Susceptibility of Candida species to photodynamic effects of photofrin

Respiratory deficiency enhances the sensitivity of the pathogenic fungus Candida to photodynamic treatment

Mucosal infections caused by the pathogenic fungus Candida are a significant infectious disease problem and are often difficult to eradicate because of the high frequency of resistance to conventional antifungal agents. Photodynamic treatment (PDT) offers an attractive therapeutic alternative. Previous studies demonstrated that filamentous forms and biofilms of Candida albicans were sensitive to PDT using Photofrin as a photosensitizer. However, early stationary phase yeast forms of C. albicans and Candida glabrata were not adversely affected by treatment. We report that the cationic porphyrin photosensitizer meso-tetra (N-methyl-4-pyridyl) porphine tetra tosylate (TMP-1363) is effective in PDT against yeast forms of C. albicans and C. glabrata. Respiratory-deficient (RD) strains of C. albicans and C. glabrata display a pleiotropic resistance pattern, including resistance to members of the azole family of antifungals, the salivary antimicrobial peptides histatins and other types of toxic stresses. In contrast to this pattern, RD mutants of both C. albicans and C. glabrata were significantly more sensitive to PDT compared to parental strains. These data suggest that intact mitochondrial function may provide a basal level of anti-oxidant defense against PDT-induced phototoxicity in Candida, and reveals pathways of resistance to oxidative stress that can potentially be targeted to increase the efficacy of PDT against this pathogenic fungus.

Study of germ tube formation by Candida albicans after photodynamic antimicrobial chemotherapy (PACT)

Due to the augmented number of immunocompromised patients, the infections associated to the pathogen of the genus Candida have increased dramatically in the recent years. In order to proliferate, Candida albicans can produce a germ tube formation extending from the cells. The germ tube formation is a transition state from budding to hyphal cells, and represents an essential stage for virulence. In this work we studied the effect of the photodynamic antimicrobial chemotherapy (PACT), a potential antimicrobial treatment on the germ tube formation by C. albicans. Germ tube formation was induced by goat serum after different treatments with Methylene blue (MB) and Laser (683nm). Our results demonstrated that photodynamic therapy using MB, as a photosensitizing drug; inhibits both the growth and the germ tube formation by C. albicans. Thus, our results suggest the possibility that Methylene blue, combined with light in a specific wavelength, can be used as a promising novel antifungal agent.

Susceptibility of Cryptococcus neoformans to photodynamic inactivation is associated with cell wall integrity

Photodynamic therapy is a rapidly developing antimicrobial technology which combines a nontoxic photoactivatable dye or photosensitizer with harmless visible light of the correct wavelength to excite the dye to its reactive triplet state to generate reactive oxygen species toxic to cells. In this report we present evidence that the fungal pathogen Cryptococcus neoformans is susceptible to photodynamic inactivation by use of a polycationic conjugate of polyethyleneimine and the photosensitizer chlorin(e6). A C. neoformans rom2 mutant, with a mutation involving a putative Rho1 guanyl nucleotide exchange factor that is part of the protein kinase C-cell wall integrity pathway, demonstrated a compromised cell wall and less (1,3)beta-d glucan than the wild-type strain and increased accumulation of PEI-ce6 as assessed by fluorescence uptake and confocal microscopy. Interestingly, C. neoformans rom2 was hypersusceptible to photodynamic inactivation and coincubation of wild-type C. neoformans strain KN99alpha with caspofungin-enhanced photoinactivation. These studies demonstrated that C. neoformans is sensitive to photodynamic therapy and illustrated the significance of cell wall integrity in microbial susceptibility to antimicrobial photodynamic inactivation.

Photodynamic activity of water-soluble phthalocyanine zinc(II) complexes against pathogenic microorganisms

Photodynamic activity of tetrakis-(3-methylpyridyloxy)- and tetrakis-(4-sulfophenoxy)-phthalocyanine zinc(II) toward the gram-positive Staphylococcus aureus, the gram-negative Pseudomonas aeruginosa, and the fungi Candida albicans was studied. The drug uptake dependency with an inverse behavior to the cell density was observed. The cationic photosensitizer completely inactivated S. aureus and C. albicans, and with 4 log10 P. aeruginosa. The photoinactivation at mild experimental conditions, such as drug dose of 1.5 microM and fluence of 50 mW cm(-2) for 10 min irradiation time, was shown.

