Performance, mechanism regulation and resource recycling of bacteria-algae symbiosis system for wastewater treatment: A review

The bacteria-algae synergistic wastewater treatment process not only efficiently eliminates nutrients and absorbs heavy metals, but also utilizes photosynthesis to convert light energy into chemical energy, generating valuable bioresource. The study systematically explores the formation, algal species, and regulatory strategies of the bacterial-algal symbiosis system. It provides a detailed analysis of various interaction mechanisms, with a particular focus on nutrient exchange, signal transduction, and gene transfer. Additionally, the efficacy of the system in removing nitrogen, phosphorus, and heavy metals, as well as its role in CO2 reduction and bioresource recycling, is thoroughly elaborated. Potential future research of bacteria-algae cell factory producing bioenergy production, feed or fertilizers are summarized. This paper clearly presents effective strategies for efficiently removing pollutants, reducing carbon emissions, and promoting resource recycling in the field of wastewater treatment. It also provides recommendations for further research on utilizing microbial-algal symbiotic systems to remove novel pollutants from wastewater and extract value-added products from the resulting biomass.

A critical review of ultra-violet light emitting diodes as a one water disinfection technology

UV light emitting diode (LED) disinfection technologies have advanced over the last decade and expanded the design space for applications in point of use, industrial, and now full-scale water treatment. This literature review examines the progression of UV LED technologies from 2007 to 2023 using key features such as total optical power, price, and wall-plug efficiency. The review found that optical power is increasing while the price per Watt is decreasing; however, the wall plug energy (WPE) is slowly improving over the last decade. These factors govern the feasibility of many UV LEDs applications and establish the current state of the art for these technologies. An analysis of inactivation rate constants for low-pressure, medium-pressure, and UV LED sources was undertaken and provides a comprehensive view of how current UV LED technologies compare to traditional technologies. This comparison found that UV LEDs perform comparably vs conventional UV technologies when disinfecting bacteria and viruses. Furthermore, comparison of reported reduction equivalent fluences for UV LED flow-through reactors at the bench-, pilot-, and full-scale were explored in this review, and it was found that LED treatment is becoming more effective at handling increased flowrates and has been proven to work at full-scale. UV LEDs do however require additional research into the impacts of water matrices at different wavelengths and the impact that each available LED wavelength has on disinfection. Overall, this work provides a broad assessment of UV disinfection technologies and serves as a state-of-the-art reference document for those who are interested in understanding this rapidly developing technology.

Oxidative stress responses in biofilms

Oxidizing agents are low-molecular-weight molecules that oxidize other substances by accepting electrons from them. They include reactive oxygen species (ROS), such as superoxide anions (O2-), hydrogen peroxide (H2O2), and hydroxyl radicals (HO-), and reactive chlorine species (RCS) including sodium hypochlorite (NaOCl) and its active ingredient hypochlorous acid (HOCl), and chloramines. Bacteria encounter oxidizing agents in many different environments and from diverse sources. Among them, they can be produced endogenously by aerobic respiration or exogenously by the use of disinfectants and cleaning agents, as well as by the mammalian immune system. Furthermore, human activities like industrial effluent pollution, agricultural runoff, and environmental activities like volcanic eruptions and photosynthesis are also sources of oxidants. Despite their antimicrobial effects, bacteria have developed many mechanisms to resist the damage caused by these toxic molecules. Previous research has demonstrated that growing as a biofilm particularly enhances bacterial survival against oxidizing agents. This review aims to summarize the current knowledge on the resistance mechanisms employed by bacterial biofilms against ROS and RCS, focussing on the most important mechanisms, including the formation of biofilms in response to oxidative stressors, the biofilm matrix as a protective barrier, the importance of detoxifying enzymes, and increased protection within multi-species biofilm communities. Understanding the complexity of bacterial responses against oxidative stress will provide valuable insights for potential therapeutic interventions and biofilm control strategies in diverse bacterial species.

Effects of ferulic acid combined with light irradiation on deoxynivalenol and its production in Fusarium graminearum

This study aimed to investigate the effects of ferulic acid (FA), a natural phenolic phytochemical, in combination with light irradiation at three wavelengths (365, 385 and 405 nm) on the concentration and toxicity of deoxynivalenol (DON), a mycotoxin produced by Fusarium graminearum. Moreover, this study examined the influence of the combination treatment on DON production in the cultured fungus. FA activated by light at a peak wavelength of 365 nm exhibited the most effective decrease in DON concentration of the tested wavelengths; a residual DON ratio of 0.23 at 24 h exposure was observed, compared with the initial concentration. The reduction in DON using 365-nm light was dependent on the concentration of FA, with a good correlation (r2 = 0.979) between the rate constants of DON decrease and FA concentration, which was confirmed by a pseudo-first-order kinetics analysis of the photoreaction with different FA concentrations (50-400 mg/L) for 3 h. The viability of HepG2 cells increased by 56.7% following in vitro treatment with a mixture containing the photoproducts obtained after treatment with 20 mg/L DON and 200 mg/L FA under 365-nm irradiation for 6 h. These results suggested that the photoreaction of FA under 365-nm irradiation induces the detoxification of DON through degradation or modification of DON. The antifungal effects of the combination (FA and 365-nm light) on F. graminearum were investigated. Conidia treated with the combination did not show additive or synergistic inhibition of fungal biomass and DON production in 7-day cultivated fungal samples compared with samples after single treatment. However, successive treatment, composed of 90 min irradiation at 365 nm and then treatment with 200 mg/L FA for 90 min in the dark, suppressed fungal growth and DON yield to 70% and 25% of the untreated sample level, respectively. This photo-technology involving the two treatment methods of 365-nm irradiation and FA addition as a food-grade phenolic acid in combination or successively, can aid in developing alternative approaches to eliminate fungal contaminants in the fields of environmental water and agriculture. However, further research is required to explore the underlying mechanisms of DON decontamination and its biosynthesis in F. graminearum.

Tertiary/quaternary treatment of urban wastewater by UV/H2O2 or ozonation: Microplastics may affect removal of E. coli and contaminants of emerging concern

The aim of this study was to investigate the interference of polyethylene microplastics (MPs) on ultraviolet irradiation/hydrogen peroxide (UV/H2O2) and ozonation processes in the inactivation of E. coli bacteria (tertiary treatment) and removal of contaminants of emerging concern (CECs) (quaternary treatment) from simulated and real secondary treated urban wastewater. Three pharmaceuticals were investigated as model CECs, namely carbamazepine, sulfamethoxazole and trimethoprim. Experimental results showed that disinfection efficiency of UV/H2O2 treatment decreased (2.4, 1.8 and 1.3 log reductions of E. coli, initial H2O2 dose of 30 mg/L, 2.5 min treatment) as the initial concentration of MPs was increased (0.25, 0.5 and 1.0 g/L, respectively). Similarly, an increase in MPs concentration (0.25, 0.5 and 1.0 g/L) reduced the inactivation (4.7, 4.1 and 3.7 log reductions) of the target bacteria after 60 min of ozonation treatment. Although the disinfection efficiency of both treatment processes was negatively affected by the presence of MPs, UV/H2O2 was more effective than the ozonation, despite ozonation being investigated at high doses to better discriminate the effect of MPs. Noteworthy, CECs degradation by UV/H2O2 under realistic operating conditions was affected to some extent by MPs, while a lower effect was observed for ozonation, at not realistic ozone dose.

UV-A Radiation: Safe Human Exposure and Antibacterial Activity

UV radiation is used for sterilization but has adverse health effects in humans. UV-A radiation has lower antimicrobial effect than UV-B and UV-C but constitutes a lower health risk, opening up the possibility to sanitize environments with human presence in controlled exposure conditions. We investigated this possibility by identifying safe exposure conditions to a UV-A lamp along with efficient sanitization of the environment. The human exposure limits were calculated following the guidelines provided by the International Commission on Non-Ionizing Radiation Protection and the International Commission on Illumination. Antibacterial activity was evaluated on Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus. The maximum human exposure duration has been identified at different irradiation distance and angle, increasing with the increase of both parameters. Bactericidal activity was observed in all microorganisms and was higher with higher exposure time and at lower distance from the source. Noteworthily, in equal conditions of radiant exposure, the exposure time impacts on the bactericidal activity more than the distance from the source. The modulation of factors such as distance from the source, exposure time and irradiation angle can enable effective antibacterial activity and human safety. Prolonged direct irradiation of the surfaces associated with indirect human exposure represents the condition of greater efficacy and safety.

