Regrowth of bacteria after light-based disinfection - What we know and where we go from here

Advanced photocatalytic disinfection mechanisms and their challenges

Currently, microbial contamination issues have globally brought out a huge health threat to human beings and animals. To be specific, microorganisms including bacteria and viruses display durable ecological toxicity and various diseases to aquatic organisms. In the past decade, the photocatalytic microorganism inactivation technique has attracted more and more concern owing to its green, low-cost, and sustainable process. A variety kinds of photocatalysts have been employed for killing microorganisms in the natural environment. However, two predominant shortcomings including low activity of photocatalysts and diverse impacts of water characteristics are still displayed in the current photocatalytic disinfection system. So far, various strategies to improve the inherent activity of photocatalysts. Other than the modification of photocatalysts, the optimization of environments of water bodies has been also conducted to enhance microorganisms inactivation. In this mini-review, we outlined the recent progress in photocatalytic sterilization of microorganisms. Meanwhile, the relevant methods of photocatalyst modification and the influences of water body characteristics on disinfection ability were thoroughly elaborated. More importantly, the relationships between strategies for constructing advanced photocatalytic microorganism inactivation systems and improved performance were correlated. Finally, the perspectives on the prospects and challenges of photocatalytic disinfection were presented. We sincerely hope that this critical mini-review can inspire some new concepts and ideas in designing advanced photocatalytic disinfection systems.

Solar/ClO2 system inactivates fungal spores in drinking water: Synergy, efficiency and mechanisms

The risk of fungal pollution in drinking water has been paid attention. Solar/chlorine dioxide (ClO2) combined system is an environment-friendly, economical and efficient disinfection method, especially for countries and regions that are economically backward and still exposed to unsafe drinking water. In this paper, the kinetics, influencing factors, mechanism and regrowth potential of inactivated Aspergillus niger (A. niger) spores by solar/ClO2 were reported for the first time. The inactivation curve can be divided into three stages: instant inactivation within 1-2 min, slow linear inactivation and finally a tail. The synergistic factors produced by solar/ClO2 in terms of log reduction and maximum inactivation rate were 1.194 and 1.112, respectively. The inhibitory effect on the regrowth of A. niger spores inactivated by solar/ClO2 was also stronger than that by ClO2 alone. Strongly oxidizing reactive species produced by solar/ClO2 accelerated the accumulation of endogenic reactive oxygen species (ROS) caused by oxidation stress of A. niger spores, improving the inactivation ability of the system. The inactivation order of A. niger spores was: loss of culturability, accumulation of intracellular ROS, loss of membrane integrity, leakage of intracellular species and change of morphology. The inactivation performance of solar/ClO2 was better than solar/chlor(am)ine according to the comparison of inactivation efficiency and regrowth potential. Results also suggested that solar/ClO2 process was more suitable for the treatment of ground water sources.

Application of UV LEDs to inactivate antibiotic resistant bacteria: Kinetics, efficiencies, and reactivations

Unregulated antibiotic use has led to the proliferation of antibiotic-resistant bacteria (ARB) in aquatic environments. Ultraviolet light-emitting diodes (UV LEDs) have evolved as an innovative technology for inactivating microorganisms offering several advantages over traditional mercury lamps. This research concentrated on utilizing UV LEDs with three distinct wavelengths (265 nm, 275 nm, and 285 nm) to inactivate E. coli DH10β encoding the ampicillin-resistant blaTEM-1 gene in its plasmid. Non-linear models, such as Geeraerd's and Weibull, provided more accurate characterization of the inactivation profiles than the traditional log-linear model due to the incorporation of both biological mechanisms and a deterministic approach within non-linear models. The inactivation rates of ARB were higher than antibiotic-sensitive bacteria (ASB) when subjected to UV LEDs. The highest inactivation rates were observed when all microorganisms were exposed to 265 nm. Photoreactivation emerged as the primary mechanism responsible for repairing DNA damage induced by UV LEDs. 285 nm showed the highest reactivation efficiencies for ARB under different fluences. At higher fluences, both 265 and 275 nm displayed similar effectiveness in suppressing reactivation, while at lower fluences, 275 nm exhibited better efficacies in controlling the reactivation. Therefore, the inhibition of reactivation was influenced by the extent of damage incurred to both DNA and enzymes. In nutrient-poor media (0.9 % NaCl), ASB did not exhibit any reactivation potential. However, the addition of Luria-Bertani (LB) broth promoted the reactivation of ASB. Lower fluence rate was more beneficial at 265 nm whereas higher fluence rates were more effective for longer wavelengths. The inactivation of ARB was enhanced by dissolved organic carbon (DOC) at low fluences. However, the removal of ARB was reduced due to the presence of DOC at higher fluences. The highest energy demand for ARB inactivation was reported at 285 nm. ENVIRONMENTAL IMPLICATION: The excessive and unregulated utilization of antibiotics has emerged as a significant issue for public health. This paper presents a comprehensive analysis of the effectiveness of UV LEDs, an emerging technology, in the inactivation of antibiotic-resistant bacteria (ARB). This research paper explores the kinetics of UV LEDs with different wavelengths to inactivate ARB along with the reactivation efficiencies. This research work also explores the impact and relevant mechanisms of the impact of dissolved organic carbon (DOC) on the inactivation of ARB by UV LEDs.

