Influence of multidrug efflux systems on methylene blue-mediated photodynamic inactivation of Candida albicans

Morphological aspects and the effectiveness of photodynamic inactivation against Rhizopus oryzae in different life cycles

Mucormycosis is an extremely aggressive fungal disease with a high mortality rate, especially in people with compromised immune systems. Most cases of mucormycosis are caused by the fungus Rhizopus oryzae. The treatments used are based on high doses of antifungals, associated with surgical resections, when it is possible. However, even with this aggressive treatment, the estimated attributable mortality rate is high. There is therefore a need to develop adjuvant treatments. Photodynamic Inactivation (PDI) may be an auxiliary therapeutic option for mucormycosis. Due to the lack of reports in the literature on the morphology and photodynamic inactivation of R. oryzae, characterization of the fungus using Confocal Microscopy and Transmission Electron Microscopy, and different protocols using Photodithazine® (PDZ), a chlorin e6 compound, as a photosensitizer, were performed. The fungus growth rate under different concentrations and incubation times of the photosensitizer and its association with the surfactant Sodium Dodecyl Sulphate (SDS) was evaluated. For the hyphae, both in the light and dark phases, in the protocols using only PDZ, no effective photodynamic response was observed. Meanwhile with the combination of SDS 0.05% and PDZ, inhibition growth rates of 98% and 72% were achieved for the white and black phase, respectively. In the conidia phase, only a 1.7 log10 reduction of the infective spores was observed. High concentration of melanin and the complex and resistant structures, especially at the black phase, results in a high limitation of the PDI inactivation response. The combined use of the SDS resulted in an improved response, when compared to the one obtained with the amphotericin B treatment.

A Uniform Design Method Can Optimize the Combinatorial Parameters of Antimicrobial Photodynamic Therapy, Including the Concentrations of Methylene Blue and Potassium Iodide, Light Dose, and Methylene Blue's Incubation Time, to Improve Fungicidal Effects on Candida Species

The optimal combinatorial parameters of antimicrobial photodynamic therapy (aPDT) mediated by methylene blue (MB) with the addition of potassium iodide (KI) against Candida species have never been defined. This study aimed to optimize the combinatorial parameters of aPDT, including the concentrations of MB (X1, 0.1-1.0 mM) and KI (X2, 100-400 mM), light dose (X3, 10-70 J/cm2), and MB's incubation time (X4, 5-35 min) for three Candida species. The best MB + KI-aPDT fungicidal effects (Y) against Candida albicans ATCC 90028 (YCa), Candida parapsilosis ATCC 22019 (YCp), and Candida glabrata ATCC 2950 (YCg) were investigated using a uniform design method. The regression models deduced using this method were YCa = 7.126 + 1.199X1X3 - 1.742X12 + 0.206X22 - 0.361X32; YCp = 10.724 - 0.867X1 - 1.497X2 + 0.560X3 + 1.298X22; and YCg = 0.892 - 0.956X1 + 2.296X3 + 1.299X42 - 3.316X3X4. The optimal combinatorial parameters inferred from the regression equations were MB 0.1 mM, KI 400 mM, a light dose of 20 J/cm2, and a 5-minute incubation time of MB for Candida albicans; MB 0.1 mM, KI 400 mM, a light dose of 70 J/cm2, and a 5-minute incubation time of MB for Candida parapsilosis; MB 0.1 mM, KI 100 mM, a light dose of 10 J/cm2, and a 35-minute incubation time of MB for Candida glabrata. The uniform design method can optimize the combinatorial parameters of aPDT mediated by MB plus KI to obtain the best aPDT fungicidal effects on Candida species, providing a new method to optimize the combinatorial parameters of aPDT for different pathogens in the future.

Photodynamic Inactivation Effectively Eradicates Candida auris Biofilm despite Its Interference with the Upregulation of CDR1 and MDR1 Efflux Genes

Candida auris, in recent years, has emerged as a dangerous nosocomial pathogen. It represents a challenge for effective treatment because of its multiresistance. Photodynamic inactivation (PDI) is a promising way to solve problems with a wide range of resistant microorganisms. This study aimed to use PDI for the eradication of C. auris biofilms. Moreover, the regulation of the CDR1, CDR2, and MDR1 resistance genes was studied. Experiments were performed on 24 h biofilms formed by three clinical isolates of C. auris in vitro. PDI was performed in the presence of the photosensitizer methylene blue (0.25 mM) and samples were irradiated with a red laser (λ = 660 nm, 190 mW/cm²) for 79, 120, and 300 s. To confirm the PDI effect, confocal laser scanning microscopy was performed after treatment. Effective PDI was achieved in all strains. The highest inhibition was observed after 300 s irradiation, with over 90% inhibition compared with the non-irradiated control sample. PDI was observed to upregulate the expression of the CDR1 gene, but mainly the MDR1 gene. Despite this observation, PDI significantly decreased the survival of C. auris biofilm cells and proved to have great potential for the eradication of problematic resistant yeasts.

Optical property recovery with spatially-resolved diffuse reflectance at short source-detector separations using a compact fiber-optic probe

We describe a compact fiber-optic probe (2 mm outside diameter) that utilizes spatially-resolved diffuse reflectance for tissue optical property recovery. Validation was performed in phantoms containing Intralipid 20% as scatterer, and methylene blue (MB), MnTPPS, and/or India ink as absorbers. Over a range of conditions, the reduced scattering coefficient was recovered with a root mean square error (RMSE) of 0.86-2.7 cm^-1 (average error = 3.8%). MB concentration was recovered with RMSE = 0.26-0.52 µM (average error = 15.0%), which did not vary with inclusion of MnTPPS (p=0.65). This system will be utilized to determine optical properties in human abscesses, in order to generate treatment plans for photodynamic therapy.

Antimicrobial Photodynamic Therapy: Latest Developments with a Focus on Combinatory Strategies

Antimicrobial photodynamic therapy (aPDT) has become a fundamental tool in modern therapeutics, notably due to the expanding versatility of photosensitizers (PSs) and the numerous possibilities to combine aPDT with other antimicrobial treatments to combat localized infections. After revisiting the basic principles of aPDT, this review first highlights the current state of the art of curative or preventive aPDT applications with relevant clinical trials. In addition, the most recent developments in photochemistry and photophysics as well as advanced carrier systems in the context of aPDT are provided, with a focus on the latest generations of efficient and versatile PSs and the progress towards hybrid-multicomponent systems. In particular, deeper insight into combinatory aPDT approaches is afforded, involving non-radiative or other light-based modalities. Selected aPDT perspectives are outlined, pointing out new strategies to target and treat microorganisms. Finally, the review works out the evolution of the conceptually simple PDT methodology towards a much more sophisticated, integrated, and innovative technology as an important element of potent antimicrobial strategies.

