Synergistic efficacy of 405 nm light and chlorinated disinfectants for the enhanced decontamination of Clostridium difficile spores

The ability of Clostridium difficile to form highly resilient spores which can survive in the environment for prolonged periods causes major contamination problems. Antimicrobial 405 nm light is being developed for environmental decontamination within hospitals, however further information relating to its sporicidal efficacy is required. This study aims to establish the efficacy of 405 nm light for inactivation of C. difficile vegetative cells and spores, and to establish whether spore susceptibility can be enhanced by the combined use of 405 nm light with low concentration chlorinated disinfectants. Vegetative cells and spore suspensions were exposed to increasing doses of 405 nm light (at 70–225 mW/cm2) to establish sensitivity. A 99.9% reduction in vegetative cell population was demonstrated with a dose of 252 J/cm2, however spores demonstrated higher resilience, with a 10-fold increase in required dose. Exposures were repeated with spores suspended in the hospital disinfectants sodium hypochlorite, Actichlor and Tristel at non-lethal concentrations (0.1%, 0.001% and 0.0001%, respectively). Enhanced sporicidal activity was achieved when spores were exposed to 405 nm light in the presence of the disinfectants, with a 99.9% reduction achieved following exposure to 33% less light dose than required when exposed to 405 nm light alone. In conclusion, C. difficile vegetative cells and spores can be successfully inactivated using 405 nm light, the sporicidal efficacy can be significantly enhanced when exposed in the presence of low concentration chlorinated disinfectants. Further research may lead to the potential use of 405 nm light decontamination in combination with selected hospital disinfectants to enhance C. difficile cleaning and infection control procedures

Nocturnal pollination: An overlooked ecosystem service vulnerable to environmental change

© 2020 The Author(s). This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and the Royal Society of Biology and distributed under the Creative Commons Attribution License 4.0 (CC BY). Existing assessments of the ecosystem service of pollination have been largely restricted to diurnal insects, with a particular focus on generalist foragers such as wild and honey bees. As knowledge of how these plant-pollinator systems function, their relevance to food security and biodiversity, and the fragility of these mutually beneficial interactions increases, attention is diverting to other, less well-studied pollinator groups. One such group are those that forage at night. In this review, we document evidence that nocturnal species are providers of pollination services (including pollination of economically valuable and culturally important crops, as well as wild plants of conservation concern), but highlight how little is known about the scale of such services. We discuss the primary mechanisms involved in night-time communication between plants and insect pollen-vectors, including floral scent, visual cues (and associated specialized visual systems), and thermogenic sensitivity (associated with thermogenic flowers). We highlight that these mechanisms are vulnerable to direct and indirect disruption by a range of anthropogenic drivers of environmental change, including air and soil pollution, artificial light at night, and climate change. Lastly, we highlight a number of directions for future research that will be important if nocturnal pollination services are to be fully understood and ultimately conserved

Electrode erosion and lifetime performance of a triggered, corona-stabilized switch in SF6, at a repetition rate of 1kHz

This paper describes the work undertaken to investigate the electrode erosion and lifetime performance of an electrically triggered, corona stabilized (TCS) switch. The switch, which had a rod-plane geometry, was filled with SF6 at a pressure of 0.8 bar (1.0bar = 100kPa) absolute and was operated at a pulse repetition frequency (PRF) of 1kHz. The erosion rates of anode and cathode electrode materials such as elkonite (a copper tungsten composite), brass, aluminum and two types of stainless steel was measured, and their surfaces were studied optically, photographed and chemically analyzed. The erosion rate of the anode (rod) electrode materials ranged from 21.39 x 10-6 cm3C-1 for elkonite to 60.4 x 10-6 cm3C-1 for brass, whilst a cathode (trigger disc) erosion rate as high as 51.23 x 10-6 cm3C-1 for aluminum was measured. The lifetime of the TCS switch was experimentally determined for each of the electrode material tested, and was found to be inversely proportional to the erosion rate of the rod electrode. The electrode surface morphology and the gas-electrode compound distribution have also been evaluated to address the erosion mechanism in effect. Additionally, the influence of rod electrode surface conditions and gas deterioration has been investigated in respect to the performance and operational behavior of the TCS switch

Comparative sensitivity of Trichophyton and Aspergillus conidia to inactivation by violet-blue light exposure

