Blue light kills Aggregatibacter actinomycetemcomitans due to its endogenous photosensitizers

Photoinactivation by UVA radiation and visible light of Candida auris compared to other fungi

In addition to the rising number of patients affected by viruses and bacteria, the number of fungal infections has also been rising over the years. Due to the increase in resistance to various antimycotics, investigations into further disinfection options are important. In this study, two yeasts (Candida auris and Saccharomyces cerevisiae) and a mold (Cladosporium cladosporioides) were irradiated at 365, 400, and 450 nm individually. The resulting log 1 reduction doses were determined and compared with other studies. Furthermore, fluorescence measurements of C. auris were performed to detect possible involved photosensitizers. A roughly exponential photoinactivation was observed for all three fungi and all irradiation wavelengths with higher D90 doses for longer wavelengths. The determined log 1 reduction doses of C. auris and S. cerevisiae converged with increasing wavelength. However, S. cerevisiae was more photosensitive than C. auris for all irradiation wavelengths and is therefore not a suitable C. auris surrogate for photoinactivation experiments. For the mold C. cladosporioides, much higher D90 doses were determined than for both yeasts. Concerning potential photosensitizers, flavins and various porphyrins were detected by fluorescence measurements. By excitation at 365 nm, another, so far unreported fluorophore and potential photosensitizer was also observed. Based on its fluorescence spectrum, we assume it to be thiamine.Graphic abstract.

Biphasic photobiomodulation of inflammation in mouse models of common wounds, infected wounds, and diabetic wounds

Bidirectional photobiomodulation (PBM) therapy is an active research area. However, most studies have focused on its dependence on optical parameters rather than on its tissue-dependent effects. We constructed mouse models of wounds in three inflammatory states (normal, low, and high levels of inflammations) to assess the bidirectional regulatory effect of PBM on inflammation. Mice were divided into three groups to prepare common wounds, diabetic wounds, and bacteria-infected wounds. The same PBM protocol was used to regularly irradiate the wounds over a 14 d period. PBM promoted healing of all three kinds of wounds, but the inflammatory manifestations in each were significantly different. In common wounds, PBM slightly increased the aggregation of inflammatory cells and expression of IL-6 but had no effect on the inflammatory score. For wounds in a high level of inflammation caused by infection, PBM significantly increased TNF-α expression in the first 3 d of treatment but quickly eliminated inflammation after the acute phase. For the diabetic wounds in a low level of inflammation, PBM intervention significantly increased inflammation scores and prevented neutrophils from falling below baseline levels at the end of the 14 d observation period. Under fixed optical conditions, PBM has a bidirectional (pro- or anti-inflammatory) effect on inflammation, depending on the immune state of the target organism and the presence of inflammatory stimulants. Our results provide a basis for the formulation of clinical guidelines for PBM application.

Can antimicrobial blue light contribute to resistance development? Genome-wide analysis revealed aBL-protective genes in Escherichia coli

IMPORTANCE

Increasing antibiotic resistance and the lack of new antibiotic-like compounds to combat bacterial resistance are significant problems of modern medicine. The development of new alternative therapeutic strategies is extremely important. Antimicrobial blue light (aBL) is an innovative approach to combat multidrug-resistant microorganisms. aBL has a multitarget mode of action; however, the full mechanism of aBL antibacterial action requires further investigation. In addition, the potential risk of resistance development to this treatment should be considered.

Photoinactivation of the bacteriophage PhiX174 by UVA radiation and visible light in SM buffer and DMEM-F12

OBJECTIVE

It has been observed that viruses can be inactivated by UVA radiation and visible light. The aim of this study is to investigate whether a medium that contains a photosensitizer might have an influence on viral reduction under irradiation by UVA, violet or blue light. Test virus is the bacteriophage PhiX174 in the photosensitizer-free SM buffer and DMEM-F12, which contains the known photosensitizer riboflavin.

RESULTS

The determined PhiX174 D90 doses in SM buffer and DMEM were 36.8 J/cm² and 13.6 J/cm² at 366 nm, 153.6 J/cm² and 129.1 J/cm² at 408 nm and 4988 J/cm² and 2477.1 J/cm² at 455 nm, respectively. It can be concluded that the medium has a large influence on the results. This might be caused by the photosensitizer riboflavin in DMEM-F12. As riboflavin is a key component in many cell culture media, irradiation experiments with viruses in cell culture media should be avoided if the investigation of intrinsical photoinactivation properties of viruses is aimed for.

The influence of growth medium on the photodynamic susceptibility of Aggregatibacter actinomycetemcomitans to antimicrobial blue light

The aim of this study was to investigate whether antimicrobial blue light (aBL) can cause the death of Aggregatibacter actinomycetemcomitans (A.a) and to determine the influence of different culture media, specifically brain heart infusion and blood agar, on bacterial survival fraction. An LED emitting at 403 ± 15 nm, with a radiant power of 1W, irradiance of 588.2 mW/cm2, and an irradiation time of 0 min, 1 min, 5 min, 10 min, 30 min, and 60 min, was used. The plates were incubated in microaerophilic conditions at 37 °C for 48 h, and the colony-forming units were counted. The photosensitizers were investigated using spectroscopy and fluorescence microscopy. There was no significant difference between the culture media (p > 0.05). However, a statistical reduction in both media was observed at 30 min (1058 J/cm2) (p < 0.05). The findings of this study suggest that aBL has the potential to kill bacteria regardless of the culture media used. Light therapy could be a promising and cost-effective strategy for preventing periodontal disease when used in combination with mechanical plaque control.

Could violet-blue lights increase the bacteria resistance against ultraviolet radiation mediated by photolyases?

Studies have demonstrated bacterial inactivation by radiations at wavelengths between 400 and 500 nm emitted by low-power light sources. The phototoxic activity of these radiations could occur by oxidative damage in DNA and membrane proteins/lipids. However, some cellular mechanisms can reverse these damages in DNA, allowing the maintenance of genetic stability. Photoreactivation is among such mechanisms able to repair DNA damages induced by ultraviolet radiation, ranging from ultraviolet A to blue radiations. In this review, studies on the effects of violet and blue lights emitted by low-power LEDs on bacteria were accessed by PubMed, and discussed the repair of ultraviolet-induced DNA damage by photoreactivation mechanisms. Data from such studies suggested bacterial inactivation after exposure to violet (405 nm) and blue (425-460 nm) radiations emitted from LEDs. However, other studies showed bacterial photoreactivation induced by radiations at 348-440 nm. This process occurs by photolyase enzymes, which absorb photons at wavelengths and repair DNA damage. Although authors have reported bacterial inactivation after exposure to violet and blue radiations emitted from LEDs, pre-exposure to such radiations at low fluences could activate the photolyases, increasing resistance to DNA damage induced by ultraviolet radiation.