Photodynamic therapy in the treatment of microbial infections: basic principles and perspective applications

BACKGROUND AND OBJECTIVES

Photodynamic therapy (PDT) appears to be endowed with several favorable features for the treatment of infections originated by microbial pathogens, including a broad spectrum of action, the efficient inactivation of antibiotic-resistant strains, the low mutagenic potential, and the lack of selection of photoresistant microbial cells. Therefore, intensive studies are being pursued in order to define the scope and field of application of this approach.

RESULTS

Optimal cytocidal activity against a large variety of bacterial, fungal, and protozoan pathogens has been found to be typical of photosensitizers that are positively charged at physiological pH values (e.g., for the presence of quaternarized amino groups or the association with polylysine moieties) and are characterized by a moderate hydrophobicity (n-octanol/water partition coefficient around 10). These photosensitizers in a micromolar concentration can induce a >4-5 log decrease in the microbial population after incubation times as short as 5-10 minutes and irradiation under mild experimental conditions, such as fluence-rates around 50 mW/cm2 and irradiation times shorter than 15 minutes.

CONCLUSIONS

PDT appears to represent an efficacious alternative modality for the treatment of localized microbial infections through the in situ application of the photosensitizer followed by irradiation of the photosensitizer-loaded infected area. Proposed clinical fields of interest of antimicrobial PDT include the treatment of chronic ulcers, infected burns, acne vulgaris, and a variety of oral infections.

Potential of photodynamic therapy in treatment of fungal infections of the mouth. Design and characterisation of a mucoadhesive patch containing toluidine blue O

Mucocutaneous oropharyngeal candidiasis is predominately caused by Candida albicans. The overall incidence of oral candidiasis in young adults has increased dramatically with the spread of HIV/AIDS. Conventional treatments have been shown to have a fungistatic rather than a fungicidal effect, resulting in an inadequate treatment outcome for patients. In addition, increasing resistance of C. albicans to antifungal agents has made effective treatment more difficult. Accordingly, interest has arisen in development of new prophylaxis/treatment regimens. One such alternative treatment is photodynamic antimicrobial chemotherapy (PACT), in which a combination of a photosensitising drug and visible light cause selective destruction of microbial cells. Due to the highly coloured nature of photosensitisers and the potential for staining of teeth, lips and buccal mucosa, administration of photosensitisers to humans as a liquid mouthwash is undesirable. Targeted delivery of the photosensitiser directly to the site of infection should be the aim. The current study, therefore, reports on a mucoadhesive patch containing toluidine blue O (TBO), as a potential delivery system for use in PACT of oropharyngeal candidiasis. Patches prepared from aqueous blends of poly(methyl vinyl ether/maleic anhydride) and tripropyleneglycol methyl ether possessed suitable properties for use as mucoadhesive drug delivery systems and were capable of resisting dissolution when immersed in artificial saliva. When releasing directly into an aqueous sink, patches containing 50 and 100mg TBO cm(-2) both generated receiver compartment concentrations exceeding the concentration (2.0-5.0 mg ml(-1)) required to produce high levels of kill (>90%) of both planktonic and biofilm-grown C. albicans upon illumination. However, the concentrations of TBO in the receiver compartments separated from patches by membranes intended to mimic biofilm structures were an order of magnitude below those inducing high levels of kill, even after 6h release. Therefore, short application times of TBO-containing mucoadhesive patches should allow treatment of recently-acquired oropharyngeal candidiasis, caused solely by planktonic cells. Longer patch application times may be required for persistent disease where biofilms are implicated.

Anti-microbial photodynamic therapy: useful in the future?

Previous chapters in this volume have focused on fundamental principles and clinical applications of PDT. This chapter will attempt to outline emerging areas of research to identify some new applications that may become useful in the future in clinical practise. The worldwide rise in antibiotic resistance has driven research to the development of novel anti-microbial strategies. Cutaneous diseases caused by MRSA are ideally suited to treatment by anti-microbial photodynamic therapy for eradicating localized infections and for modulating wound healing due to the ability to deliver photosensitizer and light with topical application. The use of photosensitizer and light as an anti-microbial agent against periodontal microbial biofilms should also represent an attractive method of eliminating oral bacteria. Suitable light sources, laser light and non-coherent light will be briefly covered. This chapter will focus on some aspects of anti-microbial photodynamic therapy that appear to be promising for dermatological indications and inactivation of pathogenic bacteria within the oral cavity.