Role of Pel and Psl polysaccharides in the response of Pseudomonas aeruginosa to environmental challenges: oxidative stress agents (UVA, H2O2, sodium hypochlorite) and its competitor Staphylococcus aureus

Pseudomonas aeruginosa is a versatile bacterium capable of adapting to a wide range of stress factors, including solar UVA radiation (400-315 nm). High UVA doses produce lethal effects due to the action of reactive oxygen species. Sublethal UVA doses also induces oxidative damage, but, in addition, it triggers a variety of adaptive responses, including the overexpression of pelA and pslA genes in P. aeruginosa. These genes encode the synthesis of Pel and Psl, which are essential polysaccharides in biofilm formation. The present study analysed the role of Pel and Psl in the adaptive responses generated by exposure to low UVA doses, and their importance in the response to lethal doses of UVA, hydrogen peroxide (H₂O₂), and sodium hypochlorite, in both planktonic cells and submerged and air-liquid interface (ALI) biofilms. It also studied the roles of Pel and Psl in P. aeruginosa-Staphylococcus aureus interaction. The results demonstrate that the capacity of sublethal UVA exposure to increase cell hydrophobicity and cell attachment and generate cross-protection phenomena in P. aeruginosa depends on the presence of Pel and Psl. The study also shows that Pel and Psl have a key role in the tolerance to lethal doses of UVA radiation, sodium hypochlorite and H₂O₂, in both biofilms and planktonic cells. Finally, co-culture assays showed total inhibition of S. aureus growth in presence of P. aeruginosa. This phenomenon depends, at least in part, on the simultaneous presence of Pel and Psl in planktonic cells and biofilms, suggesting a relevant role of these polysaccharides in the interaction between these species.

Toxic reactive oxygen species stresses for reconfiguring central carbon metabolic fluxes in foodborne bacteria: Sources, mechanisms and pathways

The toxic reactive oxygen species (toxROS) is the reactive oxygen species (ROS) beyond the normal concentration of cells, which has inactivation and disinfection effects on foodborne bacteria. However, foodborne bacteria can adapt and survive by physicochemical regulation of antioxidant systems, especially through central carbon metabolism (CCM), which is a significant concern for food safety. It is thus necessary to study the antioxidant regulation mechanisms of CCM in foodborne bacteria under toxROS stresses. Therefore, the purpose of this review is to provide an update and comprehensive overview of the reconfiguration of CCM fluxes in foodborne bacteria that respond to different toxROS stresses. In this review, two key types of toxROS including exogenous toxROS (exo-toxROS) and endogenous toxROS (endo-toxROS) are introduced. Exo-toxROS are produced by disinfectants, such as H₂O₂ and HOCl, or during food non-thermal processing such as ultraviolet (UV/UVA), cold plasma (CP), ozone (O₃), electrolyzed water (EW), pulsed electric field (PEF), pulsed light (PL), and electron beam (EB) processing. Endo-toxROS are generated by bioreagents such as antibiotics (aminoglycosides, quinolones, and β-lactams). Three main pathways for CCM in foodborne bacteria under the toxROS stress are also highlighted, which are glycolysis (EMP), pentose phosphate pathway (PPP), and tricarboxylic acid cycle (TCA). In addition, energy metabolisms throughout these pathways are discussed. Finally, challenges and future work in this area are suggested. It is hoped that this review should be beneficial in providing insights for future research on bacterial antioxidant CCM defence under both exo-toxROS stresses and endo-toxROS stresses.

Comparative Assessment of Pulsed and Continuous LED UV-A Lighting for Disinfection of Contaminated Surfaces

The germicidal efficacy of LED UV-A lighting has scarcely been compared in continuous and pulsed modes for contaminated surfaces. Herein, we compare the disinfection properties of pulsed versus continuous lighting at equal irradiances using a 365 nm LED device that replicates the doses of occupied-space continuous disinfection UV-A products. Representative organisms evaluated in this study included human-infectious enveloped and non-enveloped viruses (lentivirus and adeno-associated virus, respectively), a bacterial endospore (Bacillus atrophaeus), and a resilient gram-positive bacterium (Enterococcus faecalis). Nominal UV-A irradiances were tested at or below the UL standard limit for continuous human exposure (maximum irradiance of 10 W/m2). We observed photoinactivation properties that varied by organism type, with bacteria and enveloped virus being more susceptible to UV-A than non-enveloped virus and spores. Overall, we conclude that continuous-mode UV-A lighting is better suited for occupied-space disinfection than pulsing UV-A at equivalent low irradiances, and we draw comparisons to other studies in the literature.

Nanomaterial-enabled photothermal-based solar water disinfection processes: Fundamentals, recent advances, and mechanisms

The pathogenic microorganisms in water pose a great threat to human health. Photothermal and photothermocatalytic disinfection using nanomaterials (NPs) has offered a promising and effective strategy to address the challenges in solar water disinfection (SODIS), especially in the point-of-use operations. This review aims at providing comprehensive and state-of-the-art knowledge of photothermal-based disinfection by NPs. The fundamentals and principles of photothermal-based disinfection were first introduced. Then, recent advances in developing photothermal/photothermocatalytic catalysts were systematically summarized. The light-to-heat conversion and disinfection performance of a large variety of photothermal materials were presented. Given the complicated mechanisms of photothermal-based disinfection, the attacks from reactive oxygen species and heat, the destruction of bacterial cells, and the antibacterial effects of released metal ions were highlighted. Finally, future challenges and opportunities associated with the development of cost-effective photothermal/photothermocatalytic disinfection systems were outlined. This review will provide guidance in designing future NPs and inspire more research efforts from environmental nano-communities to move towards practical water disinfection operations.

Identifying the mediators of intracellular E. coli inactivation under UVA light: The (photo) Fenton process and singlet oxygen

Solar disinfection (SODIS) was probed for its underlying mechanism. When Escherichia coli was exposed to UVA irradiation, the dominant solar fraction acting in SODIS process, cells exhibited a shoulder before death ensued. This profile resembles cell killing by hydrogen peroxide (H2O2). Indeed, the use of specialized strains revealed that UVA exposure triggers intracellular H2O2 formation. The resultant H2O2 stress was especially impactful because UVA also inactivated the processes that degrade H2O2-peroxidases through the suppression of metabolism, and catalases through direct enzyme damage. Cell killing was enhanced when water was replaced with D2O, suggesting that singlet oxygen plays a role, possibly as a precursor to H2O2 and/or as the mediator of catalase damage. UVA was especially toxic to mutants lacking miniferritin (dps) or recombinational DNA repair (recA) enzymes, indicating that reactions between ferrous iron and UVA-generated H2O2 lead to lethal DNA damage. Importantly, experiments showed that the intracellular accumulation of H2O2 alone is insufficient to kill cells; therefore, UVA must do something more to enable death. A possibility is that UVA stimulates the reduction of intracellular ferric iron to its ferrous form, either by stimulating O2•- formation or by generating photoexcited electron donors. These observations and methods open the door to follow-up experiments that can probe the mechanisms of H2O2 formation, catalase inactivation, and iron reduction. Of immediate utility, the data highlight the intracellular pathways formed under UVA light during SODIS, and that the presence of micromolar iron accelerates the rate at which radiation disinfects water.