Blue Light-Emitting Diode Irradiation Without a Photosensitizer Alters Oral Microbiome Composition of Ligature-Induced Periodontitis in Mice

Objective: This study investigated the suppressive effects of blue light-emitting diode (LED) irradiation on bone resorption and changes in the oral microbiome of mice with ligature-induced periodontitis. Background: Wavelength of blue light has antimicrobial effects; however, whether blue LED irradiation alone inhibits the progression of periodontitis remains unclear. Methods: Nine-week-old male mice ligated ligature around the right maxillary second molar was divided into ligation alone (Li) and ligation with blue LED irradiation (LiBL) groups. The LiBL group underwent blue LED (wavelength, 455 nm) irradiation four times in a week at 150 mW/cm2 without a photosensitizer on the gingival tissue around the ligated tooth at a distance of 5 mm for 5 min. The total energy density per day was 45 J/cm2. Bone resorption was evaluated using micro-computed tomography at 8 days. Differences in the oral microbiome composition of the collected ligatures between the Li and LiBL groups were analyzed using next-generation sequencing based on the 16S rRNA gene from the ligatures. Results: Blue LED irradiation did not suppress bone resorption caused by ligature-induced periodontitis. However, in the LiBL group, the α-diversity, number of observed features, and Chao1 were significantly decreased. The relative abundances in phylum Myxococcota and Bacteroidota were underrepresented, and the genera Staphylococcus, Lactococcus, and Lactobacillus were significantly overrepresented by blue LED exposure. Metagenomic function prediction indicated an increase in the downregulated pathways related to microbial energy metabolism after irradiation. The co-occurrence network was altered to a simpler structure in the LiBL group, and the number of core genera decreased. Conclusions: Blue LED irradiation altered the composition and network of the oral microbiome of ligature-induced periodontitis in mice.

Antimicrobial photodynamic therapy against oral biofilm: influencing factors, mechanisms, and combined actions with other strategies

Oral biofilms are a prominent cause of a wide variety of oral infectious diseases which are still considered as growing public health problems worldwide. Oral biofilms harbor specific virulence factors that would aggravate the infectious process and present resistance to some traditional therapies. Antimicrobial photodynamic therapy (aPDT) has been proposed as a potential approach to eliminate oral biofilms via in situ-generated reactive oxygen species. Although numerous types of research have investigated the effectiveness of aPDT, few review articles have listed the antimicrobial mechanisms of aPDT on oral biofilms and new methods to improve the efficiency of aPDT. The review aims to summarize the virulence factors of oral biofilms, the progress of aPDT in various oral biofilm elimination, the mechanism mediated by aPDT, and combinatorial approaches of aPDT with other traditional agents.

Efficacy of ozonated water as a PS in photodynamic therapy: A tool for dental caries management? An in vitro study

Background

The most prevalent noncommunicable disease in the world is dental caries; and when it is not adequately treated, it is usually associated with tooth loss or severe dental lesions. In fact, expensive care or tooth extraction may be necessary due to the negative effects dental caries have on general health. This is due to its frequent pain and secondary bacterial infections. The aim of this study was to investigate the activity of ozonated water as such and in combination with appropriate light radiation so as to perform a photodynamic treatment (PDT) against the cariogenic bacterium Streptococcus mutans.

Design and methods

This work has been performed in vitro by using an S. mutans strain mainly structured in a biofilm status, reproducing the natural condition of the tooth infection. The ozone was tested at three different concentrations by using a commercial device able to generate different O3 formulations in water. The PDT treatment requires an appropriate light wavelength, evaluated in this work through the UV-Vis adsorption spectrum of the ozonated water.

Results

The obtained results suggested an effective and synergic property of O3 and light at 460-470 nm against this microorganism. The most antibiofilm activity was observed using a concentration of ozone of 0.06 mg/L alone as well as with PDT treatment.

Conclusions

The results are encouraging for additional research and in vitro/in vivo fresh experimental investigations to perform an exhaustive antimicrobial treatment protocol against the S. mutans tooth infection.

The Effectiveness of Laser Applications and Photodynamic Therapy on Relevant Periodontal Pathogens (Aggregatibacter actinomycetemcomitans) Associated with Immunomodulating Anti-rheumatic Drugs

Considering the current context of the increasing resistance of bacterial species to antibiotics and other antimicrobial agents, a major objective is to develop other antimicrobial approaches, which would be able to inactivate pathogens with considerable effectiveness. Two such methods are photodynamic disinfection therapy and laser irradiation. In view of the immunocompromised status of some patients under immunosuppressive therapy and potential drug interactions that can be established between systemic antimicrobial agents, the research of local, minimally invasive methods of inactivating periodontal pathogens in the context of these systemic therapies with modifying drugs of the immune response is justified. This in vitro study evaluated the antimicrobial action of a diode laser, wavelength 940 nm, and photodisinfection therapy at 670 nm (photosensitizer, 3,7 dimethyl phenothiazine chloride) on a type strain of Aggregatibacter actinomycetemcomitans, a known periodontal pathogen, in the presence and absence of active substances used in autoimmune disease therapy (Etanercept, Infliximab, Metothrexate). The association of a conventional antirheumatic drug with anti-TNF-α therapy determined a significantly greater inhibition of the strain of A. actinomycetemcomitans compared to monotherapy, in vitro. Photodisinfection caused a significant reduction in bacterial burden after a 30 s exposure in vitro, regardless of the pharmaceutical associations of biological and conventional disease-modifying antirheumatic drugs (DMARDs). Irradiation with a diode laser for 30 s at a power of 5 W caused a greater reduction compared to irradiation with 1 W. The application of laser and photodisinfection induced a significant reduction in Aggregatibacter actinomycetemcomitans in vitro and could be considered important adjunctive measures for the eradication of this oral pathogen in the context of immunomodulating therapy.

Chlorin-based photosensitizer under blue or red-light irradiation against multi-species biofilms related to periodontitis

In our previous study, Chlorin-e6 (Ce6) demonstrated a significant reduction of microorganisms' viability against single-species biofilm related to periodontitis once irradiated by red light (660 nm). Also, higher bacteria elimination was observed under blue light (450 nm) irradiation. However, the use of blue light irradiation of Ce6 for antimicrobial administration is poorly explored. This study evaluated the effect of chlorin-e6-mediated antimicrobial photodynamic therapy (aPDT) using different wavelengths (450 or 660 nm) against multi-species biofilms related to periodontitis. Streptococcus oralis, Fusobacterium nucleatum, Porphyromonas gingivalis, and Aggregatibacter actinomycetemcomitans composed the mature biofilm developed under proper conditions for five days. aPDT was performed using different concentrations of Ce6 (100 and 200 μM), wavelengths (450 or 660 nm), and comparisons were made after qPCR assay and confocal laser scanning microscopy (CLSM) analysis. The greatest bacterial elimination was observed in the groups where Ce6 was used with blue light, for S. orallis (2.05 Log₁₀ GeQ mL-1, p < 0.0001) and P. gingivalis (1.4 Log₁₀ GeQ mL-1, p < 0.0001), aPDT with red light showed significant bacteria reduction only for S. orallis. aPDT with blue light demonstrated statistically higher elimination in comparison with aPDT with red light. The aPDT did not show a statistically significant effect when tested against A. actinomycetemcomitans and F. nucleatum (p=0.776 and 0.988, respectively). The aPDT using blue light showed a promising higher photobiological effect, encouraging researchers to consider it in the irradiation of Ce6 for further investigations.