Effect of dissolved organic matter on bacterial regrowth and response after ultraviolet disinfection

The effect of dissolved organic matter (DOM) on bacterial regrowth in water after disinfection using ultraviolet (UV) light emitting diodes (UVLEDs) is still unclear. Herein, the regrowth and responses of Vibrio parahaemolyticus and Bacillus cereus were investigated after being exposed to UVLEDs at combined wavelengths (265 and 280 nm) in a phosphate-buffered saline consisting of Suwannee River natural organic matter (SRNOM) and Suwannee River fulvic acid (SRFA). Low-molecular-weight (MW) organic compounds, which may form into intermediary photoproducts, and indicate bacterial repair metabolism, were characterized through non-target screening using orbitrap mass spectrometry. This study demonstrates the ability of the UVLEDs-inactivated cells to regrow. After UV exposure, a considerable upregulation of RecA was observed in two strains. With increasing the incubation time, the expression levels of RecA in V. parahaemolyticus increased, which may be attributed to the dark repair mechanism. Coexisting anionic DOM affects both the disinfection and bacterial regrowth processes. The time required for bacterial regrowth after UV exposure reflects the time needed for the individual cells to reactivate, and it differs in the presence or absence of DOM. In the presence of DOM, the cells were less damaged and required less time to grow. The UVLEDs exposure results in the occurrence of low-MW organic compounds, including carnitine or acryl-carnitine with N-acetylmuramic acid, which are associated with bacterial repair metabolism. Overall, the results of this study expand the understanding of the effects of water matrices on bacterial health risks. This can aid in the development of more effective strategies for water disinfection.

Disinfection of wastewater by a complete equipment based on a novel ultraviolet light source of microwave discharge electrodeless lamp: Characteristics of bacteria inactivation, reactivation and full-scale studies

Ultraviolet (UV) light is widely used for wastewater disinfection. Traditional electrode-excited UV lamps, such as low-pressure mercy lamps (LPUV), encounter drawbacks like electrode aging and rapid light attenuation. A novel UV source of microwave discharge electrodeless lamp (MDEL) has aroused attention, yet its disinfection performance is unclear and still far from practical application. Here, we successfully developed a complete piece of equipment based on MDELs and achieved the application for disinfection in wastewater treatment plants (WWTPs). The light emitted by an MDEL (MWUV) shared a spectrum similar to that of LPUV, with the main emission wavelength at 254 nm. The inactivation rate of Gram-negative E. coli by MWUV reached 4.5 log at an intensity of 1.6 mW/cm2 and a dose of 20 mJ/cm2. For Gram-positive B. subtilis, an MWUV dose of 50 mJ/cm2 and a light intensity of 1.2 mW/cm2 reached an inactivation rate of 3.4 log. A higher MWUV intensity led to a better disinfection effect and a lower photoreactivation rate of E. coli. When inactivated by MWUV with an intensity of 1.2 mW/cm2 and a dose of 16 mJ/cm2, the maximum photoreactivation rate and reactivation rate constant Kmax of E. coli were 0.63 % and 0.11 % h-1 respectively. Compared with the photoreactivation, the dark repair of E. coli was insignificant. The full-scale application of the MDEL equipment was conducted in two WWTPs (10,000 m3/d and 15,000 m3/d). Generally 2-3 log inactivation rates of fecal coliforms in secondary effluent were achieved within 5-6 s contact time, and the disinfected effluent met the emission standard (1000 CFU/L). This study successfully applied MDEL for disinfection in WWTPs for the first time and demonstrated that MDEL has broad application prospects.

Underlying the mechanisms of pathogen inactivation and regrowth in wastewater using peracetic acid-based disinfection processes: A critical review

Peracetic acid (PAA) disinfection is an emerging wastewater disinfection process. Its advantages include excellent pathogen inactivation performance and little generation of toxic and harmful disinfection byproducts. The objective of this review is to comprehensively analyze the experimental data and scientific information related to PAA-based disinfection processes. Kinetic models and modeling frameworks are discussed to provide effective tools to assess pathogen inactivation efficacy. Then, the efficacy of PAA-based disinfection processes for pathogen inactivation is summarized, and the inactivation mechanisms involved in disinfection and the interactions of PAA with conventional disinfection processes are elaborated. Subsequently, the risk of pathogen regrowth after PAA-based disinfection process is clearly discussed. Finally, to address ecological risks related to PAA-based disinfection, its impact on the spread of antibiotic-resistant bacteria and the transfer of antibiotic resistance genes (ARGs) is also assessed. Among advanced PAA-based disinfection processes, ultraviolet/PAA is promising not only because it has practical application value but also because pathogen regrowth can be inhibited and ARGs transfer risk can be significantly reduced via this process. This review presents valuable and comprehensive information to provide an in-depth understanding of PAA as an alternative wastewater disinfection technology.