Photosensitizers Mediated Photodynamic Inactivation against Fungi

Superficial and systemic fungal infections are essential problems for the modern health care system. One of the challenges is the growing resistance of fungi to classic antifungals and the constantly increasing cost of therapy. These factors force the scientific world to intensify the search for alternative and more effective methods of treatment. This paper presents an overview of new fungal inactivation methods using Photodynamic Antimicrobial Chemotherapy (PACT). The results of research on compounds from the groups of phenothiazines, xanthanes, porphyrins, chlorins, porphyrazines, and phthalocyanines are presented. An intensive search for a photosensitizer with excellent properties is currently underway. The formulation based on the existing ones is also developed by combining them with nanoparticles and common antifungal therapy. Numerous studies indicate that fungi do not form any specific defense mechanism against PACT, which deems it a promising therapeutic alternative.

Influence of Incubation Time on Ortho-Toluidine Blue Mediated Antimicrobial Photodynamic Therapy Directed against Selected Candida Strains-An In Vitro Study

(1) Background and the aim: The appropriate incubation time in the antimicrobial photodynamic therapy protocol seems to have a huge impact on the efficacy of this process. This is particularly important in relation to Candida strains, due to the size of these cells and the presence of the cell wall. The aims of this study were to determine the optimal incubation time needed for the absorption of toluidine blue by cells of C. albicans, C. glabrata, C. krusei and C. parapsilosis using direct observation by optical microscopy, and to evaluate the efficacy of TBO-mediated aPDT on planktonic cells of these strains. (2) Methods: The microscopic evaluation consisted of taking a series of images at a magnification of 600× and counting the % of stained cells. The in vitro effect of TBO-mediated aPDT combined with a diode laser (635 nm, 400mW, 12 J/cm², CW) on the viability of yeast cells with different incubation times was evaluated. (3) Results: The presence of TBO within the cytoplasm was observed in all tested Candida strains and at all microscopic evaluation times. However, the highest percentages of cells were stained at 7 and 10 min. The highest % reduction of CFU/mL after TBO-mediated aPDT against Candida was obtained for the strain C. albicans ATCC 10,231 and it was 78.55%. (4) Conclusions: TBO-mediated aPDT against Candida was effective in reducing the number of CFU/mL at all assessed incubation times. However, the most efficient period for almost all strains was 7–10 min.

Antimicrobial photodynamic therapy (aPDT) for biofilm treatments. Possible synergy between aPDT and pulsed electric fields

Currently, microbial biofilms have been the cause of a wide variety of infections in the human body, reaching 80% of all bacterial and fungal infections. The biofilms present specific properties that increase the resistance to antimicrobial treatments. Thus, the development of new approaches is urgent, and antimicrobial photodynamic therapy (aPDT) has been shown as a promising candidate. aPDT involves a synergic association of a photosensitizer (PS), molecular oxygen and visible light, producing highly reactive oxygen species (ROS) that cause the oxidation of several cellular components. This therapy attacks many components of the biofilm, including proteins, lipids, and nucleic acids present within the biofilm matrix; causing inhibition even in the cells that are inside the extracellular polymeric substance (EPS). Recent advances in designing new PSs to increase the production of ROS and the combination of aPDT with other therapies, especially pulsed electric fields (PEF), have contributed to enhanced biofilm inhibition. The PEF has proven to have antimicrobial effect once it is known that extensive chemical reactions occur when electric fields are applied. This type of treatment kills microorganisms not only due to membrane rupture but also due to the formation of reactive compounds including free oxygen, hydrogen, hydroxyl and hydroperoxyl radicals. So, this review aims to show the progress of aPDT and PEF against the biofilms, suggesting that the association of both methods can potentiate their effects and overcome biofilm infections.

Molecular characteristics of the photosensitizer TONS504: Comparison of its singlet oxygen quantum yields and photodynamic antimicrobial effect with those of methylene blue

TONS504 (C51H58O5I2) is a chlorin derivative that exhibits a photodynamic antimicrobial effect (PAE) on various infectious keratitis pathogens. However, the molecular characteristics of TONS504 are not well understood. This study aimed to investigate the molecular characteristics of TONS504 by comparing its singlet oxygen (¹O₂) quantum yields and PAE with those of methylene blue (MB). To measure the ¹O₂ quantum yields, TONS504 and MB were dissolved in phosphate-buffered saline and phosphate-buffered saline containing 1% Triton X-100. The solutions were then activated by a Nd:YAG laser with an average output power of 8 mW. Near-infrared ¹O₂ luminescence was detected as an indicator of the ¹O₂ quantum yields. To evaluate the PAE, TONS504 and MB were activated by a light-emitting diode with a total light energy of 30 J/cm². We compared the minimum molar concentration of each photosensitizer to show apparent PAEs on Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. TONS504 exhibited higher ¹O₂ quantum yields than MB in PBS/Triton X-100 but not in PBS. S. aureus and C. albicans were reduced by TONS504 at lower concentrations than by MB, but this was not the case for P. aeruginosa. Our results provide insight on the molecular characteristics of TONS504 and suggest that TONS504 has excellent ¹O₂ quantum yields and PAE. Compared with MB, TONS504 in PBS has stronger efficacy toward some infectious keratitis pathogens but not others.

Verapamil inhibits efflux pumps in Candida albicans, exhibits synergism with fluconazole, and increases survival of Galleria mellonella

The emergence of resistance requires alternative methods to treat Candida albicans infections. We evaluated efficacy of the efflux pump inhibitor (EPI) verapamil (VER) with fluconazole (FLC) against FLC-resistant (CaR) and -susceptible C. albicans (CaS). The susceptibility of both strains to VER and FLC was determined, as well as the synergism of VER with FLC. Experiments were performed in vitro for planktonic cultures and biofilms and in vivo using Galleria mellonella. Larval survival and fungal recovery were evaluated after treatment with VER and FLC. Data were analyzed by analysis of variance and Kaplan-Meier tests. The combination of VER with FLC at sub-lethal concentrations reduced fungal growth. VER inhibited the efflux of rhodamine 123 and showed synergism with FLC against CaR. For biofilms, FLC and VER alone reduced fungal viability. The combination of VER with FLC at sub-lethal concentrations also reduced biofilm viability. In the in vivo assays, VER and FLC used alone or in combination increased the survival of larvae infected with CaR. Reduction of fungal recovery was observed only for larvae infected with CaR and treated with VER with FLC. VER reverted the FLC-resistance of C. albicans. Based on the results obtained, VER reverted the FLC-resistance of C. albicans and showed synergism with FLC against CaR. VER also increased the survival of G. mellonella infected with CaR and reduced the fungal recovery.