A light-guiding urinary catheter for the inhibition of Proteus mirabilis biofilm formation

Catheter-associated urinary tract infection (CAUTI) is a leading cause of hospital-acquired infections worldwide causing debilitating illness for patients as well as a significant financial and treatment burden on health services. CAUTI is linked with the build-up of biofilms on catheter surfaces which act as a reservoir for infection. Additionally, urease-producing bacteria such as Gram-negative Proteus mirabilis (PM), can form crystalline biofilms which encrust catheter surfaces ultimately leading to blockages which require immediate removal of the catheter. Currently there are limited treatments available to prevent the formation of biofilms by PM as well as other urinary tract infection causing bacteria. A novel concept for a light-guiding urinary catheter is presented where a silicone elastomer waveguide incorporated along the length of the catheter is used to irradiate the catheter surfaces with antimicrobial blue light (405 nm) to prevent biofilm formation in situ. The prototype device is mass producible while also easy to fabricate in a lab setting for research studies. The inhibitory effect of blue light on PM biofilm formation over a range of irradiances is described for the first time showing an LD90 at 192–345 J/cm2 and total inhibition at 1,700 J/cm2In vitro studies show that the light-guiding catheter (LGC) prototypes exhibit a 98% inhibition in PM biofilm formation inside the catheter lumen at an average estimated irradiance of 30–50 mW/cm2 (324–540 J/cm2 fluence) showing that the concept is highly effective, promising to be a powerful and economical antimicrobial approach to prevent catheter associated biofilm development and blockage.

Light-activated molecular machines are fast-acting broad-spectrum antibacterials that target the membrane

The increasing occurrence of antibiotic-resistant bacteria and the dwindling antibiotic research and development pipeline have created a pressing global health crisis. Here, we report the discovery of a distinctive antibacterial therapy that uses visible (405 nanometers) light-activated synthetic molecular machines (MMs) to kill Gram-negative and Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus, in minutes, vastly outpacing conventional antibiotics. MMs also rapidly eliminate persister cells and established bacterial biofilms. The antibacterial mode of action of MMs involves physical disruption of the membrane. In addition, by permeabilizing the membrane, MMs at sublethal doses potentiate the action of conventional antibiotics. Repeated exposure to antibacterial MMs is not accompanied by resistance development. Finally, therapeutic doses of MMs mitigate mortality associated with bacterial infection in an in vivo model of burn wound infection. Visible light-activated MMs represent an unconventional antibacterial mode of action by mechanical disruption at the molecular scale, not existent in nature and to which resistance development is unlikely.

Blue Light Inhibits E. coli, but Decisive Parameters Remain Hidden in the Dark: Systematic Review and Meta-Analysis

Blue light (400–500 nm) alleviates overexposure risks associated to UV light and has therefore gained increased interest in multiple applications. This meta-analysis deals with decontamination of E. coli through the use of blue light based from nine recent publications identified via a systematic literature search. In these studies, various pathogenic and non-pathogenic E. coli strains grown in nutritional broths were exposed to wavelengths ranging from 395 to 460 nm. Five meta-analyses were performed using Cochrane’s software for meta-analyses (Review Manager): one including all studies to estimate the effect of E. coli reduction and four subgroup-analyses considering reported intensities, wavelengths, exposure dose as well as serovars/pathovars. Random effects models were used. All included studies used colony-forming units to estimate the impact of E. coli reduction. None of the included studies involved an organic matrix (e.g., skin, food related surface). Exposure to blue light had a significant and large reducing effect on viable counts of E. coli. However, substantial heterogeneity across studies was observed. Among subgroups, reported intensity and wavelength showed the clearest impact on E. coli reduction. With respect to the reported exposure dose, the picture across the spectrum was scattered, but effect sizes tend to increase with increasing exposure dose. Substantial heterogeneity was also present with respect to all serovar/pathovar subgroups among the included studies. The present body of reports does not display a strong basis for recommendation of relevant intensities, wavelengths and exposure doses for superficial blue light decontamination in medical or food safety contexts. A serious shortcoming in most studies is the absence of a clear documentation of inoculum preparation and of study parameters. We suggest improvement for study protocols for future investigations.

Review of Virus Inactivation by Visible Light

The COVID-19 pandemic is driving the search for new antiviral techniques. Bacteria and fungi are known to be inactivated not only by ultraviolet radiation but also by visible light. Several studies have recently appeared on this subject, in which viruses were mainly irradiated in media. However, it is an open question to what extent the applied media, and especially their riboflavin concentration, can influence the results. A literature search identified appropriate virus photoinactivation publications and, where possible, viral light susceptibility was quantitatively determined in terms of average log-reduction doses. Sensitivities of enveloped viruses were plotted against assumed riboflavin concentrations. Viruses appear to be sensitive to visible (violet/blue) light. The median log-reduction doses of all virus experiments performed in liquids is 58 J/cm2. For the non-enveloped, enveloped and coronaviruses only, they were 222, 29 and 19 J/cm2, respectively. Data are scarce, but it appears that (among other things) the riboflavin concentration in the medium has an influence on the log-reduction doses. Experiments with DMEM, with its 0.4 mg/L riboflavin, have so far produced results with the greatest viral susceptibilities. It should be critically evaluated whether the currently published virus sensitivities are really only intrinsic properties of the virus, or whether the medium played a significant role. In future experiments, irradiation should be carried out in solutions with the lowest possible riboflavin concentration.

Pulsed blue light, saliva and curcumin significantly inactivate human coronavirus

In a recent study, we showed that pulsed blue light (PBL) inactivates as much as 52.3% of human beta coronavirus HCoV-OC43, a surrogate of SARS-CoV-2, and one of the major strains of viruses responsible for the annual epidemic of the common cold. Since curcumin and saliva are similarly antiviral and curcumin acts as blue light photosensitizer, we used Qubit fluorometry and WarmStart RT-LAMP assays to study the effect of combining 405 nm, 410 nm, 425 nm or 450 nm wavelengths of PBL with curcumin, saliva or a combination of curcumin and saliva against human beta coronavirus HCoV-OC43. The results showed that PBL, curcumin and saliva independently and collectively inactivate HCoV-OC43. Without saliva or curcumin supplementation 21.6 J/cm2 PBL reduced HCoV-OC43 RNA concentration a maximum of 32.8% (log10 = 2.13). Saliva supplementation alone inactivated the virus, reducing its RNA concentration by 61% (log10 = 2.23); with irradiation the reduction was as much as 79.1%. Curcumin supplementation alone decreased viral RNA 71.1%, and a maximum of 87.8% with irradiation. The combination of saliva and curcumin reduced viral RNA to 83.1% and decreased the RNA up to 90.2% with irradiation. The reduced levels could not be detected with qPCR. These findings show that PBL in the range of 405 nm to 450 nm wavelength is antiviral against human coronavirus HCoV-OC43, a surrogate of the COVID-19 virus. Further, it shows that with curcumin as a photosensitizer, it is possible to photodynamically inactivate the virus beyond qPCR detectable level using PBL. Since HCoV-OC43 is of the same beta coronavirus family as SARS-CoV-2, has the same genomic size, and is often used as its surrogate, these findings heighten the prospect of similarly inactivating novel coronavirus SARS-CoV-2, the virus responsible for COVID-19 pandemic.

Violet-blue Light Induces "Natural" Photodynamic Plasma Disinfection with Endogenous Sensitizers

Naturally, endogenous porphyrins can provide sensitized disinfection power, and to photobiologists' delight, violet-blue light has potential virtues. But progress is needed before violet-blue light treatment can used for microbe treatment of blood samples, and yet safeguard against plasma protein photooxidation. A report by Maclean et al. in this issue of Photochemistry & Photobiology on microbe reduction of blood samples showing negligible competing protein photooxidation may bring that goal a step closer.