The objective of this paper was to investigate the use of 405 nm light for inhibiting the growth of selected species of dermatophytic and saprophytic fungi. The increasing incidence and resilience of dermatophytic fungal infections is a major issue, and alternative treatment methods are being sought. The sensitivity of the dermatophytic fungi Trichophyton rubrum and Trichophyton mentagrophytes to 405 nm violet-blue light exposure was investigated, and the results compared with those obtained with the saprophytic fungus Aspergillus niger. Microconidia of T. rubrum and T. mentagrophytes and conidia of A. niger were seeded onto Sabauroud dextrose agar plates and irradiated with 405 nm light from an indium-gallium-nitride 99-DIE light-emitting diode (LED) array and the extent of inhibition was measured. Germination of the microconidia of the Trichophyton species was completely inhibited using an irradiance of 35 mW/cm2 for 4 h (dose of 504 J/cm2). Results: A. niger conidia showed greater resistance, and colonial growth developed after light exposure. In liquid suspension tests, 405 nm light dose levels of 360, 720, and 1440 J/cm2 resulted in complete inactivation of T. rubrum microconidia, whereas A. niger showed greater resistance, and at the highest dose level applied (1440 J/cm2 ) although A niger hyphae were completely inactivated, only a 3-log10 reduction of a 5-log10 conidial suspension was achieved. The study results demonstrate the relatively high sensitivity of Trichophyton microconidia to 405 nm violet-blue light, and this is may be of potential interest regarding the control and treatment of dermatophyte infections

New proof-of-concept in viral inactivation: virucidal efficacy of 405 nm light against feline calicivirus as a model for norovirus decontamination

The requirement for novel decontamination technologies for use in hospitals is ever present. One such system uses 405 nm visible light to inactivate microorganisms via ROS-generated oxidative damage. Although effective for bacterial and fungal inactivation, little is known about the virucidal effects of 405 nm light. Norovirus (NoV) gastroenteritis outbreaks often occur in the clinical setting, and this study was designed to investigate potential inactivation effects of 405 nm light on the NoV surrogate, feline calicivirus (FCV). FCV was exposed to 405 nm light whilst suspended in minimal and organically-rich media to establish the virucidal efficacy and the effect biologically-relevant material may play in viral susceptibility. Antiviral activity was successfully demonstrated with a 4 Log10 (99.99%) reduction in infectivity when suspended in minimal media evident after a dose of 2.8 kJ cm−2. FCV exposed in artificial faeces, artificial saliva, blood plasma and other organically rich media exhibited an equivalent level of inactivation using between 50–85% less dose of the light, indicating enhanced inactivation when the virus is present in organically-rich biologically-relevant media. Further research in this area could aid in the development of 405 nm light technology for effective NoV decontamination within the hospital environment

Development of an antimicrobial blended white LED system containing pulsed 405-nm LEDs for decontamination applications

This study details the design, build and testing of a prototype antimicrobial blended white light unit containing pulsed red, yellow, green and 405nm LEDs. With a push for alternative methods of disinfection, optical methods have become a topic of interest. Ultra-violet (UV) light is widely known for its antimicrobial properties however; 405nm light has demonstrated significant antimicrobial properties against many common hospital acquired pathogens. In this study, a pulsed, blended, white-light prototype with a high content of 405 nm antimicrobial light, was designed, built and tested. Antimicrobial efficacy testing of the prototype was conducted using Staphylococcus aureus and Pseudomonas. aeruginosa, two bacteria which are common causes of hospital acquired infections. These were exposure to 3 different light outputs from the prototype and the surviving bacteria enumerated. Results showed that the mixed light output provided a much better CRI and light output under which to work. Also, the light output containing 405 nm light provided an antimicrobial effect, with decontamination of 103 CFUml-1 populations of both bacterial species. The other light content (red, yellow, green) had no beneficial or adverse effects on the antimicrobial properties of the 405nm light. The results suggest that with further development, it could be possible to produce an antimicrobial blended white light containing pulsed 405nm light that could supplement or even replace standard white lighting in certain environments

Cytotoxic responses to 405 nm light exposure in mammalian and bacterial cells : involvement of reactive oxygen species

Light at wavelength 405 nm is an effective bactericide. Previous studies showed that exposing mammalian cells to 405 nm light at 36 J/cm2 (a bactericidal dose) had no significant effect on normal cell function, although at higher doses (54 J/cm2), mammalian cell death became evident. This research demonstrates that mammalian and bacterial cell toxicity induced by 405 nm light exposure is accompanied by reactive oxygen species production, as detected by generation of fluorescence from 6-carboxy-2’,7’- dichlorodihydrofluorescein diacetate. As indicators of the resulting oxidative stress in mammalian cells, a decrease in intracellular reduced glutathione content and a corresponding increase in the efflux of oxidised glutathione was observed from 405 nm light treated cells. The mammalian cells were significantly protected from dying at 54 J/cm2 in the presence of catalase, which detoxifies H2O2. Bacterial cells were significantly protected by sodium pyruvate (H2O2 scavenger) and by a combination of free radical scavengers (sodium pyruvate, dimethyl thiourea (OH scavenger) and catalase) at 162 and 324 J/cm2. Results herefore suggested that the cytotoxic mechanism of 405 nm light in mammalian cells and bacteria could be oxidative stress involving predominantly H2O2 generation, with other ROS contributing to the damage

Enhanced decontamination of C. difficile spores on surfaces via the synergistic action of 405nm light and disinfectants