Structural Similarities between Some Common Fluorophores Used in Biology, Marketed Drugs, Endogenous Metabolites, and Natural Products

It is known that at least some fluorophores can act as 'surrogate' substrates for solute carriers (SLCs) involved in pharmaceutical drug uptake, and this promiscuity is taken to reflect at least a certain structural similarity. As part of a comprehensive study seeking the 'natural' substrates of 'orphan' transporters that also serve to take up pharmaceutical drugs into cells, we have noted that many drugs bear structural similarities to natural products. A cursory inspection of common fluorophores indicates that they too are surprisingly 'drug-like', and they also enter at least some cells. Some are also known to be substrates of efflux transporters. Consequently, we sought to assess the structural similarity of common fluorophores to marketed drugs, endogenous mammalian metabolites, and natural products. We used a set of some 150 fluorophores along with standard fingerprinting methods and the Tanimoto similarity metric. Results: The great majority of fluorophores tested exhibited significant similarity (Tanimoto similarity > 0.75) to at least one drug, as judged via descriptor properties (especially their aromaticity, for identifiable reasons that we explain), by molecular fingerprints, by visual inspection, and via the "quantitative estimate of drug likeness" technique. It is concluded that this set of fluorophores does overlap with a significant part of both the drug space and natural products space. Consequently, fluorophores do indeed offer a much wider opportunity than had possibly been realised to be used as surrogate uptake molecules in the competitive or trans-stimulation assay of membrane transporter activities.

Global priority multidrug-resistant pathogens do not resist photodynamic therapy

Microbial drug-resistance demands immediate implementation of novel therapeutic strategies. Antimicrobial photodynamic therapy (aPDT) combines the administration of a photosensitizer (PS) compound with low-irradiance light to induce photochemical reactions that yield reactive oxygen species (ROS). Since ROS react with nearly all biomolecules, aPDT offers a powerful multitarget method to avoid selection of drug-resistant strains. In this study, we assayed photodynamic inactivation under a standardized method, combining methylene blue (MB) as PS and red light, against global priority pathogens. The species tested include Acinetobacter baumannii, Klebsiella aerogenes, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Enterococcus faecium, Enterococcus faecalis, Staphylococcus aureus, Candida albicans and Cryptococcus neoformans. Our strain collection presents resistance to all tested antimicrobials (>50). All drug-resistant strains were compared to their drug-sensitive counterparts. Regardless of resistance phenotype, MB-aPDT presented species-specific dose-response kinetics. More than 5log₁₀ reduction was observed within less than 75 s of illumination for A. baumannii, E. coli, E. faecium, E. faecalis and S. aureus and within less than 7 min for K. aerogenes, K. pneumoniae, P. aeruginosa, C. albicans and C. neoformans. No signs of correlations in between drug-resistance profiles and aPDT sensitivity were observed. Therefore, MB-aPDT can provide effective therapeutic protocols for a very broad spectrum of pathogens. Hence, we believe that this study represents a very important step to bring aPDT closer to implementation into mainstream medical practices.

Antifungal photodynamic activities of phthalocyanine derivatives on Candida albicans

Antimicrobial resistance is one of the most important causes of morbidity and mortality in the treatment of infectious diseases worldwide. Candida albicans is one of the most virulent and common species of fungi to cause invasive fungal infections on humans. Alternative treatment strategies, including photodynamic therapy, are needed for controlling these infectious diseases. The aim of this study was to investigate the antifungal photodynamic activities of phthalocyanine derivatives on C. albicans. The minimum inhibitory concentration (MIC) values of compounds were determined by the broth microdilution method. Uptake of the compounds in C. albicans and dark toxicity of the compounds were also investigated. Photodynamic inhibition of growth experiments was performed by measuring the colony-forming unit/mL (CFU/mL) of the strain. Maximum uptake into the cells was observed in the presence of 64 μg/mL concentration for each compound except for ZnPc. Compounds did not show dark toxicity/inhibitory effects at sub-MIC concentrations on C. albicans when compared to the negative control groups. Zn(II)Pc, ZnPc, and ZnPc-TiO₂ showed fungicidal effect after irradiation with the light dose of 90 J/cm² in the presence of the compounds. In addition to the fungicidal effects, SubPc, SubPc-TiO₂, Es-SiPc, and Es-SubPc compounds were also found to have inhibitory effects on the growth of yeast cells after irradiation.

The antimicrobial photodynamic inactivation resistance of Candida albicans is modulated by the Hog1 pathway and the Cap1 transcription factor

Candida albicans is the most important fungal pathogen afflicting humans, particularly immunocompromised patients. However, currently available antifungal drugs are limited and ineffective against drug-resistant strains. The development of new drugs or alternative therapeutic approaches to control fungal infections is urgent and necessary. Photodynamic inactivation (PDI) is a new promising therapy for eradicating microorganism infections through combining visible light, photosensitizers, and oxygen to generate reactive oxygen species (ROS). Although cytoprotective responses induced by photodynamic therapy (PDT) have been well studied in cancer cells, the mechanisms by which C. albicans responds to PDI are largely unknown. In this study, we first demonstrated that PDI induces C. albicans Hog1p activation. Deletion of any of the SSK2, PBS2, and HOG1 genes significantly decreased the survival rate after photochemical reactions, indicating that the Hog1 SAPK pathway is required for tolerance to PDI. Furthermore, the basic leucine zipper transcription factor Cap1 that regulates several downstream antioxidant genes was highly expressed during the response to PDI, and loss of CAP1 also resulted in decreased C. albicans survival rates. This study demonstrates the importance of the Hog1 SAPK and the Cap1 transcription factor, which regulates in resistance to PDI-mediated oxidative stress in C. albicans. Understanding the mechanisms by which C. albicans responds to PDI and consequently scavenges ROS will be very useful for the further development of therapeutics to control fungal infectious diseases, particularly those of the skin and mucosal infections.

Exploring the Galleria mellonella model to study antifungal photodynamic therapy

BACKGROUND

Antimicrobial photodynamic therapy (aPDT) shows antimicrobial activity on yeast of the genus Candida. In aPDT, the depth at which the light penetrates the tissue is extremely important for the elaboration of the treatment. The aim of this study was to evaluate the action of aPDT on experimental candidiasis and the laser impact in the tissue using Galleria mellonella as the infection model.