The effects of violet and blue light irradiation on ESKAPE pathogens and human cells in presence of cell culture media

Bacteria belonging to the group of ESKAPE pathogens are responsible for the majority of nosocomial infections. Due to the increase of antibiotic resistance, alternative treatment strategies are of high clinical relevance. In this context visible light as disinfection technique represents an interesting option as microbial pathogens can be inactivated without adjuvants. However cytotoxic effects of visible light on host cells have also been reported. We compared the cytotoxicity of violet and blue light irradiation on monocytic THP-1 and alveolar epithelium A549 cells with the inactivation effect on ESKAPE pathogens. THP-1 cells displayed a higher susceptibility to irradiation than A549 cells with first cytotoxic effects occurring at 300 J cm⁻² (405 nm) and 400 J cm⁻² (450 nm) in comparison to 300 J cm⁻² and 1000 J cm⁻², respectively. We could define conditions in which a significant reduction of colony forming units for all ESKAPE pathogens, except Enterococcus faecium, was achieved at 405 nm while avoiding cytotoxicity. Irradiation at 450 nm demonstrated a more variable effect which was species and medium dependent. In summary a significant reduction of viable bacteria could be achieved at subtoxic irradiation doses, supporting a potential use of visible light as an antimicrobial agent in clinical settings.

Efficient Inactivation of SARS-CoV-2 and Other RNA or DNA Viruses with Blue LED Light

Blue LED light has proven to have a powerful bacteria-killing ability; however, little is known about its mechanism of virucidal activity. Therefore, we analyzed the effect of blue light on different respiratory viruses, such as adenovirus, respiratory syncytial virus and SARS-CoV-2. The exposure of samples to a blue LED light with a wavelength of 420 nm (i.e., in the visible range) at 20 mW/cm² of irradiance for 15 min appeared optimal and resulted in the complete inactivation of the viral load. These results were similar for all the three viruses, demonstrating that both enveloped and naked viruses could be efficiently inactivated with blue LED light, regardless of the presence of envelope and of the viral genome nature (DNA or RNA). Moreover, we provided some explanations to the mechanisms by which the blue LED light could exert its antiviral activity. The development of such safe and low-cost light-based devices appears to be of fundamental utility for limiting viral spread and for sanitizing small environments, objects and surfaces, especially in the pandemic era.

Can Gram-Negative Bacteria Develop Resistance to Antimicrobial Blue Light Treatment?

Antimicrobial blue light (aBL) treatment is considered low risk for the development of bacterial resistance and tolerance due to its multitarget mode of action. The aim of the current study was to demonstrate whether tolerance development occurs in Gram-negative bacteria. We evaluated the potential of tolerance/resistance development in Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa and demonstrated that representative Gram-negative bacteria may develop tolerance to aBL. The observed adaption was a stable feature. Assays involving E. coli K-12 tolC-, tolA-, umuD-, and recA-deficient mutants revealed some possible mechanisms for aBL tolerance development.

High Intensity Violet Light (405 nm) Inactivates Coronaviruses in Phosphate Buffered Saline (PBS) and on Surfaces

It has been proven that visible light with a wavelength of about 405 nm exhibits an antimicrobial effect on bacteria and fungi if the irradiation doses are high enough. Hence, the question arises as to whether this violet light would also be suitable to inactivate SARS-CoV-2 coronaviruses. Therefore, a high-intensity light source was developed and applied to irradiate bovine coronaviruses (BCoV), which are employed as SARS-CoV-2 surrogates for safety reasons. Irradiation is performed in virus solutions diluted with phosphate buffered saline and on steel surfaces. Significant virus reduction by several log levels was observed both in the liquid and on the surface within half an hour with average log reduction doses of 57.5 and 96 J/cm2, respectively. Therefore, it can be concluded that 405 nm irradiation has an antiviral effect on coronaviruses, but special attention should be paid to the presence of photosensitizers in the virus environment in future experiments. Technically, visible violet radiation is therefore suitable for coronavirus reduction, but the required radiation doses are difficult to achieve rapidly.

Genetic Factors Affect the Survival and Behaviors of Selected Bacteria during Antimicrobial Blue Light Treatment

Antimicrobial resistance is a global, mounting and dynamic issue that poses an immediate threat to human, animal, and environmental health. Among the alternative antimicrobial treatments proposed to reduce the external use of antibiotics is electromagnetic radiation, such as blue light. The prevailing mechanistic model is that blue light can be absorbed by endogenous porphyrins within the bacterial cell, inducing the production of reactive oxygen species, which subsequently inflict oxidative damages upon different cellular components. Nevertheless, it is unclear whether other mechanisms are involved, particularly those that can affect the efficacy of antimicrobial blue light treatments. In this review, we summarize evidence of inherent factors that may confer protection to a selected group of bacteria against blue light-induced oxidative damages or modulate the physiological characteristics of the treated bacteria, such as virulence and motility. These include descriptions of three major photoreceptors in bacteria, chemoreceptors, SOS-dependent DNA repair and non-SOS protective mechanisms. Future directions are also provided to assist with research efforts to increase the efficacy of antimicrobial blue light and to minimize the development of blue light-tolerant phenotypes.

Pulsed blue light inactivates two strains of human coronavirus

Emerging evidence suggests that blue light has the potential to inactivate viruses. Therefore, we investigated the effect of 405 nm, 410 nm, 425 nm and 450 nm pulsed blue light (PBL) on human alpha coronavirus HCoV-229 E and human beta coronavirus HCoV-OC43, using Qubit fluorometry and RT-LAMP to quantitate the amount of nucleic acid in irradiated and control samples. Like SARS-CoV-2, HCoV-229E and HCoV-OC43 are single stranded RNA viruses transmitted by air and direct contact; they have similar genomic sizes as SARS-CoV-2, and are used as surrogates for SARS-CoV-2. Irradiation was carried out either at 32.4 J cm^-2 using 3 mW cm^-2 irradiance or at 130 J cm^-2 using 12 mW cm^-2 irradiance. Results: (1) At each wavelength tested, PBL was antiviral against both coronaviruses. (2) 405 nm light gave the best result, yielding 52.3% (2.37 log₁₀) inactivation against HCoV-OC43 (p < .0001), and a significant 1.46 log ₁₀ (44%) inactivation of HCoV-229E (p < .01). HCoV-OC43, which like SARS-CoV-2 is a beta coronavirus, was more susceptible to PBL irradiation than alpha coronavirus HCoV-229E. The latter finding suggests that PBL is potentially antiviral against multiple coronavirus strains, and that, while its potency may vary from one virus to another, it seems more antiviral against beta coronaviruses, such as HCoV-OC43. (3) Further, the antiviral effect of PBL was better at a higher irradiance than a lower irradiance, and this indicates that with further refinement, a protocol capable of yielding 100% inactivation of viruses is attainable.

Blue light inactivation of the enveloped RNA virus Phi6

Objective

Ultraviolet radiation is known for its antimicrobial properties but unfortunately, it could also harm humans. Currently, disinfection techniques against SARS-CoV-2 are being sought that can be applied on air and surfaces and which do not pose a relevant thread to humans. In this study, the bacteriophage phi6, which like SARS-CoV-2 is an enveloped RNA virus, is irradiated with visible blue light at a wavelength of 455 nm.