Novel Control Strategy for Membrane Biofouling by Surface Loading of Aerobically and Anaerobically Applicable Photocatalytic Optical Fibers Based on a Z-Scheme Heterostructure Zr-MOFs/rGO/Ag3PO4 Photocatalyst

The high replacement cost of modified membranes in antibiofouling application is inevitable. Here, surface-loaded photocatalytic optical fibers (POFs) were developed as antibiofouling strategies to replace membrane modification. To prepare aerobically and anaerobically applicable POFs, novel visible light-driven zirconium-based metal-organic framework/reduced graphene oxide/Ag₃PO₄ (Zr-MOFs/rGO/Ag₃PO₄) Z-scheme heterojunctions were constructed and coated on optical fibers. After loading the POFs on the membrane surface, the antibiofouling capability of the POFs was demonstrated during membrane filtration of bacteria and foulants under visible light irradiation. The disinfection efficiencies of the POFs against Escherichia coli and Staphylococcus aureus reached 95.7 and 92.4%, respectively, by aerobic treatment and 90.3 and 85.5%, respectively, by anaerobic treatment. For the inactivated bacteria, cell membrane and membrane-associated functions were destroyed, accompanied by antioxidant enzyme decomposition, loss of cell respiration and adenosine triphosphate (ATP) synthesis capacity, and leakage and oxidation of protein, lipid, potassium, DNA, and RNA. During membrane filtration of model foulants and membrane bioreactor sludge, the POFs significantly alleviated the membrane flux decline by foulant disintegration. By qualitative and quantitative detection and quenching tests of reactive species, aerobically generated ^•O₂^- and ^•OH and anaerobically generated ^•OH from POFs played dominant roles in the antibiofouling process. This work provides unique insights into membrane fouling control based on environmentally friendly and efficient photocatalytic technology.

Interactions of E. coli with algae and aquatic vegetation in natural waters

Both algae and bacteria are essential inhabitants of surface waters. Their presence is of ecological significance and sometimes of public health concern triggering various control actions. Interactions of microalgae, macroalgae, submerged aquatic vegetation, and bacteria appear to be important phenomena necessitating a deeper understanding by those involved in research and management of microbial water quality. Given the long-standing reliance on Escherichia coli as an indicator of the potential presence of pathogens in natural waters, understanding its biology in aquatic systems is necessary. The major effects of algae and aquatic vegetation on E. coli growth and survival, including changes in the nutrient supply, modification of water properties and constituents, impact on sunlight radiation penetration, survival as related to substrate attachment, algal mediation of secondary habitats, and survival inhibition due to the release of toxic substances and antibiotics, are discussed in this review. An examination of horizontal gene transfer and antibiotic resistance potential, strain-specific interactions, effects on the microbial, microalgae, and grazer community structure, and hydrodynamic controls is given. Outlooks due to existing and expected consequences of climate change and advances in observation technologies via high-resolution satellite imaging, unmanned aerial vehicles (drones), and mathematical modeling are additionally covered. The multiplicity of interactions among bacteria, algae, and aquatic vegetation as well as multifaceted impacts of these interactions, create a wide spectrum of research opportunities and technology developments.

Homeophasic Adaptation in Response to UVA Radiation in Pseudomonas aeruginosa: Changes of Membrane Fatty Acid Composition and Induction of desA and desB Expression

In bacteria, exposure to changes in environmental conditions can alter membrane fluidity, thereby affecting its essential functions in cell physiology. To adapt to these changes, bacteria maintain appropriate fluidity by varying the composition of the fatty acids of membrane phospholipids, a phenomenon known as homeophasic adaptation. In Pseudomonas aeruginosa, this response is achieved mainly by two mechanisms of fatty acid desaturation: the FabA-FabB and DesA-DesB systems. This study analyzed the effect of ultraviolet-A (UVA) radiation-the major fraction of solar UV radiation reaching the Earth's surface-on the homeophasic process. The prototypical strain PAO1 was grown under sublethal UVA doses or in the dark, and the profiles of membrane fatty acids were compared at early logarithmic, logarithmic and stationary growth phases. In the logarithmic growth phase, it was observed that growth under sublethal UVA doses induced the expression of the desaturase-encoding genes desA and desB and increased the proportion of unsaturated fatty acids; in addition, membrane fluidity could also increase, as suggested by the indices used as indicators of this parameter. The opposite effect was observed in the stationary growth phase. These results demonstrate the relevant role of UVA on the homeophasic response at transcriptional level.

Metabolomic analyses on microbial primary and secondary oxidative stress responses

Food safety is veryimportant in our daily life. In food processing or disinfection, microorganisms are commonly exposed to oxidative stress perturbations. However, microorganisms can adapt and respond to physicochemical interventions, leading to difficulty and complexity for food safety assurance. Therefore, understanding the response mechanisms of microbes and providing an overview of the responses under oxidative stress conditions are beneficial for ensuring food safety for the industry. The current review takes the metabolomics approach to reveal small metabolite signatures and key pathway alterations during oxidative stress at the molecular and technical levels. These alterations are involved in primary oxidative stress responses due to inactivation treatments such as using hypochlorite (HOCl), hydrogen peroxide (H₂ O₂ ), electrolyzed water (EW), irradiation, pulsed light (PL), electron beam (EB), and secondary oxidative stress responses due to exposures to excessive conditions such as heat, pressure, acid, and alkaline. Details on the putative origin of exogenous or endogenous reactive oxygen species (ROS) are discussed, with particular attention paid to their effects on lipid, amino acid, nucleotide, and carbohydrate metabolism. In addition, mechanisms on counteracting oxidative stresses, stabilization of cell osmolality as well as energy provision for microbes to survive are also discussed.

Efficacy of light-emitting diodes emitting 395, 405, 415, and 425 nm blue light for bacterial inactivation and the microbicidal mechanism

We investigated the bactericidal effects against Escherichia coli O157:H7 of light-emitting diodes (LEDs) emitting blue light of four different peak wavelengths ranging from 395 to 425 nm in water. Furthermore, we investigated inactivation in the presence of reactive oxygen species (ROS) scavengers to elucidate the contribution of bacterial inactivation. An aluminum chamber was constructed and coated in carbon to block exterior light, and a single blue light LED with a rear heat sink was attached to the chamber lid. Effective inactivation of the pathogen was observed for all blue light LED irradiation at 305, 405, 415, and 425 nm. The log-linear with shoulder and tail model and log-linear model described the survival of the bacteria after blue light LED treatments. Not just the effects of ROS but also photophysical effects were shown with the addition of mannitol, a ROS scavenger. The integrity of the cell membrane was damaged regardless of the presence of ROS, which indicates that photophysical effects were sufficient to induce damage to the cell membrane. In addition, activity of succinate-coenzyme Q reductase, which participates in respiratory metabolism to generate energy, decreased in the absence of ROS and decreased further in the presence of ROS.

Solar inactivated Salmonella Typhimurium induces an immune response in BALB/c mice

Salmonella is contracted through the consumption of untreated water and contaminated food. The contraction and spread of water-related Salmonella in resource-poor communities can be reduced by using solar disinfection (SODIS) to treat the water before its consumption. SODIS is a water sanitizing technique that relies on natural sunshine. It is a cost-effective, inexpensive, environmentally, and user-friendly means of treating microbiologically contaminated water. This water disinfection method has saved many lives in communities vulnerable to water-related infections worldwide. At present, the success of SODIS has mainly been attributed to permanent inactivation of water pathogens ability to grow. However, little to no information exists as to whether immune responses to the solar inactivated pathogens are induced in SODIS water consumers. This study assessed the potential for solar inactivated S. Typhimurium to induce an immune response in mice. Results show that solar inactivated S. Typhimurium can induce bactericidal antibodies against S. Typhimurium. Furthermore, an increase in the secretion of interferon-gamma (IFN-γ) was observed in mice given the solar inactivated S. Typhimurium. These findings suggest that solar inactivated S. Typhimurium induces a humoral and cellular immune response. However, the level of protection afforded by these responses requires further investigation.

How to Evaluate Non-Growing Cells-Current Strategies for Determining Antimicrobial Resistance of VBNC Bacteria

Thanks to the achievements in sanitation, hygiene practices, and antibiotics, we have considerably improved in our ongoing battle against pathogenic bacteria. However, with our increasing knowledge about the complex bacterial lifestyles and cycles and their plethora of defense mechanisms, it is clear that the fight is far from over. One of these resistance mechanisms that has received increasing attention is the ability to enter a dormancy state termed viable but non-culturable (VBNC). Bacteria that enter the VBNC state, either through unfavorable environmental conditions or through potentially lethal stress, lose their ability to grow on standard enrichment media, but show a drastically increased tolerance against antimicrobials including antibiotics. The inability to utilize traditional culture-based methods represents a considerable experimental hurdle to investigate their increased antimicrobial resistance and impedes the development and evaluation of effective treatments or interventions against bacteria in the VBNC state. Although experimental approaches were developed to detect and quantify VBNCs, only a few have been utilized for antimicrobial resistance screening and this review aims to provide an overview of possible methodological approaches.

