Photodynamic inactivation of Gram-negative bacteria: problems and possible solutions

Photodynamic inactivation of bacteria using polyethylenimine-chlorin(e6) conjugates: Effect of polymer molecular weight, substitution ratio of chlorin(e6) and pH

BACKGROUND AND OBJECTIVES

Antimicrobial photodynamic therapy (APDT) is a novel technique to treat local infections. Previously we reported that the attachment of chlorin(e6) to polyethylenimine (PEI) polymers to form PEI-ce6 conjugates is an effective way to improve ce6 PDT activity against bacteria. The aim of this work was to explore how the polymer molecular weight, substitution ratio (SR) of ce6 and pH value affect the PDT efficacy.

STUDY DESIGN/MATERIALS AND METHODS

We have synthesized PEI-ce6(10) (MW = 60,000, SR = 1) and PEI-ce6(11) (MW = 60,000, SR = 5) and compared these with the previous PEI-ce6(9) (MW = 10,000, SR = 1). We tested the PDT efficacy of these three conjugates against Gram-negative E. coli and Gram-positive bacteria (S. aureus and E. fecalis) at three different pH values (5.0, 7.4, 10.0) that may affect the charge on both the bacterial cells and on the conjugate (that has both basic and acidic groups).

RESULTS

PEI-ce6(9) and PEI-ce6(10) were the most effective against these tested bacteria. The PDT effect of all three conjugates depended on pH values. The effective order was pH = 10.0 > pH = 7.4 > pH = 5.0 on E. coli. For S. aureus and E. fecalis the order was pH = 5.0 > pH = 10.0 > pH = 7.4. PEI-ce6(11) PDT activity was worse than PEI-ce6(10) activity which is probably connected to the fact that ce6 molecules are self-quenched within the PEI-ce6(11) molecule. Ce6 quenching within the PEI-ce6 molecules was proved by analyzing fluorescence spectra of PEI-ce6 conjugates at different pH values. There were no differences in bacterial uptake between different pH values in three PEI-ce6 conjugates.

CONCLUSION

We assume high pH (rather than low pH as was hypothesized) disaggregates the conjugates, so the higher pH was more effective than the lower pH against E. coli. But for Gram-positive bacteria, low pH was more effective possibly due to more overall positive charge on the conjugate.

Effects of metal and the phytyl chain on chlorophyll derivatives: physicochemical evaluation for photodynamic inactivation of microorganisms

Chlorophyll compounds and their derivatives containing metal or phytyl chain can be used as photosensitizer in photodynamic inactivation of microorganisms (PDI). So, the physicochemical properties and antimicrobial effect of chlorophyll derivatives were investigated: Mg-chlorophyll (Mg-Chl), Zn-chlorophyll (Zn-Chl), Zn-chlorophyllide (Zn-Chlde), Cu-chlorophyll (Cu-Chl), pheophytin (Pheo) and pheophorbide (Pheid). The photobleaching experiments showed photostability according to Cu-Chl > Pheo ∼ Pheid ≫ Zn-Chl ∼ Zn-Chlde > Mg-Chl. This order was discussed in terms of metal and the phytyl chain presences. Pheid and Zn-Chl in aqueous Tween 80 solution exhibited highest singlet oxygen yield compared with the other derivatives. Chlorophyll derivatives (CD) with phytyl chain was limited by the self-aggregation phenomenon at high concentrations, even in micellar systems (Tween 80 and P-123). The antimicrobial effect of CD derivatives was investigated against Staphylococcus aureus, Escherichia coli, Candida albicans and Artemia salina. Pheid showed the best results against all organisms tested, Zn-Chlde was an excellent bactericide in the dark and Cu-Chl had no PDI effect. No correlation with CD uptake by microorganisms and darkness cytotoxicity was found. The physicochemical properties allied to bioassays results indicate that Mg-Chl, Pheo, Zn-Chl and Pheid are good candidates for PDI.