Results

For the first time worldwide, the antiviral properties of blue light around 455 nm can be demonstrated. With a dose of 7200 J/cm², the concentration of this enveloped RNA virus can be successfully reduced by more than three orders of magnitude. The inactivation mechanism is still unknown, but the sensitivity ratio of phi6 towards blue and violet light hints towards an involvement of photosensitizers of the host cells. Own studies on coronaviruses cannot be executed, but the results support speculations about blue-susceptibility of coronaviruses, which might allow to employ blue light for infection prevention or even therapeutic applications.

Structural membrane changes induced by pulsed blue light on methicillin-resistant Staphylococcus aureus (MRSA)

BACKGROUND

In a recent study we showed that blue light inactivates methicillin-resistant Staphylococcus aureus (MRSA) by perturbing, depolarizing, and disrupting its cell membrane.

PURPOSE

The current study presents visual evidence that the observed biochemical changes also result in cell metabolic changes and structural alteration of the cell membrane.

METHODS

Cultures of MRSA were treated with 450 nm pulsed blue light (PBL) at 3 mW/cm² irradiance, using a sub lethal dose of 2.7 J/cm² radiant exposure three times at 30-min intervals. Following 24 h incubation at 37 °C, irradiated colonies and control non-irradiated colonies were processed for light and transmission electron microscopy.

RESULTS

The images obtained revealed three major effects of PBL; (1) disruption of MRSA cell membrane, (2) alteration of membrane structure, and (3) disruption of cell replication.

CONCLUSION

These signs of bacterial inactivation at a dose deliberately selected to be sub-lethal supports our previous finding that rapid depolarization of bacterial cell membrane and disruption of cellular function comprise another mechanism underlying photo-inactivation of bacteria. Further, it affirms the potency of PBL.

Blue light absorbing pigment in Streptococcus agalactiae does not potentiate the antimicrobial effect of pulsed 450 nm light

INTRODUCTION

Recently, it was shown that Group B Streptococcus (GBS) COH1 strain, which has granadaene-an endogenous chromophore known to absorb blue light-is not susceptible to 450 nm pulsed blue light (PBL) inactivation unless the bacterium is co-cultured with exogenous porphyrin.

PURPOSE

To confirm or refute the finding, we studied the effect of blue light on NCTC, another strain of GBS with more granadaene than COH1, to determine if the abundance of granadaene-and by implication more absorption of blue light-fosters GBS susceptibility to PBL.

METHODS

We irradiated cultures of the bacterium with or without protoporphyrin, coproporphyrin, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD) or NADH. After 24-h incubation, bacterial colonies were enumerated, log₁₀ CFU/mL computed, and descriptive and inferential data analyzed and compared.

RESULTS

(1) The rich amount of granadaene in NCTC did not enhance its susceptibility to antimicrobial pulsed blue light (PBL). (2) Adding exogenous porphyrin fostered NCTC susceptibility to irradiation, resulting in 100% bacterial suppression. (3) Exogenous FMN or FAD, which strongly absorb 450 nm light, did not promote the antimicrobial effect of PBL, neither did exogenous NAD or NADH, two weak blue light-absorbing photosensitizers.

CONCLUSION

These results strengthen our previous assertion that an endogenous chromophore with the capacity to absorb and transform light energy into a biochemical process that engenders bacterial cell death, is essential for 450 nm PBL to suppress GBS.

Inactivation Effect of Violet and Blue Light on ESKAPE Pathogens and Closely Related Non-pathogenic Bacterial Species - A Promising Tool Against Antibiotic-Sensitive and Antibiotic-Resistant Microorganisms

Due to the globally observed increase in antibiotic resistance of bacterial pathogens and the simultaneous decline in new antibiotic developments, the need for alternative inactivation approaches is growing. This is especially true for the treatment of infections with the problematic ESKAPE pathogens, which include Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species, and often exhibit multiple antibiotic resistances. Irradiation with visible light from the violet and blue spectral range is an inactivation approach that does not require any additional supplements. Multiple bacterial and fungal species were demonstrated to be sensitive to this disinfection technique. In the present study, pathogenic ESKAPE organisms and non-pathogenic relatives are irradiated with visible blue and violet light with wavelengths of 450 and 405 nm, respectively. The irradiation experiments are performed at 37°C to test a potential application for medical treatment. For all investigated microorganisms and both wavelengths, a decrease in colony forming units is observed with increasing irradiation dose, although there are differences between the examined bacterial species. A pronounced difference can be observed between Acinetobacter, which prove to be particularly light sensitive, and enterococci, which need higher irradiation doses for inactivation. Differences between pathogenic and non-pathogenic bacteria of one genus are comparatively small, with the tendency of non-pathogenic representatives being less susceptible. Visible light irradiation is therefore a promising approach to inactivate ESKAPE pathogens with future fields of application in prevention and therapy.

Antimicrobial Blue Light versus Pathogenic Bacteria: Mechanism, Application in the Food Industry, Hurdle Technologies and Potential Resistance

Blue light primarily exhibits antimicrobial activity through the activation of endogenous photosensitizers, which leads to the formation of reactive oxygen species that attack components of bacterial cells. Current data show that blue light is innocuous on the skin, but may inflict photo-damage to the eyes. Laboratory measurements indicate that antimicrobial blue light has minimal effects on the sensorial and nutritional properties of foods, although future research using human panels is required to ascertain these findings. Food properties also affect the efficacy of antimicrobial blue light, with attenuation or enhancement of the bactericidal activity observed in the presence of absorptive materials (for example, proteins on meats) or photosensitizers (for example, riboflavin in milk), respectively. Blue light can also be coupled with other treatments, such as polyphenols, essential oils and organic acids. While complete resistance to blue light has not been reported, isolated evidence suggests that bacterial tolerance to blue light may occur over time, especially through gene mutations, although at a slower rate than antibiotic resistance. Future studies can aim at characterizing the amount and type of intracellular photosensitizers across bacterial species and at assessing the oxygen-independent mechanism of blue light-for example, the inactivation of spoilage bacteria in vacuum-packed meats.

Corynebacterium jeikeium Dormant Cell Formation and Photodynamic Inactivation

Pathogenic non-spore forming bacteria enter a dormant state under stressful conditions, which likely allows them to acquire resistance to various antibiotics. This work revealed the efficient formation of dormant "non-culturable" (NC) Corynebacterium jeikeium cells in stationary phase upon gradual acidification of the growth medium. Such cells were unable to form colonies and existed in a prolonged stationary phase. At an early stage of dormancy (approximately 14 days post-inoculation), dormant cells are able for resuscitation in liquid medium. However, those stored for long time in dormant state needed addition of supernatant taking from active C. jeikeium cultures for successful resuscitation. NC cells possessed low RNA synthesis and significant tolerance to antibiotics (rifampicin and vancomycin). They also accumulated free porphyrins, and 5-aminolevulinic acid addition enhanced free porphyrin accumulation which makes them potentially sensitive to photodynamic inactivation (PDI). PDI of dormant bacteria was accomplished by exposing cells to a 565 nm wavelength of light using a SOLIS-4C light-emitting diode for 60 min. This revealed that increased porphyrin concentrations were correlated with elevated PDI sensitivity. Results shown here demonstrate the potential utility of employing PDI to minimize levels of dormant, persistent corynebacteria and the C. jeikeium dormancy model developed here may be useful for finding new drugs and techniques for combatting persistent corynebacteria.