Cleaning technologies integrated in duct flows for the inactivation of pathogenic microorganisms in indoor environments: A critical review of recent innovations and future challenges

Pathogenic microorganisms are a major concern in indoor environments, particularly in sensitive facilities such as hospitals, due to their potential to cause nosocomial infections. This study evaluates the concentration of airborne bacteria and fungi in the University Hospital Complex of Albacete (Spain), comparing the results with recent literature. Staphylococcus is identified as the most prevalent bacterial genus with a percentage distribution of 35%, while Aspergillus represents the dominant fungal genus at 34%. The lack of high Technology Readiness Levels (TRL 6, TRL 7) for effective indoor air purification requires research efforts to bridge this knowledge gap. A screening of disinfection technologies for pathogenic airborne microorganisms such as bacteria and fungi is conducted. The integration of filtration, irradiation or and (electro)chemical gas treatment systems in duct flows is discussed to enhance the design of the air-conditioning systems for indoor air purification. Concerns over microbial growth have led to recent studies on coating commercial fibrous air filters with antimicrobial particles (silver nanoparticles, iron oxide nanowires) and polymeric materials (polyaniline, polyvinylidene fluoride). Promising alternatives to traditional short-wave UV-C energy for disinfection include LED and Far-UVC irradiation systems. Additionally, research explores the use of TiO2 and TiO2 doped with metals (Ag, Cu, Pt) in filters with photocatalytic properties, enabling the utilization of visible or solar light. Hybrid photocatalysis, combining TiO2 with polymers, carbon nanomaterials, or MXene nanomaterials, enhances the photocatalytic process. Chemical treatment systems such as aerosolization of biocidal agents (benzalkonium chloride, hydrogen peroxide, chlorine dioxide or ozone) with their possible combination with other technologies such as adsorption, filtration or photocatalysis, are also tested for gas disinfection. However, the limited number of studies on the use of electrochemical technology poses a challenge for further investigation into gas-phase oxidant generation, without the formation of harmful by-products, to raise its TRL for effectively inactivating airborne microorganisms in indoor environments.

The temperature-dependent kinetics and bacteria regrowth by performic acid and sodium hypochlorite disinfection

Sodium hypochlorite (NaOCl) has been widely used as a disinfectant in water and wastewater treatment, because of its high efficiency and low cost, whereas the bio-toxicity of its disinfection byproducts (DBPs) raised great concern. Performic acid (PFA) produces less DBPs and shows strong oxidation abilities. In this study, the effect of temperature on NaOCl and PFA disinfection as well as bacteria regrowth were evaluated. First, the inactivation of Escherichia coli, Staphylococcus aureus, and Bacillus subtilis by NaOCl and PFA at 4 and 20 °C, detected by cell cultured-based plate counting were fitted to kinetic models, and the predicted CTs were calculated. The results showed that NaOCl was more effective than PFA for E. coli and S. aureus inactivation, and the temperature was positively correlated to disinfection. Second, bacteria regrowth was evaluated at different temperatures (4 and 20 °C) of disinfection and storage. The results showed that the bacteria inactivated by NaOCl regrew prominently, especially for those inactivated at 4 and stored at 4 °C, probably through the mechanism of reactivation of viable but non-culturable (VBNC) bacteria. PFA was superior in suppressing bacteria regrowth, and it may be used as an alternate disinfectant in water treatment in cold environment.

Treatment of clinical laboratory sewage using a decentralized treatment unit and risk reduction for its reuse in irrigation using hybrid disinfection

The disinfection efficacy of standalone chlorine, UV and their combined approach (hybrid) was investigated for the coliform removal in BioKube 1 and 2 treated effluents collected from different environmental settings of clinical and domestic wastes. Chlorine and UV disinfection were applied to BioKube treated wastewater with doses from 0 to 4 mg L-1 and 0-166 mJ cm-2 respectively. Combined disinfection strategies were designed to reduce the dose of chlorine and UV and to exploit the synergistic effect of them. The culturable coliforms were enumerated in treated wastewater sample (control), immediately after (reduction), and 24 h post disinfections (regrowth) using culture media plating and colilert-18 media. Both the BioKube systems (1 and 2) were effective in achieving the strict norms of physicochemical parameters, but not following the coliform counts of treated effluent for reuse in irrigation. A hybrid strategy of chlorine followed by UV was found to be the most effective among various standalone and combination approaches for the removal of coliforms (>4 log ER or <1000 CFU/100 mL) from both the treated effluents. However, coliform present in treated effluent of BioKube 1 were resisting (regrowth) against all kind of applied disinfectants except chlorine followed by UV dose at or more than 0.5 mg L-1 + 41 mJ cm-2. Limited reports are available on hybrid disinfection approaches with decentralized packaged sewage treatment units and this study would help to adopt as an effective tertiary treatment strategy for reuse of treated sewage for irrigation while ensuring public health safety.