Integrating micro-algae into wastewater treatment: A review

Improving the ecological status of water sources is a growing focus for many developed and developing nations, in particular with reducing nitrogen and phosphorus in wastewater effluent. In recent years, mixotrophic micro-algae have received increased interest in implementing them as part of wastewater treatment. This is based on their ability to utilise organic and inorganic carbon, as well as inorganic nitrogen (N) and phosphorous (P) in wastewater for their growth, with the desired results of a reduction in the concentration of these substances in the water. The aim of this review is to provide a critical account of micro-algae as an important step in wastewater treatment for enhancing the reduction of N, P and the chemical oxygen demand (COD) in wastewater, whilst utilising a fraction of the energy demand of conventional biological treatment systems. Here, we begin with an overview of the various steps in the treatment process, followed by a review of the cellular and metabolic mechanisms that micro-algae use to reduce N, P and COD of wastewater with identification of when the process may potentially be most effective. We also describe the various abiotic and biotic factors influencing micro-algae wastewater treatment, together with a review of bioreactor configuration and design. Furthermore, a detailed overview is provided of the current state-of-the-art in the use of micro-algae in wastewater treatment.

Enhancing solar disinfection (SODIS) with the photo-Fenton or the Fe2+/peroxymonosulfate-activation process in large-scale plastic bottles leads to toxicologically safe drinking water

Solar disinfection (SODIS) in 2-L bottles is a well-established drinking water treatment technique, suitable for rural, peri‑urban, or isolated communities in tropical or sub-tropical climates. In this work, we assess the enlargement of the treatment volume by using cheap, large scale plastic vessels. The bactericidal performance of SODIS and two solar-Fe²⁺ based enhancements, namely photo-Fenton (light/H₂O₂/Fe²⁺) and peroxymonosulfate activation (light/PMS/Fe²⁺) were assessed in 19-L polycarbonate (PC) and 25-L polyethylene terephthalate (PET) bottles, in ultrapure and real water matrices (tap water, lake Geneva water). Although SODIS always reached total (5-logU) inactivation, under solar light, enhancement by or both Fe²⁺/H₂O₂ or Fe²⁺/PMS was always beneficial and led to an increase in bacterial elimination kinetics, as high as 2-fold in PC and PET bottles with tap water for light/H₂O₂/Fe²⁺, and 8-fold in PET bottles with Lake Geneva water. The toxicological safety of the enhancements and their effects on the plastic container materials was assessed using the E-screen assay and the Ames test, after 1-day or 1-week exposure to SODIS, photo-Fenton and persulfate activation. Although the production of estrogenic compounds was observed, we report that no treatment method, duration of exposure or material resulted in estrogenicity risk for humans, and similarly, no mutagenicity risk was measured. In summary, we suggest that SODIS enhancement by either HO^•- or SO₄^•--based advanced oxidation process is a suitable enhancement of bacterial inactivation in large scale plastic bottles, without any associated toxicity risks.

Impact of Sediment Concentration on the Survival of Wastewater-Derived blaCTX-M-15-Producing E. coli, and the Implications for Dispersal into Estuarine Waters

The environmental cycling of antibiotic-resistant bla_CTX-M-15-producing E. coli following release from wastewater treatment plants is a major public health concern. This study aimed to (i) assess the impact of sediment concentrations on the rate of their inactivation following release from human wastewater into freshwater, and (ii) simulate their subsequent dispersal to the nearby coastline during a “worst-case” event where heavy rainfall coincided with high spring tide in the Conwy Estuary, North Wales. Freshwater microcosms of low, medium and high turbidity were inoculated with bla_CTX-M-15-producing E. coli, then exposed to ultraviolet (UV) radiation. Typical regional wintertime exposure to UV was found to be insufficient to eradicate E. coli, and in highly turbid water, many bacteria survived simulated typical regional summertime UV exposure. Modelling results revealed that bla_CTX-M-15-producing E. coli concentrations reduced downstream from the discharge source, with ~30% of the source concentration capable of dispersing through the estuary to the coast, taking ~36 h. Offshore, the concentration simulated at key shellfisheries and bathing water sites ranged from 1.4% to 10% of the upstream input, depending on the distance offshore and tidal regime, persisting in the water column for over a week. Our work indicates that the survival of such organisms post-release into freshwater is extended under typical wintertime conditions, which could ultimately have implications for human health.

Evaluation of Viable Cells in Pseudomonas aeruginosa Biofilmsby Colony Count and Live/Dead Staining

Pseudomonas aeruginosa is a human pathogen capable to form robust biofilms. P. aeruginosa biofilms represent a serious problem because of the adverse effects on human health and industry, from sanitary and economic points of view. Typical strategies to break down biofilms have been long used, such as the use of disinfectants or antibiotics, but also, according to their high resistance to standard antimicrobial approaches, alternative strategies employing photocatalysis or control of biofilm formation by modifying surfaces, have been proposed. Colony forming units (cfu) counting and live/dead staining, two classic techniques used for biofilm quantification, are detailed in this work. Both methods assess cell viability, a key factor to analyze the microbial susceptibility to given treatment, then, they represent a good approach for evaluation of an antibiofilm strategy.

Employing bacterial mutations for the elucidation of photo-Fenton disinfection: Focus on the intracellular and extracellular inactivation mechanisms induced by UVA and H2O2

The bacterial inactivation mechanisms by solar light and the photo-Fenton process is still a matter of debate. In this study, we bring evidence towards the elucidation of the mechanisms that govern photo-Fenton disinfection at near-neutral pH. With the use of porin-deficient and catalase over-producing E. coli strains, in conjunction with measurements of cell wall oxidation and permeability, we are able to i) highlight the role of the aforementioned components in bacterial inactivation and ii) localize the damages in the intracellular domain, despite the addition of the Fenton reagents in the bulk. We report that H₂O₂ oxidizes cell walls but under light the process is of low significance; UVA initiated an intracellular oxidation process based on excess accumulated H₂O₂, while the UVA+H₂O₂ and UVA+H₂O₂+Fe²⁺ processes have the same effect with light, albeit enhanced, as shown by malondialdehyde (MDA) production and ONPG hydrolysis rates. Finally, compared to the UVA-assisted photo-Fenton process, its solar counterpart is enhanced by the direct UVB effects on bacterial DNA. In conclusion, we have sufficient evidence to postulate that the photo-Fenton process is intracellular and propose the pathways that form the integrated bacterial inactivation mechanism by photo-Fenton.

Identification of Genes Associated with Sensitivity to Ultraviolet A (UVA) Irradiation by Transposon Mutagenesis of Vibrio parahaemolyticus

Ultraviolet (UV) irradiation is used to disinfect water and food and can be classified as UVA (detected at wavelengths 320–400 nm), UVB (280–320 nm), and UVC (<280 nm). We developed a method for UVA sterilization of equipment with a UVA-light-emitting diode (LED); however, a high rate of fluence was needed to promote pathogen inactivation. The aim of this study was to identify genes associated with UVA sensitivity with the goal of improving UVA-LED-mediated bactericidal activity. We constructed a transposon-mutant library of Vibrio parahaemolyticus and selected six mutants with high sensitivity to UVA irradiation. Genes associated with this phenotype include F-type H+-transporting ATPases (atp), as well as those involved in general secretion (gsp), and ubiquinone and terpenoid-quinone biosynthesis (ubi). Gene complementation resulted in decreased sensitivity to UVA-LED. The atp mutants had lower intracellular adenosine triphosphate (ATP) concentrations than the wild-type treatment, with 20 mM L-serine resulting in elevated ATP concentrations and decreased sensitivity to UVA-LED. The gsp mutants exhibited high levels of extracellular protein transport and the ubi mutants exhibited significantly different intracellular concentrations of ubiquinone-8. Taken together, our results suggest that the protein products of the atp, gsp, and ubi genes may regulate sensitivity to UVA irradiation.