Photoinactivation of the Coronavirus Surrogate phi6 by Visible Light

To stop the coronavirus spread, new inactivation approaches are being sought that can also be applied in the presence of humans or even on humans. Here, we investigate the effect of visible violet light with a wavelength of 405 nm on the coronavirus surrogate phi6 in two aqueous solutions that are free of photosensitizers. A dose of 1300 J cm^-2 of 405 nm irradiation reduces the phi6 plaque-forming unit concentration by three log-levels. The next step should be similar visible light photoinactivation investigations on coronaviruses, which cannot be performed in our lab.

Light-based technologies for management of COVID-19 pandemic crisis

The global dissemination of the novel coronavirus disease (COVID-19) has accelerated the need for the implementation of effective antimicrobial strategies to target the causative agent SARS-CoV-2. Light-based technologies have a demonstrable broad range of activity over standard chemotherapeutic antimicrobials and conventional disinfectants, negligible emergence of resistance, and the capability to modulate the host immune response. This perspective article identifies the benefits, challenges, and pitfalls of repurposing light-based strategies to combat the emergence of COVID-19 pandemic.

Photoinactivation results of Enterococcus moraviensis with blue and violet light suggest the involvement of an unconsidered photosensitizer

Microorganisms can be photoinactivated with 405 and 450 nm irradiation, due to endogenous photosensitizers, which absorb light of these wavelengths and generate reactive oxygen species that destroy the cells from within. The photosensitizers assumed to be responsible are porphyrins in the spectral region around 405 nm and flavins at about 450 nm. The aim of this study was to investigate this hypothesis on enterococci, considering that they do not contain porphyrins. In photoinactivation experiments with Enterococcus moraviensis, 405 nm and 450 nm irradiation both led to a reduction of the bacterial concentration by several orders of magnitude with 405 nm irradiation being much more efficient. The measurement and analysis of the fluorescence spectra revealed no signs of porphyrins whereas flavins seemed to be rapidly converted to lumichrome by 405 nm radiation. Therefore, probably none of the usual suspects, porphyrins and flavins, was responsible for the photoinactivation of Enterococcus moraviensis during 405 nm irradiation. Fluorescence experiments revealed the spectra of lumichrome and NADH, which are both known photosensitizers. Presumably, one of them or both were actually involved here. As NADH and flavins (and therefore their photodegradation product lumichrome) are abundant in all microorganisms, they are probably also involved in 405 nm photoinactivation processes of other species.

The importance of porphyrins in blue light suppression of Streptococcus agalactiae

It is well documented that blue light absorption by bacterial chromophores triggers downstream production of reactive oxygen species (ROS), which in turn results in bacterial cell death. To elucidate the importance of chromophores in the bactericidal effect of blue light, and to determine whether blue light absorption per se or the presence of porphyrins known to engender ROS is crucial in blue light treatment, we studied the effect of 450 nm pulsed light on Streptococcus agalactiae, also known as Group B Streptococcus (GBS) strain COH1. GBS does not synthesize porphyrins but has a blue light-absorbing chromophore, granadaene. We irradiated planktonic cultures of GBS with or without exogenous chromophore supplementation using either protoporphyrin IX (PPIX), coproporphyrin III (CPIII), Nicotinamide adenine dinucleotide (NAD), reduced nicotinamide adenine dinucleotide (NADH), Flavin adenine dinucleotide (FAD), or Flavin mononucleotide (FMN). Quantification of surviving bacterial colonies, presented as percent survival and CFU/mL (log₁₀), showed that (1) 450 nm blue light does not suppress the growth of GBS, even though its endogenous chromophore, granadaene, absorbs light in the 450 nm spectrum. (2) The addition of either of the two exogenous porphyrins, PPIX or CPIII, significantly suppressed GBS, indicating the importance of porphyrins in the antimicrobial action of blue light. (3) Adding exogenous FMN or FAD, two known absorbers of 450 nm light, minimally potentiated the bactericidal effect of blue light, again confirming that mere absorption of blue light by chromophores does not necessarily result in bacterial suppression. (4) Irradiation of GBS with or without NAD+ or NADH supplementation-two weak absorbers of 450 nm light-minimally suppressed GBS, indicating that a blue light-absorbing chromophore is essential for the bactericidal action of blue light. (5) Collectively, these findings show that in addition to the presence of a blue light-absorbing chromophore in bacteria, a chromophore with the right metabolic machinery and biochemical structure, capable of producing ROS, is necessary for 450 nm blue light to suppress GBS.

Light as a potential treatment for pandemic coronavirus infections: A perspective

The recent outbreak of COVID-19, which continues to ravage communities with high death tolls and untold psychosocial and catastrophic economic consequences, is a vivid reminder of nature's capacity to defy contemporary healthcare. The pandemic calls for rapid mobilization of every potential clinical tool, including phototherapy—one of the most effective treatments used to reduce the impact of the 1918 “Spanish influenza” pandemic. This paper cites several studies showing that phototherapy has immense potential to reduce the impact of coronavirus diseases, and offers suggested ways that the healthcare industry can integrate modern light technologies in the fight against COVID-19 and other infections. The evidence shows that violet/blue (400–470 nm) light is antimicrobial against numerous bacteria, and that it accounts for Niels Ryberg Finsen's Nobel-winning treatment of tuberculosis. Further evidence shows that blue light inactivates several viruses, including the common flu coronavirus, and that in experimental animals, red and near infrared light reduce respiratory disorders, similar to those complications associated with coronavirus infection. Moreover, in patients, red light has been shown to alleviate chronic obstructive lung disease and bronchial asthma. These findings call for urgent efforts to further explore the clinical value of light, and not wait for another pandemic to serve as a reminder. The ubiquity of inexpensive light emitting lasers and light emitting diodes (LEDs), makes it relatively easy to develop safe low-cost light-based devices with the potential to reduce infections, sanitize equipment, hospital facilities, emergency care vehicles, homes, and the general environment as pilot studies have shown.

The in vitro Photoinactivation of Helicobacter pylori by a Novel LED-Based Device

The rise of antibiotic resistance is the main cause for the failure of conventional antibiotic therapy of Helicobacter pylori infection, which is often associated with severe gastric diseases, including gastric cancer. In the last years, alternative non-pharmacological approaches have been considered in the treatment of H. pylori infection. Among these, antimicrobial PhotoDynamic Therapy (aPDT), a light-based treatment able to photoinactivate a wide range of bacteria, viruses, fungal and protozoan parasites, could represent a promising therapeutic strategy. In the case of H. pylori, aPDT can exploit photoactive endogenous porphyrins, such as protoporphyrin IX and coproporphyrin I and III, to induce photokilling, without any other exogenous photosensitizers. With the aim of developing an ingestible LED-based robotic pill for minimally invasive intragastric treatment of H. pylori infection, it is crucial to determine the best illumination parameters to activate the endogenous photosensitizers. In this study the photokilling effect on H. pylori has been evaluated by using a novel LED-based device, designed for testing the appropriate LEDs for the pill and suitable to perform in vitro irradiation experiments. Exposure to visible light induced bacterial photokilling most effectively at 405 nm and 460 nm. Sub-lethal light dose at 405 nm caused morphological changes on bacterial surface indicating the cell wall as one of the main targets of photodamage. For the first time endogenous photosensitizing molecules other than porphyrins, such as flavins, have been suggested to be involved in the 460 nm H. pylori photoinactivation.