Ecology and resistance to UV light and antibiotics of microbial communities on UV cabins in the dermatology service of a Spanish hospital

Microorganisms colonize all possible ecological habitats, including those subjected to harsh stressors such as UV radiation. Hospitals, in particular the UV cabins used in phototherapy units, constitute an environment in which microbes are intermittently subjected to UV irradiation. This selective pressure, in addition to the frequent use of antibiotics by patients, may represent a threat in the context of the increasing problem of antimicrobial resistance. In this work, a collection of microorganisms has been established in order to study the microbiota associated to the inner and outer surfaces of UV cabins and to assess their resistance to UV light and the antibiotics frequently used in the Dermatology Service of a Spanish hospital. Our results show that UV cabins harbor a relatively diverse biocenosis dominated by typically UV-resistant microorganisms commonly found in sun-irradiated environments, such as Kocuria, Micrococcus or Deinococcus spp., but also clinically relevant taxa, such as Staphylococcus or Pseudomonas spp. The UV-radiation assays revealed that, although some isolates displayed some resistance, UV is not a major factor shaping the biocenosis living on the cabins, since a similar pool of resistant microorganisms was identified on the external surface of the cabins. Interestingly, some Staphylococcus spp. displayed resistance to one or more antibiotics, although the hospital reported no cases of antibiotic-resistance infections of the patients using the cabins. Finally, no association between UV and antibiotic resistances was found.

Rapid microbial viability assay using scanning electron microscopy: a proof-of-concept using Phosphotungstic acid staining

Multiple stains have been historically utilized in electron microscopy to provide proper contrast and superior image quality enabling the discovery of ultrastructures. However, the use of these stains in microbiological viability assessment has been limited. Phosphotungstic acid (PTA) staining is a common negative stain used in scanning electron microscopy (SEM). Here, we investigate the feasibility of a new SEM-PTA assay, aiming to determine both viable and dead microbes. The optimal sample preparation was established by staining bacteria with different PTA concentrations and incubation times. Once the assay conditions were set, we applied the protocol to various samples, evaluating bacterial viability under different conditions, and comparing SEM-PTA results to culture. The five minutes 10% PTA staining exhibited a strong distinction between viable micro-organisms perceived as hypo-dense, and dead micro-organisms displaying intense internal staining which was confirmed by high Tungsten (W) peak on the EDX spectra. SEM-PTA viability count after freezing, freeze-drying, or oxygen exposure, were concordant with culture. To our knowledge, this study is the first contribution towards PTA staining of live and dead bacteria. The SEM-PTA strategy demonstrated the feasibility of a rapid, cost-effective and efficient viability assay, presenting an open-view of the sample, and providing a potentially valuable tool for applications in microbiome investigations and antimicrobial susceptibility testing.

Autochthonous DOM had solar disinfection effect but nitrate counteracted with them

Pathogens in natural water can pose great threat to public health and challenge water quality. In sunlit surface water, dissolved organic matters (DOMs) can inactivate pathogens due to their photochemical activity. However, the photoreactivity of autochthonous DOM derived from different source and their interaction with nitrate on photo-inactivation remained limited understood. In this study, the composition and photoreactivity of DOM extracted from Microcystis (ADOM), submerged aquatic plant (PDOM) and river water (RDOM) were studied. Results revealed that lignin and tannin-like polyphenols and polymeric aromatic compounds negatively correlated with quantum yield of ³DOM*, whilst lignin like molecules positively correlated with •OH generation. ADOM had highest photoinactivation efficiency of E. coli, followed by RDOM and PDOM. Both the photogenerated •OH and low energy ³DOM* could inactivate bacteria damaging cell membrane and causing increase of intracellular reactive species. PDOM with more phenolic or polyphenols compounds not only weaken its photoreactivity, also increase regrowth potential of bacteria after photodisinfection. The presence of nitrate counteracted with autochthonous DOMs on photogeneration of •OH and photodisinfection activity, as well as increased the reactivation rate of PDOM and ADOM, which might be attributed to the increase of survival bacteria and more bioavailable fractions provided in systems.