METHODS

G. mellonella larvae were infected with different Candida albicans strains. After 30 min, they were treated with methylene blue-mediated aPDT and a low intensity laser (660 nm). The larvae were incubated at 37 °C for seven days and monitored daily to determine the survival curve, using the Log-rank test (Mantel Cox). To evaluate the distribution of the laser as well as its depth of action in the larva body, the Interactive 3D surface PLOT of Image J was used. The effects of aPDT on the immune system were also evaluated by the quantification of hemocytes in the hemolymph of G. mellonella after 6 h of Candida infection (ANOVA and Tukey's test).

RESULTS

In both the ATCC 18,804 strain and the C. albicans clinical strain 17, aPDT prolonged the survival of the infected G. mellonella larvae by a lethal fungal dose. There was a statistically significant difference between the aPDT and the control groups in the ATCC strain (P = 0.0056). The depth of laser action in the insect body without the photosensitizer was 2.5 mm and 2.4 mm from the cuticle of the larva with the photosensitizer. In the larvae, a uniform distribution of light occurred along 32% of the body length for the group without the photosensitizer and in 39.5% for the group with the photosensitizer. In the immunological analysis, the infection by C. albicans ATCC 18,804 in G. mellonella led to a reduction in the number of hemocytes in the hemolymph. The aPDT and laser treatment induced a slight increase in the number of hemocytes.

CONCLUSION

Both aPDT and laser treatment positively influenced the treatment of experimental candidiasis. G. mellonella larvae were a useful model for the study of light tissue penetration in antimicrobial photodynamic therapy.

Controlling methylene blue aggregation: a more efficient alternative to treat Candida albicans infections using photodynamic therapy

Methylene Blue (MB) has been widely used in antimicrobial Photodynamic Therapy (aPDT), however, the mechanisms of action (Type I or Type II) are defined by its state of aggregation. In this sense, the identification of the relationships between aggregation, the mechanisms of action and the effectiveness against microorganisms, as well as the establishment of the means and the formulations that may favor the most effective mechanisms, are essential. Thus, the objective of this study was to assess the in vitro aPDT efficacies against Candida albicans, by using MB in vehicles which may influence the aggregation and present an oral formulation (OF) containing MB, to be used in clinical aPDT procedures. The efficacy of MB at 20 mg L-1 was tested in a range of vehicles (water, physiological solution - NaCl 0.9%, phosphate saline buffer - PBS, sodium dodecyl sulfate 0.25% - SDS and urea 1 mol L-1) in a C. albicans planktonic culture, when using 4.68 J cm-2 of 640 ± 12 nm LED for the irradiations, as well as 5 minutes of pre-irradiation time, together with measuring the UFC mL-1. Based upon these analyses, an OF containing MB in the most effective vehicle was tested in the biofilms, as a proposal for clinical applications. When comparing some of the vehicles, sodium dodecyl sulfate was the only one that enhanced an MB aPDT efficacy in a planktonic C. albicans culture. This OF was tested in the biofilms and 50 mg L-1 MB was necessary, in order to achieve some reduction in the cell viabilities after the various treatments. The light dosimetries still need further adaptations, in order for this formulation to be used in clinical applications. The present research has indicated that the development of this formulation for the control of MB aggregations may result in more effective clinical protocols.

Antimicrobial photodynamic therapy as a new approach for the treatment of vulvovaginal candidiasis: preliminary results

In this work, we present the efficacy of photodynamic therapy against yeast cells in an animal model. We tested two photosensitizers, methylene blue and protoporphyrin IX. Thirty-seven female BALB-c mice with a body mass of 20-25 g were used. To achieve persistent vaginitis, estrus was induced by subcutaneous injection of 0.1 mg/mL estradiol valerate applied weekly. Three days after pseudo-estrus, intravaginal inoculation with Candida albicans was performed. Mice were anesthetized with ketamine (80 mg/kg) and xylazine (10 mg/kg) by intraperitoneal injection before inoculation, and antimicrobial photodynamic therapy (aPDT) was performed 5 days after fungal inoculation. Two photosensitizers were tested, methylene blue (MB; 100 μM) and protoporphyrin IX (PpNetNI; 10 μM). Two custom-made LEDs emitting light at 660 and 630 nm at approximately 800 mW each were used for irradiation. The aPDT treatment reduced the fungal colony-forming units (CFUs) by one order of magnitude for the MB (p = 0.020) and PpNetNI (p = 0.018) photosensitizers. Seven days after the treatment, there were significantly fewer CFUs compared to the control group (p = 0.041 and p = 0.035 for MB and PpNetNI, respectively), but this was not increased compared to the initial number immediately after aPDT. Using aPDT as a therapeutic option to decrease fungal infection in a vaginal candidiasis model resulted in a significant reduction in the C. albicans population. Both photosensitizers were effective for preventing reinfection within 7 days. The aPDT also had no effect on the vaginal mucosa at the ultrastructural level. In addition to the fungicide effect, we observed reduced swelling and lack of the formation of abscesses, microabscesses coating the cornified epithelial layer, and the accumulation of neutrophils in the submucosa.

Enhanced sterilization and healing of cutaneous pseudomonas infection using 5-aminolevulinic acid as a photosensitizer with 410-nm LED light

BACKGROUND

Pseudomonas aeruginosa (PA) frequently develops antibiotic-resistant characteristics, which is clinically problematic. The main reason behind the rise of antibiotic-resistant PA is the extensive use of antibiotics. Therefore, a novel technique is needed to treat PA infections. Photodynamic therapy (PDT) is thought to have the potential to be a non-antibiotic treatment for infections. 5-Aminolevulinic acid (ALA), which works as a photosensitizer after being metabolized into protoporphyrin IX (PpIX) in the heme synthetic pathway, is used for PDT. Thus far, the in vivo effectiveness of PDT using ALA against PA is unknown.

OBJECTIVE

In this study, we investigated PDT using ALA both in vitro and in vivo.

METHODS AND RESULTS

Although PDT with ALA alone did not show a bactericidal effect on PA, PDT with both ALA and EDTA-2Na had a bactericidal effect in vitro. In in vivo experiments, wounds healed faster in PA-infected mice treated with PDT using both EDTA-2Na and ALA compared to non-PDT.

CONCLUSION

These results suggest that PDT with EDTA-2Na and ALA is a potential novel treatment option for PA-infected wounds.