Spectrum of virucidal activity from ultraviolet to infrared radiation

The COVID-19 pandemic has sparked a demand for safe and highly effective decontamination techniques for both personal protective equipment (PPE) and hospital and operating rooms. The gradual lifting of lockdown restrictions warrants the expansion of these measures into the outpatient arena. Ultraviolet C (UVC) radiation has well-known germicidal properties and is among the most frequently reported decontamination techniques used today. However, there is evidence that wavelengths beyond the traditional 254 nm UVC - namely far UVC (222 nm), ultraviolet B, ultraviolet A, visible light, and infrared radiation - have germicidal properties as well. This review will cover current literature regarding the germicidal effects of wavelengths ranging from UVC through the infrared waveband with an emphasis on their activity against viruses, and their potential applicability in the healthcare setting for general decontamination during an infectious outbreak.

Photoinactivation of dormant Mycobacterium smegmatis due to its endogenous porphyrins

Mycobacterium tuberculosis is able to transition into a dormant state, causing a latent state of tuberculosis. Dormant mycobacteria acquire phenotypic resistance to all known antibacterial drugs; they are also able to maintain vitality in the host for decades and become active, causing the active form of the disease. In order to cure latent tuberculosis, new approaches should be developed. Earlier, we discovered accumulation in significant concentrations of porphyrins in dormant Mycobacterium smegmatis, which is a close, fast-growing relative of the causative agent of tuberculosis. In this study, we explore a new possibility to kill dormant mycobacteria by photodynamic inactivation (PDI) using accumulated porphyrins as endogenous photosensitisers. The dormant M. smegmatis were obtained under gradual acidification in Sauton's medium, for 14 days. Cells were exposed to light with different wavelengths emitted by three Spectra X light-emitting diodes (395/25, 470/24, 575/25 nm) and one separated 634-nm LED for 15 min. An increase in the concentration of coproporphyrin in M. smegmatis after 6 days of growth correlated with the beginning of a decrease in metabolic activity and formation of ovoid dormant forms. Dormant bacteria were sensitive to PDI and killed after 15-30 min of illumination, in contrast to active cells. The greatest inactivation of dormant mycobacteria occurred at 395 and 575 nm, which coincides with the main maximum of the absorption spectrum of extracted porphyrins. We, for the first time, demonstrate a successful application of PDI for inactivation of dormant mycobacteria, due to significant accumulation of endogenous photosensitisers-porphyrins.

Antimicrobial Effect of Visible Light-Photoinactivation of Legionella rubrilucens by Irradiation at 450, 470, and 620 nm

Despite the high number of legionella infections, there are currently no convincing preventive measures. Photoinactivation with visible light is a promising new approach and the photoinactivation sensitivity properties of planktonic Legionella rubrilucens to 450, 470, and 620 nm irradiation were thus investigated and compared to existing 405 nm inactivation data for obtaining information on responsible endogenous photosensitizers. Legionella were streaked on agar plates and irradiated with different doses by light emitting diodes (LEDs) of different visible wavelengths. When irradiating bacterial samples with blue light of 450 nm, a 5-log reduction could be achieved by applying a dose of 300 J cm⁻², whereas at 470 nm, a comparable reduction required about 500 J cm⁻². For red irradiation at 620 nm, no inactivation could be observed, even at 500 J cm⁻². The declining photoinactivation sensitivity with an increasing wavelength is consistent with the assumption of porphyrins and flavins being among the relevant photosensitizers. These results were obtained for L. rubrilucens, but there is reason to believe that its inactivation behavior is similar to that of pathogenic legionella species. Therefore, this photoinactivation might lead to new future concepts for legionella reduction and prevention in technical applications or even on or inside the human body.

Development of Staphylococcus aureus tolerance to antimicrobial photodynamic inactivation and antimicrobial blue light upon sub-lethal treatment

Antimicrobial photodynamic inactivation (aPDI) and antimicrobial blue light (aBL) are considered low-risk treatments for the development of bacterial resistance and/or tolerance due to their multitargeted modes of action. In this study, we assessed the development of Staphylococcus aureus tolerance to these phototreatments. Reference S. aureus USA300 JE2 was subjected to 15 cycles of both sub-lethal aPDI (employing an exogenously administered photosensitizer (PS), i.e., rose Bengal (RB)) and sub-lethal aBL (employing endogenously produced photosensitizing compounds, i.e., porphyrins). We demonstrate substantial aPDI/aBL tolerance development and tolerance stability after 5 cycles of subculturing without aPDI/aBL exposure (the development of aPDI/aBL tolerance was also confirmed with the employment of clinical MRSA and MSSA strain as well as other representatives of Gram-positive microbes, i.e. Enterococcus faecium and Streptococcus agalactiae). In addition, a rifampicin-resistant (RIFR) mutant selection assay showed an increased mutation rate in S. aureus upon sub-lethal phototreatments, indicating that the increased aPDI/aBL tolerance may result from accumulated mutations. Moreover, qRT-PCR analysis following sub-lethal phototreatments demonstrated increased expression of umuC, which encodes stress-responsive error-prone DNA polymerase V, an enzyme that increases the rate of mutation. Employment of recA and umuC transposon S. aureus mutants confirmed SOS-induction dependence of the tolerance development. Interestingly, aPDI/aBL-tolerant S. aureus exhibited increased susceptibility to gentamicin (GEN) and doxycycline (DOX), supporting the hypothesis of genetic alterations induced by sub-lethal phototreatments. The obtained results indicate that S. aureus may develop stable tolerance to studied phototreatments upon sub-lethal aPDI/aBL exposure; thus, the risk of tolerance development should be considered significant when designing aPDI/aBL protocols for infection treatments in vitro and in clinical settings.

Killing mechanism of bacteria within multi-species biofilm by blue light

Objectives: The aim of the study was to characterize the immediate and delayed effects of non-coherent blue-light treatment on the composition and viability of an in vitro biofilm composed of anaerobic multispecies, as well as the mechanisms involved. Methods: A multispecies biofilm was constructed of Streptococcus sanguinis, Actinomyces naeslundii, Porphyromonas gingivalis and Fusobacterium nucleatum, test groups were exposed to blue light. The multispecies biofilm was explored with a newly developed method based on flow cytometry and confocal microscopy. The involvement of the paracrine pathway in the phototoxic mechanism was investigated by a crossover of the supernatants between mono-species P. gingivalis and F. nucleatum biofilms. Results: Blue light led to a reduction of about 50% in the viable pathogenic bacteria P. gingivalis and F. nucleatum, vs that in the non-exposed biofilm. Biofilm thickness was also reduced by 50%. The phototoxic effect of blue light on mono-species biofilm was observed in P. gingivalis, whereas F. nucleatum biofilm was unaffected. A lethal effect was obtained when the supernatant of P. gingivalis biofilm previously exposed to blue light was added to the F. nucleatum biofilm. The effect was circumvented by the addition of reactive oxygen species (ROS) scavengers to the supernatant. Conclusion: Blue-light has an impact on the bacterial composition and viability of the multispecies biofilm. The phototoxic effect of blue light on P. gingivalis in biofilm was induced directly and on F. nucleatum via ROS mediators of the paracrine pathway. This phenomenon may lead to a novel approach for 'replacement therapy,' resulting in a less periodonto-pathogenic biofilm.