Production of singlet oxygen from photosensitizer erythrosine for facile inactivation of coronavirus on mask

The global health crisis caused by the COVID-19 pandemic has led to a surge in demand and use of personal protective equipment (PPE) such as masks, putting great pressure on social production and the environment.It is urgent to find an efficient and non-destructive disinfection method for the safe reuse of PPE. This study proposes a PPE disinfection method that uses erythrosine, a U.S. Food and Drug Administration-approved food dye, as photosensitizer to produce singlet oxygen for virus inactivation, and indicates the completion of disinfection by its photobleaching color change.After spraying 100 μL of 10 μM erythrosine on the surface of the mask for 3 times and light exposure for 25 min, the titer of coronavirus decreased by more than 99.999%, and the color of erythrosine on the mask surface disappeared. In addition, the structure of the mask was intact and the filtration efficiency was maintained at > 95% after 10 cycles of erythrosine treatment.Therefore, this disinfection method can provide at least 10 cycles of reuse with the advantages of high safety and convenient, and the completion of disinfection can be indicated by its photobleaching, which is suitable for hospitals and daily life to reduce the consumption of PPE.

Evaluation of the disinfection effect and mechanism of SO4•- and HO• UV/persulfate salts

Although ultraviolet (UV) and persulfate (PS) have been widely used in water disinfection process, their incompleteness of disinfection, such as inducing the production of viable but non-culturable cells (VBNC), has attracted extensive attention. In this study, the disinfection effect of combined UV and PS was evaluated, and the roles of SO₄^•- and HO^• radicals in UV/PS disinfection were also analyzed. UV/PS more effectively inactivated cells and reduced the number of culturable cells. Also, the test of bacterial dark activation suggested that UV/PS disinfection inhibited the recovery of VBNC bacteria. The mechanisms of UV/PS disinfection were the increase of membrane permeability and oxidative stress, where SO₄^•- radicals played more role than HO^• radicals. Furthermore, UV/PS disinfection more significantly perturbed the metabolism of Pseudomonas aeruginosa (p < 0.05), mainly involving glyoxylate and dicarboxylic acid metabolism, aminoacyl-tRNA biosynthesis, Alanine, aspartate and glutamate metabolism, and citric acid cycle (TCA cycle). In short, UV/PS disinfection can not only significantly reduce the number of culturable bacteria (kill bacteria) but also inhibits the recovery of VBNC bacteria.

Optimizing ozone dose and contact time for removal of antibiotic-resistant P. aeruginosa, A. baumannii, E. coli, and associated resistant genes in effluent of an activated sludge process in a municipal WWTP

This study aimed to investigate the impact of ozonation on inactivation of antibiotic-resistant bacteria (ARB) including E. coli, P. aeruginosa, and A. baumannii, as well as on removal of 16S-rRNA gene and their associated antibiotic-resistant genes (ARGs) indigenously present in effluent of municipal wastewater treatment plant. The Chick-Watson model was used to describe bacterial inactivation rates at specific ozone doses. Maximum reduction of total cultivable A. baumannii, E. coli, and P. aeruginosa were found to be 7.6, 7.1, and 4.7 log, respectively, with the highest ozone dose of 0.48 gO₃/gCOD at 12 min contact time. According to the study results, complete inactivation of ARB and bacterial regrowth was not observed after 72 h incubation. The culture methods overestimated the performance of disinfection processes and propidium monoazide combined with qPCR, and showed the presence of viable but non-culturable bacteria after ozonation. ARGs were more persistent to ozone than ARB. The results of this study highlighted the significance of specific ozone dose and contact time in ozonation process considering the bacterial species and associated ARGs as well as the wastewater physicochemical characteristics, in order to help diminish the entrance of the biological microcontaminants into the environment.

Challenges on solar oxidation as post-treatment of municipal wastewater from UASB systems: Treatment efficiency, disinfection and toxicity

The application of solar photo-Fenton as post-treatment of municipal secondary effluents (MSE) in developing tropical countries is the main topic of this review. Alternative technologies such as stabilization ponds and upflow anaerobic sludge blanket (UASB) are vastly applied in these countries. However, data related to the application of solar photo-Fenton to improve the quality of effluents from UASB systems are scarce. This review gathered main achievements and limitations associated to the application of solar photo-Fenton at neutral pH and at pilot scale to analyze possible challenges associated to its application as post-treatment of MSE generated by alternative treatments. To this end, the literature review considered studies published in the last decade focusing on CECs removal, toxicity reduction and disinfection via solar photo-Fenton. Physicochemical characteristics of effluents originated after UASB systems alone and followed by a biological post-treatment show significant difference when compared with effluents from conventional activated sludge (CAS) systems. Results obtained for solar photo-Fenton as post-treatment of MSE in developed countries indicate that remaining organic matter and alkalinity present in UASB effluents may pose challenges to the performance of solar advanced oxidation processes (AOPs). This drawback could result in a more toxic effluent. The use of chelating agents such as Fe³⁺-EDDS to perform solar photo-Fenton at neutral pH was compared to the application of intermittent additions of Fe²⁺ and both of these strategies were reported as effective to remove CECs from MSE. The latter strategy may be of greater interest in developing countries due to costs associated to complexing agents. In addition, more studies are needed to confirm the efficiency of solar photo-Fenton on the disinfection of effluent from UASB systems to verify reuse possibilities. Finally, future research urges to evaluate the efficiency of solar photo-Fenton at natural pH for the treatment of effluents from UASB systems.