Antimicrobial photodynamic therapy - what we know and what we don't

Considering increasing number of pathogens resistant towards commonly used antibiotics as well as antiseptics, there is a pressing need for antimicrobial approaches that are capable of inactivating pathogens efficiently without the risk of inducing resistances. In this regard, an alternative approach is the antimicrobial photodynamic therapy (aPDT). The antimicrobial effect of aPDT is based on the principle that visible light activates a per se non-toxic molecule, the so-called photosensitizer (PS), resulting in generation of reactive oxygen species that kill bacteria unselectively via an oxidative burst. During the last 10-20 years, there has been extensive in vitro research on novel PS as well as light sources, which is now to be translated into clinics. In this review, we aim to provide an overview about the history of aPDT, its fundamental photochemical and photophysical mechanisms as well as photosensitizers and light sources that are currently applied for aPDT in vitro. Furthermore, the potential of resistances towards aPDT is extensively discussed and implications for proper comparison of in vitro studies regarding aPDT as well as for potential application fields in clinical practice are given. Overall, this review shall provide an outlook on future research directions needed for successful translation of promising in vitro results in aPDT towards clinical practice.

Photodynamic antimicrobial chemotherapy (PACT) using toluidine blue inhibits both growth and biofilm formation by Candida krusei

Among non-albicans Candida species, the opportunistic pathogen Candida krusei emerges because of the high mortality related to infections produced by this yeast. The Candida krusei is an opportunistic pathogen presenting an intrinsic resistance to fluconazol. In spite of the reduced number of infections produced by C. krusei, its occurrence is increasing in some groups of patients submitted to the use of fluconazol for prophylaxis. Photodynamic antimicrobial chemotherapy (PACT) is a potential antimicrobial therapy that combines visible light and a nontoxic dye, known as a photosensitizer, producing reactive oxygen species (ROS) that can kill the treated cells. The objective of this study was to investigate the effects of PACT, using toluidine blue, as a photosensitizer on both growth and biofilm formation by Candida krusei. In this work, we studied the effect of the PACT, using TB on both cell growth and biofilm formation by C. krusei. PACT was performed using a light source with output power of 0.068 W and peak wavelength of 630 nm, resulting in a fluence of 20, 30, or 40 J/cm². In addition, ROS production was determined after PACT. The number of samples used in this study varied from 6 to 8. Statistical differences were evaluated by analysis of variance (ANOVA) and post hoc comparison with Tukey-Kramer test. PACT inhibited both growth and biofilm formation by C. krusei. It was also observed that PACT stimulated ROS production. Comparing to cells not irradiated, irradiation was able to increase ROS production in 11.43, 6.27, and 4.37 times, in the presence of TB 0.01, 0.02, and 0.05 mg/mL, respectively. These results suggest that the inhibition observed in the cell growth after PACT could be related to the ROS production, promoting cellular damage. Taken together, these results demonstrated the ability of PACT reducing both cell growth and biofilm formation by C. krusei.

Photodynamic Antimicrobial Chemotherapy (PACT), using Toluidine blue O inhibits the viability of biofilm produced by Candida albicans at different stages of development

BACKGROUND

Candida albicans is an opportunistic fungus producing both superficial and systemic infections, especially in immunocompromised individuals. It has been demonstrated that C. albicans ability to form biofilms is a crucial process for colonization and virulence. Furthermore, a correlation between the development of drug resistance and biofilm maturation at Candida biofilms has been shown. Photodynamic Antimicrobial Chemotherapy (PACT) is a potential antimicrobial therapy that combines visible light and a non-toxic dye, known as a photosensitizer, producing reactive oxygen species (ROS) that can kill the treated cells. The objective of this study was to investigate the effects of PACT, using Toluidine Blue O (TBO) on the viability of biofilms produced by C. albicans at different stages of development.

METHODS

In this study, the effects of PACT on both biofilm formation and viability of the biofilm produced by C. albicans were studied. Biofilm formation and viability were determined by a metabolic assay based on the reduction of XTT assay. In addition, the morphology of the biofilm was observed using light microscopy.

RESULTS

PACT inhibited both biofilm formation and viability of the biofilm produced by C. albicans. Furthermore, PACT was able to decrease the number of both cells and filamentous form present in the biofilm structure. This inhibitory effect was observed in both early and mature biofilms.

CONCLUSIONS

The results obtained in this study demonstrated the potential of PACT (using TBO) as an effective antifungal therapy, including against infections associated with biofilms at different stages of development.

A Closer Look at Dark Toxicity of the Photosensitizer TMPyP in Bacteria

Photodynamic inactivation of bacteria (PIB) is based on photosensitizers which absorb light and generate reactive oxygen species (ROS), killing cells via oxidation. PIB is evaluated by comparing viability with and without irradiation, where reduction of viability in the presence of the photosensitizer without irradiation is considered as dark toxicity. This effect is controversially discussed for photosensitizers like TMPyP (5,10,15,20-Tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluensulfonate). TMPyP shows a high absorption coefficient for blue light and a high yield of ROS production, especially singlet oxygen. Escherichia coli and Bacillus atrophaeus were incubated with TMPyP and irradiated with different light sources at low radiant exposures (μW per cm²), reflecting laboratory conditions of dark toxicity evaluation. Inactivation of E. coli occurs for blue light, while no effect was detectable for wavelengths >450 nm. Being more susceptible toward PIB, growth of B. atrophaeus is even reduced for light with emission >450 nm. Decreasing the light intensities to nW per cm² for B. atrophaeus, application of TMPyP still caused bacterial killing. Toxic effects of TMPyP disappeared after addition of histidine, quenching residual ROS. Our experiments demonstrate that the evaluation of dark toxicity of a powerful photosensitizer like TMPyP requires low light intensities and if necessary additional application of substances quenching any residual ROS.

Advances in antimicrobial photodynamic inactivation at the nanoscale

The alarming worldwide increase in antibiotic resistance amongst microbial pathogens necessitates a search for new antimicrobial techniques, which will not be affected by, or indeed cause resistance themselves. Light-mediated photoinactivation is one such technique that takes advantage of the whole spectrum of light to destroy a broad spectrum of pathogens. Many of these photoinactivation techniques rely on the participation of a diverse range of nanoparticles and nanostructures that have dimensions very similar to the wavelength of light. Photodynamic inactivation relies on the photochemical production of singlet oxygen from photosensitizing dyes (type II pathway) that can benefit remarkably from formulation in nanoparticle-based drug delivery vehicles. Fullerenes are a closed-cage carbon allotrope nanoparticle with a high absorption coefficient and triplet yield. Their photochemistry is highly dependent on microenvironment, and can be type II in organic solvents and type I (hydroxyl radicals) in a biological milieu. Titanium dioxide nanoparticles act as a large band-gap semiconductor that can carry out photo-induced electron transfer under ultraviolet A light and can also produce reactive oxygen species that kill microbial cells. We discuss some recent studies in which quite remarkable potentiation of microbial killing (up to six logs) can be obtained by the addition of simple inorganic salts such as the non-toxic sodium/potassium iodide, bromide, nitrite, and even the toxic sodium azide. Interesting mechanistic insights were obtained to explain this increased killing.