Solar inactivated Vibrio cholerae induces maturation of JAWS II dendritic cell line in vitro

Solar disinfection (SODIS) has been shown to reduce the risk associated with the contraction of water borne diseases such as cholera. However, little or no research has been undertaken in exploring the role played by the immune system following the consumption of solar inactivated water pathogens. This study investigated the potential for solar inactivated Vibrio cholerae to induce the maturation of dendritic cells in vitro. Dendritic cells are professional antigen presenting cells found in mammals. However, only in their mature form are dendritic cells able to play their role towards a long lasting immune response. Three strains of V. cholerae were solar irradiated for 7 hours. Thereafter, the solar irradiated, non-solar irradiated, phosphate buffered saline prepared and heat/chemically inactivated cultures of V. cholerae as well as lipopolysaccharide and cholerae toxin-β subunit were used to stimulate immature dendritic cells. After 48 hours, the dendritic cells were assessed for the expression of CD54, CD80, CD83, CD86, MHC-I and MHC-II cell surface markers. Results show that solar inactivated V. cholerae was able to induce maturation of the dendritic cells in vitro. These findings suggest that there may be an immunological benefit in consuming SODIS treated water.

Mechanistic insights into UV-A mediated bacterial disinfection via endogenous photosensitizers

UV-A and visible light are thought to excite endogenous photosensitizers in microbes, thereby initiating complex chemical interactions that ultimately kill cells. Natural solar-based disinfection methods have been adapted into commercial lighting technologies with varying degrees of reported efficacy and associated safety hazards for human exposure. Here we utilize a narrow-spectrum UV-A LED prototype (currently in development for health care applications) to investigate the mechanism of bacterial photoinactivation using 365 nm light. Using a combination of reverse genetics and biochemical investigation, we report mechanistic evidence that 365nm light initiates a chain-reaction of superoxide-mediated damage via auto-excitation of vitamin-based electron carriers, specifically vitamin K2 menaquinones and the FAD flavoprotein in Complex II in the electron transport chain. We observe that photoinactivation is modifiable through supplementation of the environment to bypass cell damage. Lastly, we observe that bacteria forced into metabolic dormancy by desiccation become hypersensitized to the effects of UV-A light, thereby permitting photoinactivation at fluences that are significantly lower than the industry threshold for safe human exposure. In total, these results substantiate the mechanism and potential application of narrow- spectrum UV-A light for bacterial disinfection purposes.

Visible light plays a significant role during bacterial inactivation by the photo-fenton process, even at sub-critical light intensities

The aim of this research is to clarify the contribution of sunlight wavelengths, irradiance and Fe²⁺/H₂O₂ during bacterial disinfection by the photo-Fenton process in clear surface waters. We considered different solar spectrum distributions (visible, UVA-Visible), sub-critical irradiances (0-400 W/m²), focusing on the action modes of E. coli inactivation by the constituents involved in the composite process, at low μM reactants concentration (Fe²⁺/H₂O₂) in in ultrapure (MQ) water. We report that solar disinfection improved with Fenton reagents (photo-Fenton process) is a reality from very low light irradiance values (200 W/m²), and made possible even without the presence of UVA radiation, even when using low quantities of the Fenton reagents (0.5 mg/L Fe²⁺, 5 mg/L H₂O₂). Under light exposure, H₂O₂ was found to augment the intracellular Fenton process and Fe²⁺ to initiate further, distinct oxidative actions. Finally, validation was performed in Lake Geneva water over a wider irradiance range, where the photo-Fenton process was found to be reagent-dependent in low irradiance, shifting to light-driven in the higher values.

Mechanisms of Enhanced Antibacterial Activity by Reduced Chitosan-Intercalated Nontronite

Previous studies have documented the antibacterial activity of certain iron-containing clays. However, the repulsion between negatively charged bacteria and the clay surface makes this process inefficient. The objective of this study is to improve the bactericidal efficiency of clays by reversing their surface charge from negative to positive. To achieve this objective, positively charged chitosan, a nontoxic and biodegradable polymer, was intercalated into nontronite NAu-2. Chitosan-intercalated NAu-2 (C-NAu-2) was chemically reduced to obtain reduced C-NAu-2 (rC-NAu-2). Relative to reduced nontronite (rNAu-2), the antibacterial activity of rC-NAu-2 is higher and more persistent over a pH range of 6-8. The close spatial association between positively charged rC-NAu-2 and negatively charged bacteria increases the chances of cell membrane attack by extracellular ROS, the influx of soluble Fe²⁺ into the bacterial cell, and the yield of intracellular ROS. All these factors contribute to the enhanced antibacterial activity of rC-NAu-2. In contrast to rNAu-2 treated E. coli cells, where membrane damage and intracellular ROS/Fe accumulation are restricted to the polar regions, the close bacteria-clay association in rC-NAu-2 results in nonselective membrane damage and more uniform intracellular ROS/Fe distribution across whole bacterial cells. These results advance the antibacterial model by highlighting the importance of bacteria-clay interactions to the antibacterial activity of Fe-bearing clays.

Photocatalytic Bacterial Inactivation by a Rape Pollen-MoS2 Biohybrid Catalyst: Synergetic Effects and Inactivation Mechanisms

A novel and efficient 3D biohybrid photocatalyst, defective MoS₂ nanosheets encapsulated carbonized rape pollen, was fabricated and applied to water disinfection. The rape pollen-MoS₂ (PM) biohybrid showed excellent dispersibility, high stability, and efficient charge-carrier separation and migration ability, resulting in the highly enhanced photocatalytic inactivation performance toward various waterborne bacteria under different light sources. The inactivation mechanisms were systematically investigated. Reactive species (RSs), including electrons, holes, and reactive oxygen species (•O₂^- and •OH), played major roles in inactivating bacteria. The antioxidant system of bacteria exhibited a self-protection capacity by eliminating the photogenerated RSs from PM biohybrid at the early stage of inactivation. With the accumulation of RSs, the cell membrane and membrane-associated functions were destroyed, as suggested by the collapse of cell envelope and subsequent loss of cell respiration and ATP synthesis capacity. The microscopic images further confirmed the destruction of the bacterial membrane. After losing the membrane barrier, the oxidation of cytoplasmic proteins and lipids caused by invaded RSs occurred readily. Finally, the leakage of DNA and RNA announced the irreversible death of bacteria. These results indicated that the bacterial inactivation began with the membrane rupture, followed by the oxidation and leakage of intracellular substances. This work not only provided a new insight into the combination of semiconductors with earth-abundant biomaterials for fabricating high-performance photocatalysts, but also revealed the underlying mechanisms of photocatalytic bacterial inactivation in depth.

Photoelectrocatalytic bacterial inactivation of Acinetobacter baumannii on Cu2O/TiO2@Cu mesh photoanodes

A Cu₂O/TiO₂@Cu mesh acting as a photoanode presented strong photoelectrochemical activity for bacterial inactivation of Acinetobacter baumannii.

Antibiofilm effect of mesoporous titania coatings on Pseudomonas aeruginosa biofilms

Activation of photocatalytic titania by ultraviolet-A (UVA) radiation has been proposed as a good approach for combating bacteria. Titania powder, in solution or immobilized on a surface, has excellent UVA-assisted killing properties on several microorganisms. However, these properties could not be demonstrated in biofilms of Pseudomonas aeruginosa, a resistant opportunistic human pathogen that can cause severe complications in patients who are immunocompromised or have burn wounds or cystic fibrosis. P. aeruginosa biofilms have detrimental effects on health and industry, causing serious economic damage. In this study, the effect of titania photocatalysis for controlling P. aeruginosa biofilms was investigated by employing different coatings obtained through sol-gel and evaporation-induced self-assembly. Biofilms were grown on non-mesoporous and mesoporous titania surfaces with different pore sizes, which were achieved based on the use of surfactants Brij-58 and Pluronics-F127. In addition, two structural forms of titania were assayed: amorphous and anatase. As well as inhibiting biofilm formation, these coatings significantly enhanced the bactericidal effect of UVA on P. aeruginosa biofilms. The most efficient surface with regard to total antibacterial effect was the mesoporous Brij-58-templated anatase film, which, compared to control biofilms, decreased the number of viable bacteria by about 5 orders, demonstrating the efficacy of this methodology as a disinfection system.

