (ROS), while simultaneously reducing electron–hole recombination. As a result, photocatalytic activity increases, particularly under visible-light conditions. Ultrasonication further strengthens the disinfection process through acoustic cavitation, the formation and implosion of microbubbles in the liquid medium. This phenomenon generates extreme local temperatures and pressures, producing shock waves and microjets capable of disrupting microbial cell membranes. Additionally, these high-energy conditions stimulate ROS production, creating a potent chemical–mechanical synergy when coupled with photocatalysis. This dual effect has proven particularly effective in removing pharmaceutical contaminants and multidrug-resistant bacteria from wastewater.

Natural biocides, such as plant-derived terpenes, introduce a third layer of action. Compounds like terpinolene disrupt bacterial membranes and metabolic activity through their lipophilic interactions with the lipid bilayer. When used in tandem with photocatalysis and ultrasonication, these agents significantly accelerate bacterial inactivation while maintaining a low environmental impact. This multi-pronged approach shortens treatment times and minimizes chemical inputs.

Long-term benefits of reusable photocatalysts

One of the most compelling advantages of hybrid disinfection systems lies in the long-term stability and reusability of their photocatalytic nanomaterials. These materials retain high levels of photocatalytic activity even after prolonged exposure to UV or visible light. This property is largely attributed to their robust crystalline structure and resistance to photodegradation. Their chemical and mechanical resilience makes them suitable for use in varied aqueous environments. They can withstand continuous operational stress, such as filtration and agitation, without losing performance.

This inherent durability enables the recovery and reuse of photocatalysts over multiple treatment cycles, minimizing both the environmental impact and the economic cost of the process. Reusability is particularly valuable in large-scale wastewater treatment, where frequent material replacement would be impractical and costly.

From an energy perspective, hybrid systems that combine photocatalysis with technologies like LED irradiation and ultrasonication optimize performance. They do so without demanding excessive energy input. The use of energy-efficient blue LEDs, for instance, reduces reliance on traditional, less sustainable light sources and supports continuous operation under controlled conditions.

Towards large-scale deployment

Looking ahead, hybrid photocatalytic disinfection systems show promise for practical application, particularly in large-scale infrastructure such as municipal and industrial plants. Their modularity allows for adaptable configurations tailored to specific wastewater sources, whether from hospitals, pharmaceuticals, or urban infrastructure. Moreover, the compatibility of hybrid systems with renewable energy sources, such as using solar power to drive LED irradiation, further strengthens their feasibility for sustainable implementation.

However, several challenges must be addressed to enable wide-scale adoption. The potential environmental toxicity of nanomaterials, particularly concerning their long-term accumulation and interactions with ecosystems, remains a critical concern. Regulatory gaps also hinder progress, as most current frameworks are not yet equipped to evaluate or oversee the use of nanostructured materials in water treatment. Economic barriers, including the costs associated with catalyst production, energy consumption, and system maintenance, can limit accessibility, especially in low-resource settings. Furthermore, ensuring consistent efficiency across larger treatment volumes requires optimization in reactor design and operational standardization.

Future research must focus on enhancing catalyst performance through advanced material engineering. This can include constructing Z-scheme heterojunctions or employing selective metal doping. These strategies aim to boost ROS generation while minimizing environmental risks. Continued efforts in refining the integration of photocatalysis with complementary technologies like ultrasonication and natural biocides will be crucial to improving treatment efficacy and energy efficiency.

References: 

• Mandal, T. K. (2024). Nanomaterial-Enhanced Hybrid Disinfection: A Solution to Combat Multidrug-Resistant Bacteria and Antibiotic Resistance Genes in Wastewater. Nanomaterials, 14(22), 1847.
• Rezai, B., & Allahkarami, E. (2021). Wastewater treatment processes—Techniques, technologies, challenges faced, and alternative solutions. In R. R. Karri, G. Ravindran, & M. H. Dehghani (Eds.), Soft computing techniques in solid waste and wastewater management (pp. 35–53). Elsevier
• Wang, J., & Chen, H. (2020). Catalytic ozonation for water and wastewater treatment: Recent advances and perspective. Science of The Total Environment