Antimicrobial Photodynamic Therapy against Endodontic Enterococcus faecalis and Candida albicans Mono and Mixed Biofilms in the Presence of Photosensitizers: A Comparative Study with Classical Endodontic Irrigants

Endodontic biofilms eradication from the infected root canal system remains as the primary focus in endodontic field. In this study, it was assessed the efficacy of antimicrobial Photodynamic Therapy (aPDT) with the Zn(II)chlorin e6 methyl ester (Zn(II)e₆Me) activated by red light against monospecies and mixed biofilms of Enterococcus faecalis and Candida albicans. The results were compared with the ones obtained with Rose Bengal (RB), Toluidine Blue-O (TBO), the synthetic tetracationic porphyrin (TMPyP) as well as classical endodontic irrigants (3% NaOCl, 17% EDTA and 2% CHX). The antimicrobial efficacy of aPDT toward monospecies and mixed biofilms was quantified resorting to safranin red method. The changes of biofilm organization and of cellular ultrastructure were evaluated through several microscopy techniques (light, laser confocal and transmission electron microscopy). Zn(II)e₆Me once activated with light for 60 or 90 s was able to remove around 60% of the biofilm’s biomass. It was more efficient than TBO and RB and showed similar efficiency to TMPyP and classical irrigants, CHX and EDTA. As desirable in a PS, Zn(II)e₆Me in the dark showed smaller activity than TMPyP. Only NaOCl revealed higher efficiency, with 70–90% of the biofilm’s biomass removal. The organization of biofilms and the normal microbial cell ultrastructure were extensively damaged by the presence of Zn(II)e₆Me. aPDT with Zn(II)e₆Me showed to be an efficient antimicrobial strategy deserving further studies leading to a future clinical usage in endodontic disinfection.

Glucose modulates antimicrobial photodynamic inactivation of Candida albicans in biofilms

Candida albicans biofilm is a main cause of infections associated with medical devices such as catheters, contact lens and artificial joint prosthesis. The current treatment comprises antifungal chemotherapy that presents low success rates. Photodynamic inactivation (PDI) involves the combination of a photosensitizing compound (PS) and light to generate oxidative stress that has demonstrated effective antimicrobial activity against a broad-spectrum of pathogens, including C. albicans. This fungus senses glucose inducing an upregulation of membrane transporters that can facilitate PS uptake into the cell. The aim of this study was to evaluate the effects of glucose on methylene blue (MB) uptake and its influence on PDI efficiency when combined to a red LED with central wavelength at λ=660nm. C. albicans biofilms were grown on hydrogel disks. Prior to PDI assays, MB uptake tests were performed with and without glucose-sensitization. In this system, the optimum PS administration was determined as 500μM of MB in contact with the biofilm during 30min before irradiation. Irradiation was performed during 3, 6, 9, 12, 15 and 18min with irradiance of 127.3mW/cm². Our results showed that glucose was able to increase MB uptake in C. albicans cells. In addition, PDI without glucose showed a higher viability reduction until 6min; after 9min, glucose group demonstrated a significant decrease in cell viability when compared to glucose-free group. Taken together, our data suggest that glucose is capable to enhance MB uptake and modulate photodynamic inactivation of C. albicans biofilm.

Photoantimicrobials-are we afraid of the light?

Although conventional antimicrobial drugs have been viewed as miraculous cure-alls for the past 80 years, increasing antimicrobial drug resistance requires a major and rapid intervention. However, the development of novel but still conventional systemic antimicrobial agents, having only a single mode or site of action, will not alleviate the situation because it is probably only a matter of time until any such agents will also become ineffective. To continue to produce new agents based on this notion is unacceptable, and there is an increasing need for alternative approaches to the problem. By contrast, light-activated molecules called photoantimicrobials act locally via the in-situ production of highly reactive oxygen species, which simultaneously attack various biomolecular sites in the pathogenic target and therefore offer both multiple and variable sites of action. This non-specificity at the target circumvents conventional mechanisms of resistance and inhibits the development of resistance to the agents themselves. Photoantimicrobial therapy is safe and easy to implement and, unlike conventional agents, the activity spectrum of photoantimicrobials covers bacteria, fungi, viruses, and protozoa. However, clinical trials of these new, truly broad-spectrum, and minimally toxic agents have been few, and the funding for research and development is almost non-existent. Photoantimicrobials constitute one of the few ways forward through the morass of drug-resistant infectious disease and should be fully explored. In this Personal View, we raise awareness of the novel photoantimicrobial technologies that offer a viable alternative to conventional drugs in many relevant application fields, and could thus slow the pace of resistance development.

Photodynamic therapy as an antifungal treatment

Photodynamic therapy (PDT) involves the systemic or topical application of a photosensitizer (PS), alongside the selective illumination of the target lesion with light of an appropriate wavelength, in order to promote localized oxidative photodamage and subsequent cell death. Numerous studies have demonstrated that PDT is highly effective in the destruction of fungi in vitro. The mechanism underlying the effects of PDT results from the photons of visible light of an appropriate wavelength interacting with the intracellular molecules of the PS. Reactive species are produced as a result of the oxidative stress caused by the interaction between the visible light and the biological tissue. At present, no antifungal treatment based on PDT has been licensed. However, antifungal PDT is emerging as an area of interest for research.

Evaluation of the Effects of Photodynamic Therapy Alone and Combined with Standard Antifungal Therapy on Planktonic Cells and Biofilms of Fusarium spp. and Exophiala spp