Modifications of the Aerobic Respiratory Chain of Paracoccus Denitrificans in Response to Superoxide Oxidative Stress

Paracoccus denitrificans is a strictly respiring bacterium with a core respiratory chain similar to that of mammalian mitochondria. As such, it continuously produces and has to cope with superoxide and other reactive oxygen species. In this work, the effects of artificially imposed superoxide stress on electron transport were examined. Exposure of aerobically growing cells to paraquat resulted in decreased activities of NADH dehydrogenase, succinate dehydrogenase, and N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) oxidase. Concomitantly, the total NAD(H) pool size in cells was approximately halved, but the NADH/NAD⁺ ratio increased twofold, thus partly compensating for inactivation losses of the dehydrogenase. The inactivation of respiratory dehydrogenases, but not of TMPD oxidase, also took place upon treatment of the membrane fraction with xanthine/xanthine oxidase. The decrease in dehydrogenase activities could be fully rescued by anaerobic incubation of membranes in a mixture containing 2-mercaptoethanol, sulfide and ferrous iron, which suggests iron–sulfur clusters as targets for superoxide. By using cyanide titration, a stress-sensitive contribution to the total TMPD oxidase activity was identified and attributed to the cbb₃-type terminal oxidase. This response (measured by both enzymatic activity and mRNA level) was abolished in a mutant defective for the FnrP transcription factor. Therefore, our results provide evidence of oxidative stress perception by FnrP.

Simultaneous Effects of UV-A and UV-B Irradiation on the Survival of Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes in Buffer Solution and Apple Juice

The objective of this study was to evaluate the efficacy of simultaneous UV-A and UV-B irradiation (UV-A+B) for inactivating Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes in both phosphate-buffered saline (PBS) and apple juice. A cocktail of the three pathogens was inoculated into PBS and apple juice, and then the suspensions were irradiated with UV lamps of 356 nm (UV-A) and 307 nm (UV-B). Significant (P < 0.05) log reductions of the three pathogens in PBS and apple juice were observed after a maximum dose of UV-B alone or the UV-A+B treatment, but few reductions were observed upon UV-A treatment alone. At all irradiation times, antagonistic effects were observed for the application of UV-A+B against in E. coli O157:H7, Salmonella Typhimurium, and L. monocytogenes in PBS and apple juice. The degree of antagonistic effect in apple juice was greater than that in PBS. The results of this study suggest that the combined treatment of commercial UV-A and UV-B lamps would be impractical for disinfecting juice products.

Mechanisms investigation on bacterial inactivation through combinations of UV wavelengths

Recently, ultraviolet light-emitting diodes (UV-LEDs) have emerged as a new UV source, bringing flexibility for various UV wavelength combinations due to their unique feature of wavelength diversity. In this study, we investigated inactivation mechanisms of representative microorganisms at different wavelength combinations using UV-LEDs. Two types of indicator microorganisms were examined, namely Escherichia coli (E. coli) as a representative bacteria and bacteriophage MS2 as a representative virus. Different inactivation effects were observed, and the results for UVA pretreatment followed by UVC inactivation were particularly interesting. While a substantial shoulder in the E. coli UVC inactivation curve was observed, this was reduced by UVA pretreatment (365 nm) at 17 J/cm². Further, 52 J/cm² UVA eliminated the shoulder in the fluence-response curves, resulting in improved UVC (265 nm) inactivation of E. coli by over two orders of magnitude. No inactivation improvement was observed for MS2. Moreover, UVA pretreatment eliminated photoreactivation of E. coli but did not affect dark repair. Detailed investigation of inactivation mechanisms revealed that hydroxyl radicals (•OH) played a significant role in the effects of UVA pretreatment. This study demonstrated that •OH radicals were generated inside E. coli cells during UVA pretreatment, which accounted for the subsequent effects on E. coli. The impact of UVA pretreatment on E. coli inactivation and reactivation was mainly due to increased levels of •OH radicals in E. coli cells, impairing cell functions such as DNA self-repair.

The involvement of superoxide radicals in medium pressure UV derived inactivation

Today, two types of lamp systems dominate the UV disinfection industry: low-pressure (LP) UV lamps and medium-pressure (MP) polychromatic lamps. Both lamp types have their advantages and disadvantages in microorganism inactivation, with LP lamps being cheaper, having longer life, and working at lower temperature, hence reducing fouling, and MP lamps showing better inactivation per germicidal dose for certain microorganisms. Bacterium-based biosensors were used to compare LP and MP irradiation. These biosensors were Escherichia coli bacteria carrying the lux operon genes under the control of different stress-responding promoters, where activation of the specific promoter is manifested as bioluminescence. MP irradiation, considerably more than LP irradiation, resulted in activation of the superoxide dismutase expression, indicating the formation of superoxide radicals inside the cells. Accordingly, pre-exposure (immunization) of the bacteria to an activator that produces superoxide radicals resulted in lower inactivation and increased resistance to MP irradiation, but not to LP irradiation. This study shows that the difference in germicidal efficiency may result from the production of intracellular superoxide radicals by MP irradiation, at wavelengths other than 254 nm, as emitted by LP lamps.

Effects of intense pulsed light on Cronobacter sakazakii and Salmonella surrogate Enterococcus faecium inoculated in different powdered foods

Cronobacter sakazakii and Salmonella spp. are foodborne pathogens associated with low moisture foods. An intense pulsed light (IPL) system is being developed as an alternative novel method to pasteurize powdered food. The aim of the study is to investigate the microorganism inactivation in different powdered foods and a variety of related variables using a vibratory-assisted IPL system. The results showed that C. sakazakii on non-fat dry milk (NFDM), wheat flour, and egg white powder were significantly inactivated by 5.27, 4.92, and 5.30 log₁₀ CFU/g, respectively, after 3 or 4 passes of IPL treatments. For decontamination of E. faecium, 3-4 passes of IPL treatments reduced the E. faecium level on NFDM, wheat flour, and egg white by 3.67, 2.79, 2.74 log₁₀ CFU/g, respectively. These results demonstrated that the enhanced microbiological inactivation can be achieved using this vibratory-assisted IPL system after multiple passes.

Analogies and differences among bacterial and viral disinfection by the photo-Fenton process at neutral pH: a mini review

Over the last years, the photo-Fenton process has been established as an effective, green alternative to chemical disinfection of waters and wastewaters. Microorganisms' inactivation is the latest success story in the application of this process at near-neutral pH, albeit without clearly elucidated inactivation mechanisms. In this review, the main pathways of the combined photo-Fenton process against the most frequent pathogen models (Escherichia coli for bacteria and MS2 bacteriophage for viruses) are analyzed. Firstly, the action of solar light is described and the specific inactivation mechanisms in bacteria (internal photo-Fenton) and viruses (genome damage) are presented. The contribution of the external pathways due to the potential presence of organic matter in generating reactive oxygen species (ROS) and their effects on microorganism inactivation are discussed. Afterwards, the effects of the gradual addition of Fe and H₂O₂ are assessed and the differences among bacterial and viral inactivation are highlighted. As a final step, the simultaneous addition of both reagents induces the photo-Fenton in the bulk, focusing on the differences induced by the homogeneous or heterogeneous fraction of the process and the variation among the two respective targets. This work exploits the accumulated evidence on the mechanisms of bacterial inactivation and the scarce ones towards viral targets, aiming to bridge this knowledge gap and make possible the further application of the photo-Fenton process in the field of water/wastewater treatment.

Sunlight-mediated inactivation of health-relevant microorganisms in water: a review of mechanisms and modeling approaches

Health-relevant microorganisms present in natural surface waters and engineered treatment systems that are exposed to sunlight can be inactivated by a complex set of interacting mechanisms. The net impact of sunlight depends on the solar spectral irradiance, the susceptibility of the specific microorganism to each mechanism, and the water quality; inactivation rates can vary by orders of magnitude depending on the organism and environmental conditions. Natural organic matter (NOM) has a large influence, as it can attenuate radiation and thus decrease inactivation by endogenous mechanisms. Simultaneously NOM sensitizes the formation of reactive intermediates that can damage microorganisms via exogenous mechanisms. To accurately predict inactivation and design engineered systems that enhance solar inactivation, it is necessary to model these processes, although some details are not yet sufficiently well understood. In this critical review, we summarize the photo-physics, -chemistry, and -biology that underpin sunlight-mediated inactivation, as well as the targets of damage and cellular responses to sunlight exposure. Viruses that are not susceptible to exogenous inactivation are only inactivated if UVB wavelengths (280-320 nm) are present, such as in very clear, open waters or in containers that are transparent to UVB. Bacteria are susceptible to slightly longer wavelengths. Some viruses and bacteria (especially Gram-positive) are susceptible to exogenous inactivation, which can be initiated by visible as well as UV wavelengths. We review approaches to model sunlight-mediated inactivation and illustrate how the environmental conditions can dramatically shift the inactivation rate of organisms. The implications of this mechanistic understanding of solar inactivation are discussed for a range of applications, including recreational water quality, natural treatment systems, solar disinfection of drinking water (SODIS), and enhanced inactivation via the use of sensitizers and photocatalysts. Finally, priorities for future research are identified that will further our understanding of the key role that sunlight disinfection plays in natural systems and the potential to enhance this process in engineered systems.