Chitosan augments photodynamic inactivation of gram-positive and gram-negative bacteria

Antimicrobial photodynamic inactivation (PDI) was shown to be a promising treatment modality for microbial infections. This study explores the effect of chitosan, a polycationic biopolymer, in increasing the PDI efficacy against Gram-positive bacteria, including Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pyogenes, and methicillin-resistant S. aureus (MRSA), as well as the Gram-negative bacteria Pseudomonas aeruginosa and Acinetobacter baumannii. Chitosan at <0.1% was included in the antibacterial process either by coincubation with hematoporphyrin (Hp) and subjection to light exposure to induce the PDI effect or by addition after PDI and further incubation for 30 min. Under conditions in which Hp-PDI killed the microbe on a 2- to 4-log scale, treatment with chitosan at concentrations of as low as 0.025% for a further 30 min completely eradicated the bacteria (which were originally at ∼10(8) CFU/ml). Similar results were also found with toluidine blue O (TBO)-mediated PDI in planktonic and biofilm cells. However, without PDI treatment, chitosan alone did not exert significant antimicrobial activity with 30 min of incubation, suggesting that the potentiated effect of chitosan worked after the bacterial damage induced by PDI. Further studies indicated that the potentiated PDI effect of chitosan was related to the level of PDI damage and the deacetylation level of the chitosan. These results indicate that the combination of PDI and chitosan is quite promising for eradicating microbial infections.

Cationic porphycenes as potential photosensitizers for antimicrobial photodynamic therapy

Structures of typical photosensitizers used in antimicrobial photodynamic therapy are based on porphyrins, phthalocyanines, and phenothiazinium salts, with cationic charges at physiological pH values. However, derivatives of the porphycene macrocycle (a structural isomer of porphyrin) have barely been investigated as antimicrobial agents. Therefore, we report the synthesis of the first tricationic water-soluble porphycene and its basic photochemical properties. We successfully tested it for in vitro photoinactivation of different Gram-positive and Gram-negative bacteria, as well as a fungal species (Candida) in a drug-dose and light-dose dependent manner. We also used the cationic porphycene in vivo to treat an infection model comprising mouse third degree burns infected with a bioluminescent methicillin-resistant Staphylococcus aureus strain. There was a 2.6-log(10) reduction (p < 0.001) of the bacterial bioluminescence for the PDT-treated group after irradiation with 180 J·cm(-2) of red light.

Photodynamic antimicrobial chemotherapy by liposome-encapsulated water-soluble photosensitizers

Photodynamic antimicrobial chemotherapy is an alternative method for killing bacterial cells in view of the increasing problem of multi-antibiotic resistance. We examined the effect of three water-soluble photosensitizers (PhS): methylene blue (MB), neutral red (NR) and rose bengal (RB) on Gram-positive and Gram-negative bacteria. We compared the efficacy of PhS in their free form and encapsulated in liposomal formulations against various bacterial strains, and determined conditions for the effective use of encapsulated PhS. We found that all three PhS were able to eradicate the Gram-positive microbes Staphylococcus aureus and Sarcina lutea; and MB and RB were effective against St. epidermidis. In the case of the Gram-negative species, MB and RB were cytotoxic against the Shigella flexneri, NR-inactivated Escherichia coli and Salmonella para B, and BR was effective in killing Pseudomonas aeruginosa. None of the examined PhS showed activity against Klebsiella pneumoniae. MB and NR enclosed in liposomes gave a stronger antimicrobial effect than free PhS for all tested prokaryotes, whereas encapsulation of RB led to no increase in its activity. We suggest that encapsulation of PhS can increase the photoinactivation of bacteria.