Infections of Fusarium spp. and Exophiala spp. are often chronic, recalcitrant, resulting in significant morbidity, causing discomfort, disfigurement, social isolation. Systemic disseminations happen in compromised patients, which are often refractory to available antifungal therapies and thereby lead to death. The antimicrobial photodynamic therapy (aPDT) has been demonstrated to effectively inactivate multiple pathogenic fungi and is considered as a promising alternative treatment for mycoses. In the present study, we applied methylene blue (8, 16, and 32 μg/ml) as a photosensitizing agent and light emitting diode (635 ± 10 nm, 12 and 24 J/cm²), and evaluated the effects of photodynamic inactivation on five strains of Fusarium spp. and five strains of Exophiala spp., as well as photodynamic effects on in vitro susceptibility to itraconazole, voriconazole, posaconazole and amphotericin B, both planktonic and biofilm forms. Photodynamic therapy was efficient in reducing the growth of all strains tested, exhibiting colony forming unit-reductions of up to 6.4 log₁₀ and 5.6 log₁₀ against planktonic cultures and biofilms, respectively. However, biofilms were less sensitive since the irradiation time was twice longer than that of planktonic cultures. Notably, the photodynamic effects against Fusarium strains with high minimal inhibitory concentration (MIC) values of ≥16, 4-8, 4-8, and 2-4 μg/ml for itraconazole, voriconazole, posaconazole and amphotericin B, respectively, were comparable or even superior to Exophiala spp., despite Exophiala spp. showed relatively better antifungal susceptibility profile. MIC ranges against planktonic cells of both species were up to 64 times lower after aPDT treatment. Biofilms of both species showed high sessile MIC50 (SMIC50) and SMIC80 of ≥16 μg/ml for all azoles tested and variable susceptibilities to amphotericin B, with SMIC ranging between 1 and 16 μg/ml. Biofilms subjected to aPDT exhibited a distinct reduction in SMIC50 and SMIC80 compared to untreated groups for both species, except SMIC80 of itraconazole against Fusarium biofilms. In conclusion, in vitro photodynamic therapy was efficient in inactivation of Fusarium spp. and Exophiala spp., both planktonic cultures and biofilms. In addition, the combination of aPDT and antifungal drugs represents an attractive alternative to the current antifungal strategies. However, further investigations are warranted for the reliable and safe application in clinical practice.

Cyclodextrin induced controlled delivery of a biological photosensitizer from a nanocarrier to DNA

Addition of β-cyclodextrin to the micelle bound phenosafranin in the presence of ctDNA leads to quantitative transfer of the fluorophore from the micelle to the DNA.

Photodynamic Inactivation of Candida albicans with Imidazoacridinones: Influence of Irradiance, Photosensitizer Uptake and Reactive Oxygen Species Generation

The increasing applicability of antifungal treatments, the limited range of available drug classes and the emergence of drug resistance in Candida spp. suggest the need for new treatment options. To explore the applicability of C. albicans photoinactivation, we examined nine structurally different imidazoacridinone derivatives as photosensitizing agents. The most effective derivatives showed a >10(4)-fold reduction of viable cell numbers. The fungicidal action of the three most active compounds was compared at different radiant powers (3.5 to 63 mW/cm2), and this analysis indicated that 7 mW/cm2 was the most efficient. The intracellular accumulation of these compounds in fungal cells correlated with the fungicidal activity of all 9 derivatives. The lack of effect of verapamil, an inhibitor targeting Candida ABC efflux pumps, suggests that these imidazoacridinones are not substrates for ABC transporters. Thus, unlike azoles, a major class of antifungals used against Candida, ABC transporter-mediated resistance is unlikely. Electron paramagnetic resonance (EPR)-spin trapping data suggested that the fungicidal light-induced action of these derivatives might depend on the production of superoxide anion. The highest generation rate of superoxide anion was observed for 1330H, 1610H, and 1611. Singlet oxygen production was also detected upon the irradiation of imidazoacridinone derivatives with UV laser light, with a low to moderate yield, depending on the type of compound. Thus, imidazoacridinone derivatives examined in the present study might act via mixed type I/type II photodynamic mechanism. The presented data indicate lack of direct correlation between the structures of studied imidazoacridinones, cell killing ability, and ROS production. However, we showed for the first time that for imidazoacridinones not only intracellular accumulation is necessary prerequisite of lethal photosensitization of C. albicans, but also localization within particular cellular structures. Our findings present IA derivatives as efficient antifungal photosensitizers with a potential to be used in local treatment of Candida infection.

Antimicrobial photodynamic therapy combined with periodontal treatment for metabolic control in patients with type 2 diabetes mellitus: study protocol for a randomized controlled trial

Background

The relationship between diabetes mellitus (DM) and periodontal disease is bidirectional. DM is a predisposing and modifying factor of periodontitis, which, in turn, worsens glycemic control and increases proteins found in the acute phase of inflammation, such as C-reactive protein. The gold standard for the treatment of periodontal disease is oral hygiene orientation, scaling and planing. Moreover, systemic antibiotic therapy may be employed in some cases. In an effort to minimize the prescription of antibiotics, photodynamic therapy (PDT) has been studied as an antimicrobial technique and has demonstrated promising results. The aim of the proposed study is to determine whether PDT as a complement to periodontal therapy (PT) is helpful in the metabolic control of individuals with type 2 diabetes and the reduction of acute-phase inflammatory markers.

Methods/Design

The patients will be randomized using a proper software program into two groups: 1) PT + placebo PDT or 2) PT + active PDT. All patients will first be examined by a specialist, followed by PT performed by two other healthcare professionals. At the end of each session, PDT (active or placebo) will be administered by a fourth healthcare professional. The following will be the PDT parameters: diode laser (660 nm); power output = 110 mW; exposure time = 90 s per point (9 J/point); and energy density = 22 J/cm². The photosensitizer will be methylene blue (50 μg/mL). The patients will be re-evaluated 15, 30, 90 and 180 days after treatment. Serological examinations with complete blood count, fasting glucose, glycated hemoglobin and salivary examinations to screen for tumor necrosis factor alpha, interleukin 1, interleukin 6, ostelocalcin, and osteoprotegerin/RANKL will be performed at each evaluation. The data will be statistically evaluated using the most appropriate tests.

Discussion

The results of this study will determine the efficacy of photodynamic therapy as an adjuvant to periodontal treatment in diabetic patients.

Trial registration

The protocol for this trial was registered with Clinical Trials registration number NCT01964833 on 14 October 2013.

Antimicrobial photodynamic therapy: an effective alternative approach to control fungal infections

Skin mycoses are caused mainly by dermatophytes, which are fungal species that primarily infect areas rich in keratin such as hair, nails, and skin. Significantly, there are increasing rates of antimicrobial resistance among dermatophytes, especially for Trichophyton rubrum, the most frequent etiologic agent worldwide. Hence, investigators have been developing new therapeutic approaches, including photodynamic treatment. Photodynamic therapy (PDT) utilizes a photosensitive substance activated by a light source of a specific wavelength. The photoactivation induces cascades of photochemicals and photobiological events that cause irreversible changes in the exposed cells. Although photodynamic approaches are well established experimentally for the treatment of certain cutaneous infections, there is limited information about its mechanism of action for specific pathogens as well as the risks to healthy tissues. In this work, we have conducted a comprehensive review of the current knowledge of PDT as it specifically applies to fungal diseases. The data to date suggests that photodynamic treatment approaches hold great promise for combating certain fungal pathogens, particularly dermatophytes.