New envelope stress factors involved in σE activation and conditional lethality of rpoE mutations in Salmonella enterica

Salmonella enterica serovar Typhimurium (S. typhimurium) can cause food- and water-borne illness with diverse clinical manifestations. One key factor for S. typhimurium pathogenesis is the alternative sigma factor σ^E, which is encoded by the rpoE gene and controls the transcription of genes required for outer-membrane integrity in response to alterations in the bacterial envelope. The canonical pathway for σ^E activation involves proteolysis of the antisigma factor RseA, which is triggered by unfolded outer-membrane porins (OMPs) and lipopolysaccharides (LPS) that have accumulated in the periplasm. This study reports new stress factors that are able to activate σ^E expression. We demonstrate that UVA radiation induces σ^E activity in a pathway that is dependent on the stringent response regulator ppGpp. Survival assays revealed that rpoE has a role in the defence against lethal UVA doses that is mediated by functions that are dependent on and independent of the alternative sigma factor RpoS. We also report that the envelope stress generated by phage infection requires a functional rpoE gene for optimal bacterial tolerance and that it is able to induce σ^E activity in an RseA-dependent fashion. σ^E activity is also induced by hypo-osmotic shock in the absence of osmoregulated periplasmic glucans (OPGs). It is known that the rpoE gene is not essential in S. typhimurium. However, we report here two cases of the conditional lethality of rpoE mutations in this micro-organism. We demonstrate that rpoE mutations are not tolerated in the absence of OPGs (at low to moderate osmolarity) or LPS O-antigen. The latter case resembles that of the prototypic Escherichia coli strain K12, which neither synthesizes a complete LPS nor tolerates null rpoE mutations.

The role of operating parameters and oxidative damage mechanisms of advanced chemical oxidation processes in the combat against antibiotic-resistant bacteria and resistance genes present in urban wastewater

An upsurge in the study of antibiotic resistance in the environment has been observed in the last decade. Nowadays, it is becoming increasingly clear that urban wastewater is a key source of antibiotic resistance determinants, i.e. antibiotic-resistant bacteria and antibiotic resistance genes (ARB&ARGs). Urban wastewater reuse has arisen as an important component of water resources management in the European Union and worldwide to address prolonged water scarcity issues. Especially, biological wastewater treatment processes (i.e. conventional activated sludge), which are widely applied in urban wastewater treatment plants, have been shown to provide an ideal environment for the evolution and spread of antibiotic resistance. The ability of advanced chemical oxidation processes (AOPs), e.g. light-driven oxidation in the presence of H₂O₂, ozonation, homogeneous and heterogeneous photocatalysis, to inactivate ARB and remove ARGs in wastewater effluents has not been yet evaluated through a systematic and integrated approach. Consequently, this review seeks to provide an extensive and critical appraisal on the assessment of the efficiency of these processes in inactivating ARB and removing ARGs in wastewater effluents, based on recent available scientific literature. It tries to elucidate how the key operating conditions may affect the process efficiency, while pinpointing potential areas for further research and major knowledge gaps which need to be addressed. Also, this review aims at shedding light on the main oxidative damage pathways involved in the inactivation of ARB and removal of ARGs by these processes. In general, the lack and/or heterogeneity of the available scientific data, as well as the different methodological approaches applied in the various studies, make difficult the accurate evaluation of the efficiency of the processes applied. Besides the operating conditions, the variable behavior observed by the various examined genetic constituents of the microbial community, may be directed by the process distinct oxidative damage mechanisms in place during the application of each treatment technology. For example, it was shown in various studies that the majority of cellular damage by advanced chemical oxidation may be on cell wall and membrane structures of the targeted bacteria, leaving the internal components of the cells relatively intact/able to repair damage. As a result, further in-depth mechanistic studies are required, to establish the optimum operating conditions under which oxidative mechanisms target internal cell components such as genetic material and ribosomal structures more intensively, thus conferring permanent damage and/or death and preventing potential post-treatment re-growth.

Controlling Bacterial Pathogens in Water for Reuse: Treatment Technologies for Water Recirculation in the Blue Diversion Autarky Toilet

The Blue Diversion AUTARKY Toilet is a urine-diverting toilet with on-site treatment. The toilet is being developed to provide a safe and affordable sanitation technology for people who lack access to sewer-based sanitation. Water used for personal hygiene, hand washing, and flushing to rinse urine- and feces-collection bowls is treated, stored, and recycled for reuse to reduce reliance on external water supplies. The system provides an opportunity to investigate hygiene of water for reuse following treatment. Treatment in the toilet includes a Biologically Activated Membrane Bioreactor (BAMBi) followed by a secondary treatment technology. To identify effective secondary treatment, three options, including granular activated carbon (GAC) only, GAC+chlorine (sodium hypochlorite), and GAC+electrolysis are considered based on the bacterial inactivation and growth inhibition efficiency. Four different hygiene-relevant bacteria are tested: Escherichia coli, Enterococcus faecalis, Pseudomonas aeruginosa, and Salmonella typhimurium. Our evaluation demonstrates that-despite treatment of water with the BAMBi-E. coli, P aeruginosa, and S. typhimurium have the potential to grow during storage in the absence of microbial competition. Including the indigenous microbial community influences bacterial growth in different ways: E. coli growth decreases but P. aeruginosa growth increases relative to no competition. The addition of the secondary treatment options considerably improves water quality. A column of GAC after the BAMBi reduces E. coli growth potential by 2 log₁₀, likely due to the reduction of carbon sources. Additional treatments including chlorination and electrolysis provide further safety margins, with more than 5 log-₁₀ inactivation of E. coli. However, reactivation and/or regrowth of E. coli and P. aeruginosa occurs under in the absence of residual disinfectant. Treatment including the BAMBi, GAC, and electrolysis appear to be promising technologies to control bacterial growth during storage in water intended for reuse.

Staphylococcus aureus Strain Newman Photoinactivation and Cellular Response to Sunlight Exposure

Sunlight influences microbial water quality of surface waters. Previous studies have investigated photoinactivation mechanisms and cellular photostress responses of fecal indicator bacteria (FIB), including Escherichia coli and enterococci, but further work is needed to characterize photostress responses of bacterial pathogens. Here we investigate the photoinactivation of Staphylococcus aureus (strain Newman), a pigmented, waterborne pathogen of emerging concern. We measured photodecay using standard culture-based assays and cellular membrane integrity and investigated photostress response by measuring the relative number of mRNA transcripts of select oxidative stress, DNA repair, and metabolism genes. Photoinactivation experiments were performed in both oxic and anoxic systems to further investigate the role of oxygen-mediated and non-oxygen-mediated photoinactivation mechanisms. S. aureus lost culturability much faster in oxic systems than in anoxic systems, indicating an important role for oxygen in photodecay mechanisms. S. aureus cell membranes were damaged by sunlight exposure in anoxic systems but not in oxic systems, as measured by cell membrane permeability to propidium iodide. After sunlight exposure, S. aureus increased expression of a gene coding for methionine sulfoxide reductase after 12 h of sunlight exposure in the oxic system and after 6 h of sunlight exposure in the anoxic system, suggesting that methionine sulfoxide reductase is an important enzyme for defense against both oxygen-dependent and oxygen-independent photostresses. This research highlights the importance of oxygen in bacterial photoinactivation in environmentally relevant systems and the complexity of the bacterial photostress response with respect to cell structure and transcriptional regulation.IMPORTANCEStaphylococcus aureus is a pathogenic bacterium that causes gastrointestinal, respiratory, and skin infections. In severe cases, S. aureus infection can lead to life-threatening diseases, including pneumonia and sepsis. Cases of community-acquired S. aureus infection have been increasing in recent years, pointing to the importance of considering S. aureus transmission pathways outside the hospital environment. Associations have been observed between recreational water contact and staphylococcal skin infections, suggesting that recreational waters may be an important environmental transmission pathway for S. aureus However, prediction of human health risk in recreational waters is hindered by incomplete knowledge of pathogen sources, fate, and transport in this environment. This study is an in-depth investigation of the inactivation of a representative strain of S. aureus by sunlight exposure, one of the most important factors controlling the fate of microbial contaminants in clear waters, which will improve our ability to predict water quality changes and human health risk in recreational waters.