Antimicrobial Blue Light Inactivation of Neisseria gonorrhoeae: Roles of Wavelength, Endogenous Photosensitizer, Oxygen, and Reactive Oxygen Species

BACKGROUND AND OBJECTIVES

The aim of this study was to investigate the efficacy, safety, and mechanism of action of antimicrobial blue light (aBL) for the inactivation of Neisseria gonorrhoeae, the etiological agent of gonorrhea.

STUDY DESIGN/MATERIALS AND METHODS

The susceptibilities of N. gonorrhoeae (ATCC 700825) in planktonic suspensions to aBL at 405- and 470-nm wavelengths were compared. The roles of oxygen in the anti-gonococcal activity of aBL were studied by examining the effects of hypoxic condition (blowing N₂ ) on the anti-gonococcal efficiency of 405-nm aBL. The presence, identification, and quantification of endogenous photosensitizers in N. gonorrhoeae cells and human vaginal epithelial cells (VK2/E6E7 cells) were determined using fluorescence spectroscopy and ultra-performance liquid chromatography (UPLC). Finally, the selectivity of aBL inactivation of N. gonorrhoeae over the host cells were investigated by irradiating the co-cultures of N. gonorrhoeae and human vaginal epithelial cells using 405-nm aBL.

RESULTS

About 3.12-log₁₀ reduction of bacterial colony forming units (CFU) was achieved by 27 J/cm ² exposure at 405 nm, while about 3.70-log₁₀ reduction of bacterial CFU was achieved by 234 J/cm² exposure at 470 nm. The anti-gonococcal efficacy of 405-nm aBL was significantly suppressed under hypoxic condition. Spectroscopic and UPLC analyses revealed the presence of endogenous porphyrins and flavins in N. gonorrhoeae. The concentrations of endogenous photosensitizers in N. gonorrhoeae (ATCC 700825) cells were more than 10 times higher than those in the VK2/E6E7 cells. In the co-cultures of N. gonorrhoeae and VK2/E6E7 cells, 405-nm aBL at 108 J/cm² preferentially inactivated N. gonorrhoeae cells while sparing the vaginal epithelial cells.

CONCLUSIONS

aBL at 405-nm wavelength is more effective than 470-nm wavelength in inactivating N. gonorrhoeae while sparing the vaginal epithelial cells. Reactive oxygen species generated from the photochemical reactions between aBL and endogenous photosensitizers play a vital role in the anti-gonococcal activity of 405-nm aBL. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Photosensitizers in antibacterial photodynamic therapy: an overview

Antibacterial Photodynamic therapy (APDT) is a process utilizing light and light sensitive agents (named photosensitizer (PS)) and is usually applied in an oxygen-rich environment. The energy of the photons is absorbed by the photosensitizer and subsequently transferred to surrounding molecules. Consequently, reactive oxygen species and free radicals are formed. These oxidative molecules can damage bacterial macromolecules such as proteins, lipids and nucleic acids and may result in bacterial killing. Unlike antibiotics, APDT as a novel technique does not lead to the selection of mutant resistant strains, hence it has appealed to researchers in this field. The type of PS used in APDT is a major determinant regarding outcome. In this review, various types of PS that are used in antimicrobial Photodynamic therapy will be discussed. PSs are classified based on their chemical structure and origin. Synthetic dyes such as methylene blue and toluidine blue are the most commonly used photosensitizers in Antibacterial Photodynamic therapy (APDT). Other photosensitizers including natural PSs (e.g. curcumin and hypericin) and tetra-pyrrole structures like phthalocyanines and porphyrins have also been studied. Furthermore, nanostructures and their probable contribution to APDT will be discussed.

Water Filtered Infrared A and Visible Light (wIRA/VIS) Irradiation Reduces Chlamydia trachomatis Infectivity Independent of Targeted Cytokine Inhibition

Chlamydia trachomatis is the major cause of infectious blindness and represents the most common bacterial sexually transmitted infection worldwide. Considering the potential side effects of antibiotic therapy and increasing threat of antibiotic resistance, alternative therapeutic strategies are needed. Previous studies showed that water filtered infrared A alone (wIRA) or in combination with visible light (wIRA/VIS) reduced C. trachomatis infectivity. Furthermore, wIRA/VIS irradiation led to secretion of pro-inflammatory cytokines similar to that observed upon C. trachomatis infection. We confirmed the results of previous studies, namely that cytokine secretion (IL-6, IL-8, and RANTES/CCL5) upon wIRA/VIS treatment, and the subsequent reduction of chlamydial infectivity, are independent of the addition of cycloheximide, a host protein synthesis inhibitor. Reproducible cytokine release upon irradiation indicated that cytokines might be involved in the anti-chlamydial mechanism of wIRA/VIS. This hypothesis was tested by inhibiting IL-6, IL-8, and RANTES secretion in C. trachomatis or mock-infected cells by gene silencing or pharmaceutical inhibition. Celastrol, a substance derived from Trypterygium wilfordii, used in traditional Chinese medicine and known for anti-cancer and anti-inflammatory effects, was used for IL-6 and IL-8 inhibition, while Maraviroc, a competitive CCR5 antagonist and anti-HIV drug, served as a RANTES/CCL5 inhibitor. HeLa cell cytotoxicity and impact on chlamydial morphology, size and inclusion number was evaluated upon increasing inhibitor concentration, and concentrations of 0.1 and 1 μM Celastrol and 10 and 20 μM Maraviroc were subsequently selected for irradiation experiments. Celastrol at any concentration reduced chlamydial infectivity, an effect only observed for 20 μM Maraviroc. Triple dose irradiation (24, 36, 40 hpi) significantly reduced chlamydial infectivity regardless of IL-6, IL-8, or RANTES/CCL5 gene silencing, Celastrol or Maraviroc treatment. Neither gene silencing nor pharmaceutical cytokine inhibition provoked the chlamydial stress response. The anti-chlamydial effect of wIRA/VIS is independent of cytokine inhibition under all conditions evaluated. Thus, factors other than host cell cytokines must be involved in the working mechanism of wIRA/VIS. This study gives a first insight into the working mechanism of wIRA/VIS in relation to an integral component of the host immune system and supports the potential of wIRA/VIS as a promising new tool for treatment in trachoma.

Antimicrobial blue light photoinactivation of Pseudomonas aeruginosa: Quorum sensing signaling molecules, biofilm formation and pathogenicity

Pseudomonas aeruginosa is a common causative bacterium of acute and chronic infections that have been responsible for high mortality over the past decade. P. aeruginosa produces many virulence factors such as toxins, enzymes and dyes that are strongly dependent on quorum sensing (QS) signaling systems. P. aeruginosa has three major QS systems (las, rhl and Pseudomonas quinolone signal) that regulate the expression of genes encoding virulence factors as well as biofilm production and maturation. Antimicrobial blue light (aBL) is considered a therapeutic option for bacterial infections and has other benefits, such as reducing bacterial virulence. Therefore, this study investigated the efficacy of aBL to reduce P. aeruginosa pathogenicity. aBL treatment resulted in the reduced activity of certain QS signaling molecules in P. aeruginosa and inhibited biofilm formation. in vivo tests using a Caenorhabditis elegans infection model indicated that sublethal aBL decreased the pathogenicity of P. aeruginosa. aBL may be a new virulence-targeting therapeutic approach.