Inactivation of food-borne spoilage and pathogenic micro-organisms on the surface of a photoactive polymer

The photodynamic action of a novel photoactive polymer comprising covalently bound anthraquinone (AQ) moieties was evaluated after developing a methodology to reliably immobilize viable micro-organisms onto polymer film surfaces. The survival of Escherichia coli, Bacillus cereus (vegetative cells and spores), Fusarium oxysporum and Saccharomyces cerevisiae microbes inoculated on the surface of inert polymeric substrates was assessed to determine the effect of inoculum composition, drying rate and exposure to ultraviolet (UV-A) radiation. Their survival was highly dependent on microbial genus, with E. coli consistently displaying markedly shorter survival times than the other microbes, and B. cereus spores being the most resistant. Inoculation of the microbes onto the surface of the photoactive polymer films, followed by exposure to UV-A radiation, dramatically accelerated the inactivation of all microbial types studied compared with their survival on the surface of inert polymer substrates. Simultaneous exposure to both oxygen and UV-A radiation is required to affect cell survival, which is consistent with this effect most likely originating from the photoinduced production of singlet oxygen by the photoactive polymer. These results provide further compelling evidence that singlet oxygen produced exogenously by this photoactive polymeric substrate can successfully inactivate a broad spectrum of microbes on the substrate's surface.

Stable synthetic cationic bacteriochlorins as selective antimicrobial photosensitizers

Photodynamic inactivation is a rapidly developing antimicrobial treatment that employs a nontoxic photoactivatable dye or photosensitizer in combination with harmless visible light to generate reactive oxygen species that are toxic to cells. Tetrapyrroles (e.g., porphyrins, chlorins, bacteriochlorins) are a class of photosensitizers that exhibit promising characteristics to serve as broad-spectrum antimicrobials. In order to bind to and efficiently penetrate into all classes of microbial cells, tetrapyrroles should have structures that contain (i) one or more cationic charge(s) or (ii) a basic group. In this report, we investigate the use of new stable synthetic bacteriochlorins that have a strong absorption band in the range 720 to 740 nm, which is in the near-infrared spectral region. Four bacteriochlorins with 2, 4, or 6 quaternized ammonium groups or 2 basic amine groups were compared for light-mediated killing against a gram-positive bacterium (Staphylococcus aureus), a gram-negative bacterium (Escherichia coli), and a dimorphic fungal yeast (Candida albicans). Selectivity was assessed by determining phototoxicity against human HeLa cancer cells under the same conditions. All four compounds were highly active (6 logs of killing at 1 microM or less) against S. aureus and showed selectivity for bacteria over human cells. Increasing the cationic charge increased activity against E. coli. Only the compound with basic groups was highly active against C. albicans. Supporting photochemical and theoretical characterization studies indicate that (i) the four bacteriochlorins have comparable photophysical features in homogeneous solution and (ii) the anticipated redox characteristics do not correlate with cell-killing ability. These results support the interpretation that the disparate biological activities observed stem from cellular binding and localization effects rather than intrinsic electronic properties. These findings further establish cationic bacteriochlorins as extremely active and selective near-infrared activated antimicrobial photosensitizers, and the results provide fundamental information on structure-activity relationships for antimicrobial photosensitizers.

Quenching of singlet oxygen by pyocyanin and related phenazines

Pseudomonas aeruginosa is a human pathogen, which causes infections of various organs, including lung, skin and eye, particularly in individuals who are immunocompromised. Pyocyanin (1-hydroxy-5-methylphenazine), a cytotoxic pigment secreted by the bacterium, is among the factors that contribute to virulence of this pathogen. We have previously shown that rose bengal and riboflavin photosensitize oxidation of pyocyanin to a product(s) with diminished reactivity and toxicity. Singlet oxygen was suggested as the major oxidant, based on the inhibitory effect of sodium azide. In the present study, we used the time resolved technique to investigate direct interaction of pyocyanin and related phenazines (1-hydroxyphenazine [1-OH-Phen], 1-methoxy-5-methylphenazine [1-MeO-PCN] and phenazine methosulfate [PMS]) with (1)O(2). The rate constants for the (1)O(2) quenching (physical + chemical) by pyocyanin and 1-OH-Phen in D(2)O buffer (pD approximately 7.2) have been determined to be 4.8 x 10(8) and 6.8 x 10(8) M(-1) s(-1), respectively. 1-MeO-PCN and PMS were markedly less efficient (1)O(2) quenchers. Among the phenazines studied only phenazine methosulfate photogenerated (1)O(2) (Phi((1)O(2)) = 0.56 in acetonitrile). Interaction of (1)O(2) with pyocyanin and other related phenazines produced by the bacteria may be important in determining the potential utility of photochemical/pharmacological approaches to eradicate P. aeruginosa from infected tissues.