Inactivation of fungal spores in water by CuO-activated peracetic acid: Kinetics, mechanism and regrowth

The disinfection of pathogenic microorganisms in water treatment by peracetic acid (PAA)-based advanced oxidation processes (AOPs) has been gaining increasing concern. In this work, the inactivation mechanism, influencing factors and regrowth of two pathogenic Aspergillus species in the system of CuO-activated PAA were studied for the first time. The k values of A. niger and A. flavus inactivated by PAA/CuO system were 3.9 and 2.1-fold higher than those inactivated by PAA alone. PAA concentration and CuO dose were positively correlated with the inactivation efficiency, while humic acid and pH were negatively correlated. The main active species that contributed to the inactivation of fungal spores in PAA/CuO system were •OH, CH₃C(O)OO• and ¹O₂. PAA/CuO system had more intense oxidative stimulation and more serious damage to fungal spores according to the analysis of cell membrane integrity and intracellular ROS levels. In addition, the PAA/CuO system was less impacted by the water matrix and kept a good inactivation efficiency in real water samples. The regrowth potential of fungal spores after disinfection was also reduced in PAA/CuO system so as to avoid the risk of biological regrowth. This study provides a feasible PAA-based advanced oxidation method for activating PAA and inactivating fungal spores.

Inactivation and photoreactivation of blaNDM-1-carrying super-resistant bacteria by UV, chlorination and UV/chlorination

The excessive dissemination of New Delhi metallo-β-lactamase-1 (NDM-1), which mediates resistance to a majority of clinical β-lactam antibiotics, has created a major public health problem worldwide. Herein, a bla_NDM-1-carrying (plasmid encoded) super-resistant bacterium, Acinetobacter sp. CS-2, was selected to reveal its mechanisms of inactivation and photoreactivation during UV, chlorination and UV/chlorination disinfection. The inactivated CS-2 underwent a certain photoreactivation after UV and chlorination. The logistic model precisely fitted the data obtained in the photoreactivation experiments by UV treatment, with the estimated kinetic parameters S_m (0.530%-12.071%) and k₂ (0.0009-0.0471). The photoreactivation of Acinetobacter sp. CS-2 was observed when treated by chlorination at a dosage of 0.5 mg/L with a survival ratio of 34.04%. UV/chlorination not only resulted in the high-efficiency reduction of CS-2 but also effectively controlled its photoreactivation with a survival ratio of 0%- 0.87%. UV/chlorination showed great advantages in causing the irreversible destruction of bacterial surface structures by making the cell membranes wrinkled and incomplete compared with UV disinfection. The singlet oxygen (¹O₂) generated during UV/chlorination treatment played a vital role in bla_NDM-1 removal. This study proposed new insights into the mechanism of inactivation and the characteristics of photoreactivation for the super-resistant bacteria by UV, chlorination and UV/chlorination.

Bi-based photocatalysts for bacterial inactivation in water: Inactivation mechanisms, challenges, and strategies to improve the photocatalytic activity

Bi-based photocatalysts have been considered suitable materials for water disinfection under natural solar light due to their outstanding optical and electronic properties. However, until now, there are not extensive reviews about the development of Bi-based materials and their application in bacterial inactivation in aqueous solutions. For this reason, this work has focused on summarizing the state of the art related to the inactivation of Gram- and Gram + pathogenic bacteria under visible light irradiation using different Bi-based micro and nano structures. In this sense, the photocatalytic bacterial inactivation mechanisms are analyzed, considering several modifications. The factors that can affect the photocatalytic performance of these materials in real conditions and at a large scale (e.g., water characteristics, pH, light intensity, photocatalyst dosage, and bacteria level) have been studied. Furthermore, current alternatives for improving the photocatalytic antibacterial activity and reuse of Bi-based materials (e.g., surface engineering, crystal facet engineering, doping, noble metal coupling, heterojunctions, Z-scheme junctions, coupling with graphene derivatives, magnetic composites, immobilization) have been explored. According to several reports, inactivation rate values higher than 90% can be achieved by using the modified Bi-based micro/nano structures, which become them excellent candidates for photocatalytic water disinfection. However, these innovative photocatalytic materials bring a variety of future difficulties and opportunities in water disinfection.