On mechanism behind UV-A light enhanced antibacterial activity of gallic acid and propyl gallate against Escherichia coli O157:H7

Possible mechanisms behind the enhanced antimicrobial activity of gallic acid (GA) and its ester propyl gallate (PG) in the presence of UV-A light against Escherichia coli O157:H7 were investigated. GA by itself is a mild antimicrobial and has a pro-oxidant ability. We found that the presence of UV-A light increases the uptake of GA by the bacteria. Once GA is internalized, the interaction between GA and UV-A induces intracellular ROS formation, leading to oxidative damage. Concurrently, GA + UV-A also inhibits the activity of superoxide dismutase (SOD), magnifying the imbalance of redox status of E. coli O157:H7. In addition to ROS induced damage, UV-A light and GA also cause injury to the cell membrane of E. coli O157:H7. UV-A exposed PG caused oxidative damage to the cell and significantly higher damage to the cell membrane than GA + UV-A treatment, explaining its higher effectiveness than GA + UV-A treatment. The findings presented here may be useful in developing new antimicrobial sanitation technologies for food and pharmaceutical industries.

ATP-synthesis capacity of pulsed light-exposed bacteria

The ability of four different bacteria to synthesize new ATP upon exposure to different doses of pulsed-light (PL) irradiation was investigated. The bacterial cells were PL treated on a gel surface, resuspended in phosphate buffered saline (PBS) and subsequently incubated in Tryptic Soy Broth (TSB) at 37°C. Cellular ATP levels were monitored during a 2h incubation period and compared to the respective colony count data. Although PL affected ATP production in a dose dependent manner, the results showed that bacteria, which had rendered unculturable after PL exposure, are still capable of generating significant quantities of ATP. Escherichia coli and Listeria innocua proved to be more resistant to PL than Salmonella enterica and Staphylococcus aureus, which was supported by the colony count data and the ATP synthesis capacity. These findings underline that bacteria undetectable by culture-based methods may still show cellular activity and synthesize new ATP.

Enhanced Antimicrobial Activity Based on a Synergistic Combination of Sublethal Levels of Stresses Induced by UV-A Light and Organic Acids

The reduction of microbial load in food and water systems is critical for their safety and shelf life. Conventionally, physical processes such as heat or light are used for the rapid inactivation of microbes, while natural compounds such as lactic acid may be used as preservatives after the initial physical process. This study demonstrates the enhanced and rapid inactivation of bacteria based on a synergistic combination of sublethal levels of stresses induced by UV-A light and two food-grade organic acids. A reduction of 4.7 ± 0.5 log CFU/ml in Escherichia coli O157:H7 was observed using a synergistic combination of UV-A light, gallic acid (GA), and lactic acid (LA), while the individual treatments and the combination of individual organic acids with UV-A light resulted in a reduction of less than 1 log CFU/ml. Enhanced inactivation of bacteria on the surfaces of lettuce and spinach leaves was also observed based on the synergistic combination. Mechanistic investigations suggested that the treatment with a synergistic combination of GA plus LA plus UV-A (GA+LA+UV-A) resulted in significant increases in membrane permeability and intracellular thiol oxidation and affected the metabolic machinery of E. coli In addition, the antimicrobial activity of the synergistic combination of GA+LA+UV-A was effective only against metabolically active E. coli O157:H7. In summary, this study illustrates the potential of simultaneously using a combination of sublethal concentrations of natural antimicrobials and a low level of physical stress in the form of UV-A light to inactivate bacteria in water and food systems.IMPORTANCE There is a critical unmet need to improve the microbial safety of the food supply, while retaining optimal nutritional and sensory properties of food. Furthermore, there is a need to develop novel technologies that can reduce the impact of food processing operations on energy and water resources. Conventionally, physical processes such as heat and light are used for inactivating microbes in food products, but these processes often significantly reduce the sensory and nutritional properties of food and are highly energy intensive. This study demonstrates that the combination of two natural food-grade antimicrobial agents with a sublethal level of physical stress in the form of UV-A light can greatly increase microbial load inactivation. In addition, this report elucidates the potential mechanisms for this synergistic interaction among physical and chemical stresses. Overall, these results provide a novel approach to develop antimicrobial solutions for food and water systems.

Molecular Viability Testing of UV-Inactivated Bacteria

PCR is effective in detecting bacterial DNA in samples, but it is unable to differentiate viable bacteria from inactivated cells or free DNA fragments. New PCR-based analytical strategies have been developed to address this limitation. Molecular viability testing (MVT) correlates bacterial viability with the ability to rapidly synthesize species-specific rRNA precursors (pre-rRNA) in response to brief nutritional stimulation. Previous studies demonstrated that MVT can assess bacterial inactivation by chlorine, serum, and low-temperature pasteurization. Here, we demonstrate that MVT can detect inactivation of Escherichia coli, Aeromonas hydrophila, and Enterococcus faecalis cells by UV irradiation. Some UV-inactivated E. coli cells transiently retained the ability to synthesize pre-rRNA postirradiation (generating false-positive MVT results), but this activity ceased within 1 h following UV exposure. Viable but transiently undetectable (by culture) E. coli cells were consistently detected by MVT. An alternative viability testing method, viability PCR (vPCR), correlates viability with cell envelope integrity. This method did not distinguish viable bacteria from UV-inactivated bacteria under some conditions, indicating that the inactivated cells retained intact cell envelopes. MVT holds promise as a means to rapidly assess microbial inactivation by UV treatment.IMPORTANCE UV irradiation is increasingly being used to disinfect water, food, and other materials for human use. Confirming the effectiveness of UV disinfection remains a challenging task. In particular, microbiological methods that rely on rapid detection of microbial DNA can yield misleading results, due to the detection of remnant DNA associated with dead microbial cells. This report describes a novel method that rapidly distinguishes living microbial cells from dead microbial cells after UV disinfection.

Activation of persulfates by natural magnetic pyrrhotite for water disinfection: Efficiency, mechanisms, and stability

This study introduces natural occurring magnetic pyrrhotite (NP) as an environmentally friendly, easy available, and cost-effective alternative catalyst to activate persulfate (PS) of controlling microbial water contaminants. The E. coli K-12 inactivation kinetics observed in batch experiments was well described with first-order reaction. The optimum inactivation rate (k = 0.47 log/min) attained at a NP dose of 1 g/L and a PS dose of 1 mM, corresponding to total inactivation of 7 log₁₀ cfu/mL cells within 15 min. Measured k increased > 2-fold when temperature increased from 20 to 50 °C; and > 4-fold when pH decreased from 9 to 3. Aerobic conditions were more beneficial to cell inactivation than anaerobic conditions due to more reactive oxygen species (ROS) generated. ROS responsible for the inactivation were identified to be SO₄^- > OH > H₂O₂ based on a positive scavenging test and in situ ROS determination. In situ characterization suggested that PS effectively bind to NP surface was likely to form charge transfer complex (≡Fe(II)⋯O₃SOOSO₃^-), which mediated ROS generation and E. coli K-12 oxidation. The increased cell-envelope lesions consequently aggravated intracellular protein depletion and genome damage to cause definite bacterial death. The NP still maintained good physiochemical structure and stable activity even after 4 cycle. Moreover, NP/PS system also exhibited good E. coli K-12 inactivation efficiency in authentic water matrices like surface water and effluents of secondary wastewater.