Porphyrin-apidaecin conjugate as a new broad spectrum antibacterial agent

The conjugation of the cationic antimicrobial peptide, apidaecin Ib, to the anionic photosensitizer, 5(4'-carboxyphenyl)-10,15,20-triphenylporphyrin (cTPP), afforded a new antibacterial agent effective, under light activation, against both Gram-positive and Gram-negative bacteria. At low concentrations (1.5-15 μM) the conjugate was able to reduce the survival of Escherichia coli cells by 3-4 log10, and most notably, it resulted photoactive also against hard-to-treat Pseudomonas aeruginosa, although at higher concentration (60 μM). Under similar conditions, the photosensitizer alone was only photoactive against Staphylococcus aureus while the unconjugated peptide was inactive against all the bacterial strains tested. This study shows the possibility of obtaining new broad-spectrum apidaecin-photosensitizer conjugates with potent antibacterial activity.

In vitro and in vivo photodynamic therapy of otitis media in gerbils

OBJECTIVES/HYPOTHESIS

The aim of this study was to evaluate the antibacterial effects of photodynamic therapy (PDT) on common bacteria causing otitis media with effusion (OME).

METHODS

An in vitro study was carried out using a hematoporphyrin derivative sensitizer (Photogem; Lemonosov Institute of Fine Chemical, Moscow, Russia) and a 632-nm diode laser on Haemophilus influenzae, Moraxella catarrhalis, and Streptococcus pneumoniae. The presence of colony-forming units of the bacteria was examined, the microscopic structures of the bacteria were examined by transmission electron microscopy (TEM), and flow cytometry of the bacteria was performed. An in vivo PDT study was performed using gerbils. S. pneumoniae or H. influenzae were injected into bullae. The Photogem was injected into the bullae 2 days later when OME developed, and transcanal irradiation with the 632-nm diode laser (90 J) was performed. Middle ear and bulla were washed with Dulbecco's phosphate buffered saline (DPBS) and the washed DPBS was cultured. The presence of bacterial colonies was examined.

RESULTS

The PDT was effective in killing all three kinds of bacteria. TEM showed damaged bacterial cell membranes and cytoplasmic structures, and the flow cytometry showed a lower number of viable bacteria in the PDT group compared to the control group. PDT was effective in killing S. pneumoniae in 87% of the infected bullae with OME, whereas it was effective in eradicating H. influenzae in 50% of the infected bullae with OME.

CONCLUSIONS

The results of these studies demonstrated that PDT may be effective to treat otitis media. PDT may have clinical implications in the treatment of otitis media that is resistant to antibiotic therapy.

Antimicrobial photodynamic inactivation and photodynamic therapy for infections

Photodynamic therapy (PDT) was initially discovered over 100 years ago by its ability to kill microorganisms, but its use to treat infections clinically has not been much developed. However, the present relentless increase in antibiotic resistance worldwide and the emergence of strains that are resistant to all known antibiotics has stimulated research into novel antimicrobial strategies such as PDT that are thought to be unlikely to lead to the development of resistance. In this chapter we will cover the use of PDT to kill pathogenic microbial cells in vitro and describe a mouse model of localized infection and its treatment by PDT without causing excessive damage to the host tissue.