The ability of C. difficile to form spores which can survive for prolonged periods causes significant environmental contamination problems. 405nm light has wide antimicrobial activity against vegetative bacteria, and is being developed for environmental decontamination within hospitals. As expected, spores are more resilient to inactivation. This study aims to establish whether spore susceptibility can be enhanced by combining 405nm light with low concentration chlorinated disinfectants: sodium hypochlorite, Actichlor and Tristel. Spore suspensions were seeded onto surfaces including PVC, stainless steel and vinyl flooring. Disinfectant was added to the surface, and the samples were then exposed to 405nm light at irradiances of ~0.2-225 mWcm-2. Control samples were exposed to 405nm light alone, and disinfectants alone, to establish the sporicidal activity of each agent, and to demonstrate the synergistic effect when combined. Results demonstrated increased sporicidal activity of 405nm light and low-concentration sodium hypochlorite and Actichlor against C. difficile seeded on vinyl flooring and PVC surfaces, with approximately 3-log10 reductions achieved with up to 66% lower doses than achieved with light alone. Tristel demonstrated limited synergy on vinyl and PVC, whilst all three disinfectants demonstrated minimal synergy on stainless steel. Results are also reported for lower intensity light, as used in the clinical environment. In conclusion, the sporicidal efficacy of 405nm light is enhanced when used alongside chlorinated disinfectants. Further research could potentially lead to the use of lower strength chlorinated disinfectants in combination with 405nm light to provide enhanced decontamination of C. difficile spores in the clinical environment

Hydroxyl radical production in DC streamer discharge

Plasma-induced advanced oxidation processes do not suffer from the drawbacks, such as carcinogenic by-products, associated with conventional water treatment, and enable the removal of micro-pollutants. The high oxidation strength of hydroxyl radicals enables degradation of resistant contaminants. Many reactions are known to occur at the plasma-water interface; however, the mechanisms of hydroxyl radical production are still not clear. To understand the physical and chemical processes occurring at the plasma-water interface, this research involved investigation of the hydroxyl radicals produced during d.c. streamer discharges. A needle-plate electrode configuration in atmospheric air was used, with the treated solution used as the ground electrode. To understand the effects of polarity and gas type on hydroxyl radical production, both positive- and negative-polarity energization in air, nitrogen and helium were investigated. Plasma filaments were developed from the needle electrode, which was in contact with the solution. Terephthalic acid (TA) was used as a scavenger of hydroxyl (OH) radicals, with OH density subsequently being quantified by fluorescence emission from 2-hydroxyterephthalic acid (HTA), which is formed through specific reaction between TA and OH. The power inputs in positive pulsed streamer discharges were 0.125 W, 0.18 W and 0.26 W in air, nitrogen and helium, respectively; the corresponding hydroxyl radical production efficiencies were 0.56 mmol/kWh, 1.1 mmol/kWh and 5.94 mmol/kWh, respectively. For negative pulsed streamer discharges in air, the power input was 0.063 W and the efficiency was 1 mmol/kWh. The hydroxyl radical production rates were 2.6× 10-7 Ms-1 in negative air discharges, and 2.7× 10-7 Ms-1, 1.8× 10-6 Ms-1, and 2.2× 10-6 Ms-1 in positive air, nitrogen and helium discharges, respectively

Efficacy of antimicrobial 405 nm blue-light for inactivation of airborne bacteria

Airborne transmission of infectious organisms is a considerable concern within the healthcare environment. A number of novel methods for ‘whole room’ decontamination, including antimicrobial 405 nm blue light, are being developed. To date, research has focused on its effects against surface-deposited contamination; however, it is important to also establish its efficacy against airborne bacteria. This study demonstrates evidence of the dose-response kinetics of airborne bacterial contamination when exposed to 405 nm light and compares bacterial susceptibility when exposed in three different media: air, liquid and surfaces. Bacterial aerosols of Staphylococcus epidermidis, generated using a 6-Jet Collison nebulizer, were introduced into an aerosol suspension chamber. Aerosolized bacteria were exposed to increasing doses of 405 nm light, and air samples were extracted from the chamber using a BioSampler liquid impinger, with viability analysed using pour-plate culture. Results have demonstrated successful aerosol inactivation, with a 99.1% reduction achieved with a 30 minute exposure to high irradiance (22 mWcm-2) 405 nm light (P=0.001). Comparison to liquid and surface exposures proved bacteria to be 3-4 times more susceptible to 405 nm light inactivation when in aerosol form. Overall, results have provided fundamental evidence of the susceptibility of bacterial aerosols to antimicrobial 405 nm light treatment, which offers benefits in terms of increased safety for human exposure, and eradication of microbes regardless of antibiotic resistance. Such benefits provide advantages for a number of applications including ‘whole room’ environmental decontamination, in which reducing levels of airborne bacteria should reduce the number of infections arising from airborne contamination