Rare UV-resistant cells in clonal populations of Escherichia coli

Water disinfection is one of the most important applications of ultraviolet light-emitting diodes (UV-LEDs), though bacterial regrowth remains a serious problem. In this study, we showed that UV-resistant cells, though rare, exist in an Escherichia coli clonal population. The UV-resistance of stationary phase cells was higher than that of exponential phase cells. Regrowth cell populations showed identical UV sensitivity before and after UV treatment, indicating that UV resistance is not acquired genetically, but is generated stochastically. The characteristics of these UV-resistant cells are similar to those of non-heritable antibiotic-resistant cells, termed persisters. The induction of persister formation increased the number of viable cells after UV treatment. The toxin-antitoxin system gene hipA (high persistence A) is a key factor in persister cell formation. We observed that hipA was strongly expressed in the stationary phase cells, while regrowth cells after UV treatment lost hipA expression, suggesting that the regrowth cells lost their persistence. Compared to UV batch radiation, we demonstrated that intermittent UV irradiation, which included the induction of regrowth between UV treatments, significantly reduced the number of viable E. coli cells.

Regrowth of Escherichia coli in environmental waters after chlorine disinfection: shifts in viability and culturability

Bacterial regrowth after water/wastewater disinfection poses severe risks to public health. However, regrowth studies under realistic water conditions that might critically affect bacterial regrowth are scarce. This study aimed to assess for the first time the regrowth of Escherichia coli (E. coli) in terms of its viability and culturability in environmental waters after chlorine disinfection, which is the most widely used disinfection method. Post-chlorination regrowth tests were conducted in 1) standard 0.85% NaCl solution, 2) river water receiving domestic wastewater effluents, and 3) river water that is fully recharged by domestic wastewater effluents. The multiplex detection of plate count and fluorescence-based viability test was adopted to quantify the culturable and viable E. coli to monitor the regrowth process. The results confirmed that chlorine treatment (0.2, 0.5 and 1.0 mg L-1 initial free chlorine) induced more than 99.95% of E. coli to enter a viable but non-culturable (VBNC) state and the reactivation of VBNC E. coli is presumably the major process of the regrowth. A second-order regrowth model well described the temporal shift of the survival ratio of culturable E. coli after the chlorination (R2: 0.73-1.00). The model application also revealed that the increase in initial chlorine concentration and chlorine dose limited the maximum regrowth rate and the maximum survival ratio, and the regrowth rate and percentage also changed with the water type. This study gives a better understanding of the potential regrowth after chlorine disinfection and highlights the need for investigating the detailed relation of the regrowth to environmental conditions such as major components of water matrices.

Novel fluorescence-based method for rapid quantification of live bacteria in river water and treated wastewater

Monitoring bacteria is essential for ensuring microbial safety of water sources, including river water and treated wastewater. The plate count method is common for monitoring bacterial abundance, although it cannot detect all live bacteria such as viable but non-culturable bacteria, causing underestimation of microbial risks. Live/Dead BacLight kit, involving fluorochromes SYTO 9 and propidium iodide (PI), provides an alternative to assess bacterial viability using flow cytometry or microscopy. However, its application is limited due to the high cost of flow cytometry and the inapplicability of microscopy to most environmental waters. Thus, this study introduces the combination of BacLight kit and fluorescence spectroscopy for quantifying live bacteria in river water and treated wastewater. Mixtures of live and dead Escherichia coli (E. coli) with various ratios and total cell concentrations were stained with SYTO 9 and PI and measured by fluorescence spectroscopy. The fluorescence emission peak area of SYTO 9 in the range of 500-510 nm at the excitation wavelength of 470 nm correlates linearly with the viable cell counts (R ² > 0.99, p < 0.0001) with only slight variations in the complex water matrix. The tested method can quantify the live E. coli from 3.67 × 10⁴ to 2.70 × 10⁷ cells per mL. This method is simple, sensitive and reliable for quantifying live bacteria in environmental water, which can be later integrated into real-time monitoring systems.

Advances in application of ultraviolet irradiation for biofilm control in water and wastewater infrastructure

Biofilms are ubiquitous in aquatic environment. While so far, most of the ultraviolet (UV) disinfection studies focus on planktonic bacteria, and only limited attention has been given to UV irradiation on biofilms. To enrich this knowledge, the present paper reviews the up-to-date studies about applying UV to control biofilms in water and wastewater infrastructure. The development of UV light sources from the conventional mercury lamp to the light emitting diode (LED), and the resistance mechanisms of biofilms to UV are summarized, respectively. Then the feasibility to control biofilms with UV is discussed in terms of three technical routes: causing biofilm slough, inhibiting biofilm formation, and inactivating bacteria in the established biofilm. A comprehensive evaluation of the biofilm-targeted UV technologies currently used or potentially useful in water industry is provided as well, after comparative analyses on single/combined wavelengths, continuous/pulsed irradiation, and instant/chronic disinfection effects. UV LEDs are emerging as competitive light sources because of advantages such as possible selection of wavelengths, adjustable emitting mode and the designable configuration. They still, however, face challenges arising from the low wall plug efficiency and power output. At last, the implementation of the UV-based advanced oxidation processes in controlling biofilms on artificial surfaces is overviewed and their synergistic mechanisms are proposed, which further enlightens the prospective of UV in dealing with the biofilm issue in water infrastructure.