Innovative cationic fullerenes as broad-spectrum light-activated antimicrobials

Photodynamic inactivation is a rapidly developing antimicrobial technology that combines a nontoxic photoactivatable dye or photosensitizer in combination with harmless visible light of the correct wavelength to excite the dye to its reactive-triplet state that will then generate reactive oxygen species that are highly toxic to cells. Buckminsterfullerenes are closed-cage molecules entirely composed of sp2-hybridized carbon atoms, and although their main absorption is in the UV, they also absorb visible light and have a long-lived triplet state. When C(60) fullerene is derivatized with cationic functional groups it forms molecules that are more water-soluble and can mediate photodynamic therapy efficiently upon illumination; moreover, cationic fullerenes can selectively bind to microbial cells. In this report we describe the synthesis and characterization of several new cationic fullerenes. Their relative effectiveness as broad-spectrum antimicrobial photosensitizers against gram-positive and gram-negative bacteria, and a fungal yeast was determined by quantitative structure-function relationships.

FROM THE CLINICAL EDITOR

Photodynamic inactivation (PDI) is a rapidly developing antimicrobial technology in which a non-toxic photoactivatable dye or photosensitizer is excited with harmless visible light to its reactive state, where it will generate highly toxic reactive oxygen species. Buckminsterfullerenes derivatized with cationic functional groups form molecules that are water-soluble and mediate PDI efficiently. These fullerenes can also selectively bind to microbial cells. Several new cationic fullerenes are presented in this paper, and their efficacy against Gram-positive, Gram-negative bacteria, and a fungal yeast is also demonstrated.

Highly efficient in vitro photodynamic inactivation of Mycobacterium smegmatis

OBJECTIVES

Efforts to control tuberculosis (TB) have been hampered by the emergence of multiple-drug resistant strains, necessitating pursuit of alternative approaches to the current antibiotic-based treatments. Herein, we explore the feasibility of photodynamic inactivation (PDI) of mycobacteria.

METHODS

In vitro PDI studies employing Mycobacterium smegmatis as a surrogate for Mycobacterium tuberculosis were performed examining photosensitizer (PS) type, concentration and light dose. M. smegmatis was grown to a concentration of 10(8) colony forming units (cfu) per mL, resuspended in PBS-0.5% Tween-80-containing buffer, incubated with the PS for 5 min and subsequently illuminated with white light (400-700 nm) at a fluence rate of 60 mW/cm(2) for 1, 5, 15 or 30 min (equivalent to 3.4, 18, 54 or 108 J/cm(2)). The percentage survival was determined by the ratio of the colony count from illuminated and non-illuminated control cell suspensions. The PSs examined were 5,10,15,20-tetrakis(1-methyl-4-pyridinyl)porphyrin tetratosylate (TMPyP), 5,10,15,20-tetrakis(4-N,N,N-trimethylanilinium)porphyrin tetrachloride (TNMAP), methylene blue (MB), 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrin (TSPP), 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin-Pd(II) (TCPP-Pd) and phthalocyanine tetrasulphonic acid (PhCS).

RESULTS

Our best results demonstrate that PDI of M. smegmatis can achieve a noteworthy 5-6 log unit reduction in cfu (99.999% + viable cell eradication) when cationic PSs are employed in the nanomolar concentration range. Anionic PSs did not effectively mediate PDI of mycobacteria due to their inability to associate with the negatively charged mycobacterial cell membrane.

CONCLUSIONS

PDI of M. smegmatis was found to be highly efficient in reducing the number of viable cells in vitro when cationic PSs were employed.

Phage therapy and photodynamic therapy: low environmental impact approaches to inactivate microorganisms in fish farming plants

Owing to the increasing importance of aquaculture to compensate for the progressive worldwide reduction of natural fish and to the fact that several fish farming plants often suffer from heavy financial losses due to the development of infections caused by microbial pathogens, including multidrug resistant bacteria, more environmentally-friendly strategies to control fish infections are urgently needed to make the aquaculture industry more sustainable. The aim of this review is to briefly present the typical fish farming diseases and their threats and discuss the present state of chemotherapy to inactivate microorganisms in fish farming plants as well as to examine the new environmentally friendly approaches to control fish infection namely phage therapy and photodynamic antimicrobial therapy.