Protease-stable polycationic photosensitizer conjugates between polyethyleneimine and chlorin(e6) for broad-spectrum antimicrobial photoinactivation

We previously showed that covalent conjugates between poly-L-lysine and chlorin(e6) were efficient photosensitizers (PS) of both gram-positive and gram-negative bacteria. The polycationic molecular constructs increased binding and penetration of the PS into impermeable gram-negative cells. We have now prepared a novel set of second-generation polycationic conjugates between chlorin(e6) and three molecular forms of polyethyleneimine (PEI): a small linear, a small cross-linked, and a large cross-linked molecule. The conjugates were characterized by high-pressure liquid chromatography and tested for their ability to kill a panel of pathogenic microorganisms, the gram-positive Staphylococcus aureus and Streptococcus pyogenes, the gram-negative Escherichia coli and Pseudomonas aeruginosa, and the yeast Candida albicans, after exposure to low levels of red light. The large cross-linked molecule efficiently killed all organisms, while the linear conjugate killed gram-positive bacteria and C. albicans. The small cross-linked conjugate was the least efficient antimicrobial PS and its remarkably low activity could not be explained by reduced photochemical quantum yield or reduced cellular uptake. In contrast to polylysine conjugates, the PEI conjugates were resistant to degradation by proteases such as trypsin that hydrolyze lysine-lysine peptide bonds, The advantage of protease stability combined with the ready availability of PEI suggests these molecules may be superior to polylysine-PS conjugates for photodynamic therapy of localized infections.

Photosensitization of different Candida species by low power laser light

The aim of this study was to evaluate the effects of the laser radiation (685 nm) associated with photosensitizers on viability of different species of Candida genus. Suspensions of Candida albicans, Candida dubliniensis, Candida krusei and Candida tropicalis, containing 10(6) viable cells per milliliter were obtained with the aid of a Neubauer's chamber. From each species, 10 samples of the cell suspension were irradiated with diode laser (685 nm) with 28 J/cm2 in the presence of methylene blue (0.1 mg/ml), 10 samples were only treated with methylene blue, 10 samples were irradiated with laser in the absence of the dye, 10 samples were treated with the dye and irradiated with laser light and 10 samples were exposed to neither the laser light nor to the methylene blue dye. From each sample, serial dilutions of 10(-2) and 10(-3) were obtained and aliquots of 0.1 ml of each dilution were plated in duplicate on Sabouraud dextrose agar. After incubation at 37 degrees C for 48 h, the number of colony-forming units (CFU/ml) was obtained and data were submitted to ANOVA and Tukey's test (p<0.05). Laser radiation in the presence of methylene blue reduced the number of CFU/ml in 88.6% for C. albicans, 84.8% for C. dubliniensis, 91.6% for C. krusei and 82.3% for C. tropicalis. Despite this, only laser radiation or methylene blue did not reduce significantly the number of CFU/ml of Candida samples, except for C. tropicalis. It could be concluded that the photo activation of methylene blue by the red laser radiation at 685 nm presented fungicide effect on all Candida species studied.

Sensitivity of Candida albicans germ tubes and biofilms to photofrin-mediated phototoxicity

Treatment of mucocutaneous and cutaneous Candida albicans infections with photosensitizing agents and light, termed photodynamic therapy (PDT), offers an alternative to conventional treatments. Initial studies using the clinically approved photosensitizer Photofrin demonstrated the susceptibility of C. albicans to its photodynamic effects. In the present study, we have further refined parameters for Photofrin-mediated photodynamic action against C. albicans and examined whether mechanisms commonly used by microorganisms to subvert either antimicrobial oxidative defenses or antimicrobial therapy, including biofilm formation, were operative. In buffer and defined medium, germ tubes preloaded with Photofrin retained their photosensitivity for up to 2 hours, indicating the absence of degradation or export of Photofrin by the organism. The addition of serum resulted in a gradual loss of photosensitivity over 2 hours. In contrast to an adaptive response by germ tubes to oxidative stress by hydrogen peroxide, there was no adaptive response to singlet oxygen-mediated stress by photodynamic action. C. albicans biofilms were sensitive to Photofrin-mediated phototoxicity in a dose-dependent manner. Finally, the metabolic activity of C. albicans biofilms following photodynamic insult was significantly lower than that of biofilms treated with amphotericin B for the same time period. These results demonstrate that several of the mechanisms microorganisms use to subvert either antimicrobial oxidative defenses or antimicrobial therapy are apparently not operative during Photofrin-mediated photodynamic treatment of C. albicans. These observations provide support and rationale for the continued investigation of PDT as an adjunctive, or possibly alternative, mode of therapy against cutaneous and mucocutaneous candidiasis.