Metagenomic analysis of MWWTP effluent treated via solar photo-Fenton at neutral pH: Effects upon microbial community, priority pathogens, and antibiotic resistance genes

The effectiveness of advanced technologies on eliminating antibiotic resistant bacteria (ARB) and resistance genes (ARGs) from wastewaters have been recently investigated. Solar photo-Fenton has been proven effective in combating ARB and ARGs from Municipal Wastewater Treatment Plant effluent (MWWTPE). However, most of these studies have relied solely on cultivable methods to assess ARB removal. This is the first study to investigate the effect of solar photo-Fenton upon ARB and ARGs in MWWTPE by high throughput metagenomic analysis (16S rDNA sequencing and Whole Genome Sequencing). Treatment efficiency upon priority pathogens and resistome profile were also investigated. Solar photo-Fenton (30 mg L^-1 of Fe²⁺ intermittent additions and 50 mg L^-1 of H₂O₂) reached 76-86% removal of main phyla present in MWWTPE. An increase in Proteobacteria abundance was observed after solar photo-Fenton and controls in which H₂O₂ was present as an oxidant (Fenton, H₂O₂ only, solar/H₂O₂). Hence, tolerance mechanisms presented by this group should be further assessed. Solar photo-Fenton achieved complete removal of high priority Staphylococcus and Enterococcus, as well as Klebsiella pneumoniae and Pseudomonas aeruginosa. Substantial reduction of intrinsically multi-drug resistant bacteria was detected. Solar photo-Fenton removed nearly 60% of ARGs associated with sulfonamides, macrolides, and tetracyclines, and complete removal of ARGs related to β-lactams and fluoroquinolones. These results indicate the potential of using solar-enhanced photo-Fenton to limit the spread of antimicrobial resistance, especially in developing tropical countries.

Risks, characteristics, and control strategies of disinfection-residual-bacteria (DRB) from the perspective of microbial community structure

The epidemic of COVID-19 has aroused people's particular attention to biosafety. A growing number of disinfection products have been consumed during this period. However, the flaw of disinfection has not received enough attention, especially in water treatment processes. While cutting down the quantity of microorganisms, disinfection processes exert a considerable selection effect on bacteria and thus reshape the microbial community structure to a great extent, causing the problem of disinfection-residual-bacteria (DRB). These systematic and profound changes could lead to the shift in regrowth potential, bio fouling potential, as well as antibiotic resistance level and might cause a series of potential risks. In this review, we collected and summarized the data from the literature in recent 10 years about the microbial community structure shifting of natural water or wastewater in full-scale treatment plants caused by disinfection. Based on these data, typical DRB with the most reporting frequency after disinfection by chlorine-containing disinfectants, ozone disinfection, and ultraviolet disinfection were identified and summarized, which were the bacteria with a relative abundance of over 5% in the residual bacteria community and the bacteria with an increasing rate of relative abundance over 100% after disinfection. Furthermore, the phylogenic relationship and potential risks of these typical DRB were also analyzed. Twelve out of fifteen typical DRB genera contain pathogenic strains, and many were reported of great secretion ability. Pseudomonas and Acinetobacter possess multiple disinfection resistance and could be considered as model bacteria in future studies of disinfection. We also discussed the growth, secretion, and antibiotic resistance characteristics of DRB, as well as possible control strategies. The DRB phenomenon is not limited to water treatment but also exists in the air and solid disinfection processes, which need more attention and more profound research, especially in the period of COVID-19.

Worldwide Research Trends on Solar-Driven Water Disinfection

“Ensure access to water for all”, states Goal 6 of the UN’s Sustainable Development Goals. This worldwide challenge requires identifying the best water disinfection method for each scenario. Traditional methods have limitations, which include low effectiveness towards certain pathogens and the formation of disinfection byproducts. Solar-driven methods, such as solar water disinfection (SODIS) or solar photocatalysis, are novel, effective, and financially and environmentally sustainable alternatives. We have conducted a critical study of publications in the field of water disinfection using solar energy and, hereby, present the first bibliometric analysis of scientific literature from Elsevier’s Scopus database within the last 20 years. Results show that in this area of growing interest USA, Spain, and China are the most productive countries in terms of publishing, yet Europe hosts the most highly recognized research groups, i.e., Spain, Switzerland, Ireland, and UK. We have also reviewed the journals in which researchers mostly publish and, using a systematic approach to determine the actual research trends and gaps, we have analyzed the capacity of these publications to answer key research questions, pinpointing six clusters of keywords in relation to the main research challenges, open areas, and new applications that lie ahead. Most publications focused on SODIS and photocatalytic nanomaterials, while a limited number focused on ensuring adequate water disinfection levels, testing regulated microbial indicators and emerging pathogens, and real-world applications, which include complex matrices, large scale processes, and exhaustive cost evaluation.