Fast and effective photodynamic inactivation of multiresistant bacteria by cationic riboflavin derivatives

Photodynamic inactivation of bacteria (PIB) proves to be an additional method to kill pathogenic bacteria. PIB requires photosensitizer molecules that effectively generate reactive oxygen species like singlet oxygen when exposed to visible light. To allow a broad application in medicine, photosensitizers should be safe when applied in humans. Substances like vitamin B2, which are most likely safe, are known to produce singlet oxygen upon irradiation. In the present study, we added positive charges to flavin derivatives to enable attachment of these molecules to the negatively charged surface of bacteria. Two of the synthesized flavin derivatives showed a high quantum yield of singlet oxygen of approximately 75%. Multidrug resistant bacteria like MRSA (Methicillin resistant Staphylococcus aureus), EHEC (enterohemorrhagic Escherichia coli), Pseudomonas aeruginosa, and Acinetobacter baumannii were incubated with these flavin derivatives in vitro and were subsequently irradiated with visible light for seconds only. Singlet oxygen production in bacteria was proved by detecting its luminescence at 1270 nm. After irradiation, the number of viable bacteria decreased up to 6 log10 steps depending on the concentration of the flavin derivatives and the light dosimetry. The bactericidal effect of PIB was independent of the bacterial type and the corresponding antibiotic resistance pattern. In contrast, the photosensitizer concentration and light parameters used for bacteria killing did not affect cell viability of human keratinocytes (therapeutic window). Multiresistant bacteria can be safely and effectively killed by a combination of modified vitamin B2 molecules, oxygen and visible light, whereas normal skin cells survive. Further work will include these new photosensitizers for topical application to decolonize bacteria from skin and mucosa.

Photodynamic inactivation of oropharyngeal Candida strains

Oropharyngeal candidiasis (OPC) is an infection frequent in immunocompromised patients. Photodynamic therapy is an alternative to conventional treatments, based on the utilization of compounds that inhibit or kill microorganisms only under the effect of light, process known as Photodynamic Inactivation (PDI). In the present study, PDI of Candida spp. by the natural product α-terthienyl (α-T) was investigated following the guidelines of CLSI M27-A3, under UV-A light irradiation. The optimal values of two variables, exposure irradiation time (ET) and distance to the irradiation source (DIS) were established by employing Design Expert Software (DES). For this purpose, a panel of Candida strains isolated from OPC (C. albicans, C. tropicalis, C. parapsilosis and C. krusei) was employed and optimal values were 5 min (ET) and between 6.06 and 6.43 cm (DIS) with a desirability factor of 0.989. α-T plus UV-A light in the optimal conditions caused a complete reduction in viable cells in 5 min which was demonstrated by viable cells reduction assays and confocal microscopy after vital staining (propidium iodide/fluorescein diacetate). The germ tube formation of C. albicans was inhibited by α-T at sub-inhibitory concentrations. Results showed that α-T plus UV-A light could constitute an alternative for OPC treatments at the optimal conditions determined here.

Antimicrobial photodynamic therapy for inactivation of biofilms formed by oral key pathogens

With increasing numbers of antibiotic-resistant pathogens all over the world there is a pressing need for strategies that are capable of inactivating biofilm-state pathogens with less potential of developing resistances in pathogens. Antimicrobial strategies of that kind are especially needed in dentistry in order to avoid the usage of antibiotics for treatment of periodontal, endodontic or mucosal topical infections caused by bacterial or yeast biofilms. One possible option could be the antimicrobial photodynamic therapy (aPDT), whereby the lethal effect of aPDT is based on the principle that visible light activates a photosensitizer (PS), leading to the formation of reactive oxygen species, e.g., singlet oxygen, which induce phototoxicity immediately during illumination. Many compounds have been described as potential PS for aPDT against bacterial and yeast biofilms so far, but conflicting results have been reported. Therefore, the aim of the present review is to outline the actual state of the art regarding the potential of aPDT for inactivation of biofilms formed in vitro with a main focus on those formed by oral key pathogens and structured regarding the distinct types of PS.

Selective photoinactivation of Candida albicans in the non-vertebrate host infection model Galleria mellonella

Background

Candida spp. are recognized as a primary agent of severe fungal infection in immunocompromised patients, and are the fourth most common cause of bloodstream infections. Our study explores treatment with photodynamic therapy (PDT) as an innovative antimicrobial technology that employs a nontoxic dye, termed a photosensitizer (PS), followed by irradiation with harmless visible light. After photoactivation, the PS produces either singlet oxygen or other reactive oxygen species (ROS) that primarily react with the pathogen cell wall, promoting permeabilization of the membrane and cell death. The emergence of antifungal-resistant Candida strains has motivated the study of antimicrobial PDT (aPDT) as an alternative treatment of these infections. We employed the invertebrate wax moth Galleria mellonella as an in vivo model to study the effects of aPDT against C. albicans infection. The effects of aPDT combined with conventional antifungal drugs were also evaluated in G. mellonella.

Results

We verified that methylene blue-mediated aPDT prolonged the survival of C. albicans infected G. mellonella larvae. The fungal burden of G. mellonella hemolymph was reduced after aPDT in infected larvae. A fluconazole-resistant C. albicans strain was used to test the combination of aPDT and fluconazole. Administration of fluconazole either before or after exposing the larvae to aPDT significantly prolonged the survival of the larvae compared to either treatment alone.

Conclusions

G. mellonella is a useful in vivo model to evaluate aPDT as a treatment regimen for Candida infections. The data suggests that combined aPDT and antifungal therapy could be an alternative approach to antifungal-resistant Candida strains.

Microbial efflux systems and inhibitors: approaches to drug discovery and the challenge of clinical implementation

Conventional antimicrobials are increasingly ineffective due to the emergence of multidrug-resistance among pathogenic microorganisms. The need to overcome these deficiencies has triggered exploration for novel and unconventional approaches to controlling microbial infections. Multidrug efflux systems (MES) have been a profound obstacle in the successful deployment of antimicrobials. The discovery of small molecule efflux system blockers has been an active and rapidly expanding research discipline. A major theme in this platform involves efflux pump inhibitors (EPIs) from natural sources. The discovery methodologies and the available number of natural EPI-chemotypes are increasing. Advances in our understanding of microbial physiology have shed light on a series of pathways and phenotypes where the role of efflux systems is pivotal. Complementing existing antimicrobial discovery platforms such as photodynamic therapy (PDT) with efflux inhibition is a subject under investigation. This core information is a stepping stone in the challenge of highlighting an effective drug development path for EPIs since the puzzle of clinical implementation remains unsolved. This review summarizes advances in the path of EPI discovery, discusses potential avenues of EPI implementation and development, and underlines the need for highly informative and comprehensive translational approaches.