Effect of cell-photosensitizer binding and cell density on microbial photoinactivation

Photodynamic therapy involves the use of nontoxic dyes called photosensitizers and visible light to produce reactive oxygen species and cell killing. It is being studied as an alternative method of killing pathogens in localized infections due to the increasing problem of multiantibiotic resistance. Although much has been learned about the mechanisms of microbial killing, there is still uncertainty about whether dyes must bind to and penetrate various classes of microbe in order to produce effective killing after illumination. In this report, we compare the interactions of three antimicrobial photosensitizers: rose bengal (RB), toluidine blue O (TBO), and a poly-L-lysine chlorin(e6) conjugate (pL-ce6) with representative members of three classes of pathogens; Escherichia coli (gram-negative bacteria), Staphylococcus aureus (gram-positive bacteria), Candida albicans (yeast). We compared fluence-dependent cell survival after illumination with the appropriate wavelengths of light before and after extracellular dye had been washed out and used three 10-fold dilutions of cell concentration. pL-ce6 was overall the most powerful photosensitizer, was equally effective with and without washing, and showed a strong dependence on cell concentration. TBO was less effective in all cases after washing, and the dependence on cell concentration was less pronounced. RB was ineffective after washing (except for S. aureus) but still showed a dependence on cell concentration. The overall order of susceptibility was S. aureus>E. coli>C. albicans, but C. albicans cells were 10 to 50 times bigger than the bacteria. We conclude that the number and mass of the cells compete both for available dye binding and for extracellularly generated reactive oxygen species.

Mechanistic study of the photodynamic inactivation of Candida albicans by a cationic porphyrin

The growing resistance against antifungal agents has renewed the search for alternative treatment modalities, and antimicrobial photodynamic inactivation (PDI) is a potential candidate. The cationic porphyrin 5-phenyl-10,15,20-Tris(N-methyl-4-pyridyl)porphyrin chloride (TriP[4]) is a photosensitizer that in combination with light can inactivate bacteria, fungi, and viruses. For future improvement of the efficacy of PDI of clinically relevant fungi such as Candida albicans, we sought to understand the working mechanism by following the response of C. albicans exposed to PDI using fluorescence confocal microscopy and freeze-fracture electron microscopy. The following events were observed under dark conditions: TriP[4] binds to the cell envelope of C. albicans, and none or very little TriP[4] enters the cell. Upon illumination the cell membrane is damaged and eventually becomes permeable for TriP[4]. After lethal membrane damage, a massive influx of TriP[4] into the cell occurs. Only the vacuole membrane is resistant to PDI-induced damage once TriP[4] passes the plasma membrane. Increasing the incubation time of C. albicans with TriP[4] prior to illumination did not increase the influx of TriP[4] into the cell or the efficacy of PDI. After the replacement of 100% phosphate-buffered saline (PBS) by 10% PBS as the medium, C. albicans became permeable for TriP[4] during dark incubation and the efficacy of PDI increased dramatically. In conclusion, C. albicans can be successfully inactivated by the cationic porphyrin TriP[4], and the cytoplasmic membrane is the target organelle. TriP[4] influx occurred only after cell death.

Effect of albumin on the photodynamic inactivation of microorganisms by a cationic porphyrin

BACKGROUND

Photodynamic inactivation (PDI) employs visible light and a photosensitizer to inactivate cells. The technique is currently clinically used for the treatment of several malignancies. However, the PDI of microorganisms still remains in the research phase.

PURPOSE

To study the effect of human blood plasma and human serum albumin (HSA) on the PDI of Staphylococcus aureus, Pseudomonas aeruginosa and Candida albicans.

METHODS

PDI experiments were performed using white light (30 mW cm-2) and the cationic 5-phenyl-10,15,20-tris(N-methyl-4-pyridyl)porphyrin chloride (TriP[4]) as photosensitizer.

RESULTS

The microorganisms could be successfully photoinactivated by TriP[4] when suspended in phosphate buffered saline (PBS). In this medium, P. aeruginosa was the most resistant microorganism. Changing the suspending medium from PBS to human blood plasma reduced the PDI of all three microorganisms. In human blood plasma C. albicans was the most resistant microorganism. The same results were obtained with 4.5% and 7% HSA/PBS suspensions.

CONCLUSIONS

Albumin inhibits the PDI of S. aureus, P. aeruginosa and C. albicans in a dose dependent manner. However, our results are encouraging towards the potential future application of PDI for the treatment of superficial wound infections caused by S. aureus, P. aeruginosa and C. albicans.