Photodynamic therapy for Acinetobacter baumannii burn infections in mice

Multidrug-resistant Acinetobacter baumannii infections represent a growing problem, especially in traumatic wounds and burns suffered by military personnel injured in Middle Eastern conflicts. Effective treatment with traditional antibiotics can be extremely difficult, and new antimicrobial approaches are being investigated. One of these alternatives to antimicrobials could be the combination of nontoxic photosensitizers (PSs) and visible light, known as photodynamic therapy (PDT). We report on the establishment of a new mouse model of full-thickness thermal burns infected with a bioluminescent derivative of a clinical Iraqi isolate of A. baumannii and its PDT treatment by topical application of a PS produced by the covalent conjugation of chlorin(e6) to polyethylenimine, followed by illumination of the burn surface with red light. Application of 10(8) A. baumannii cells to the surface of 10-s burns made on the dorsal surface of shaved female BALB/c mice led to chronic infections that lasted, on average, 22 days and that were characterized by a remarkably stable bacterial bioluminescence. PDT carried out on day 0 soon after application of the bacteria gave over 3 log units of loss of bacterial luminescence in a light exposure-dependent manner, while PDT carried out on day 1 and day 2 gave an approximately 1.7-log reduction. The application of PS dissolved in 10% or 20% dimethyl sulfoxide without light gave only a modest reduction in the bacterial luminescence from mouse burns. Some bacterial regrowth in the treated burn was observed but was generally modest. It was also found that PDT did not lead to the inhibition of wound healing. The data suggest that PDT may be an effective new treatment for multidrug-resistant localized A. baumannii infections.

Charge effect on the photoinactivation of Gram-negative and Gram-positive bacteria by cationic meso-substituted porphyrins

BACKGROUND

In recent times photodynamic antimicrobial therapy has been used to efficiently destroy Gram (+) and Gram (-) bacteria using cationic porphyrins as photosensitizers. There is an increasing interest in this approach, namely in the search of photosensitizers with adequate structural features for an efficient photoinactivation process. In this study we propose to compare the efficiency of seven cationic porphyrins differing in meso-substituent groups, charge number and charge distribution, on the photodynamic inactivation of a Gram (+) bacterium (Enterococcus faecalis) and of a Gram (-) bacterium (Escherichia coli). The present study complements our previous work on the search for photosensitizers that might be considered good candidates for the photoinactivation of a large spectrum of environmental microorganisms.

RESULTS

Bacterial suspension (10(7) CFU mL(-1)) treated with different photosensitizers concentrations (0.5, 1.0 and 5.0 microM) were exposed to white light (40 W m(-2)) for a total light dose of 64.8 J cm(-2). The most effective photosensitizers against both bacterial strains were the Tri-Py+-Me-PF and Tri-Py+-Me-CO2Me at 5.0 microM with a light fluence of 64.8 J cm(-2), leading to > 7.0 log (> 99,999%) of photoinactivation. The tetracationic porphyrin also proved to be a good photosensitizer against both bacterial strains. Both di-cationic and the monocationic porphyrins were the least effective ones.

CONCLUSION

The number of positive charges, the charge distribution in the porphyrins' structure and the meso-substituent groups seem to have different effects on the photoinactivation of both bacteria. As the Tri-Py+-Me-PF porphyrin provides the highest log reduction using lower light doses, this photosensitizer can efficiently photoinactivate a large spectrum of environmental bacteria. The complete inactivation of both bacterial strains with low light fluence (40 W m(-2)) means that the photodynamic approach can be applied to wastewater treatment under natural light conditions which makes this technology cheap and feasible in terms of the light source.

Uptake pathways of anionic and cationic photosensitizers into bacteria

The effect of divalent cations (calcium and magnesium) and a permeabilizing agent (EDTA) on the uptake of a cationic photosensitizer (PS), methylene blue (MB), and two anionic PSs, rose bengal (RB) and indocyanine green (ICG), by Gram-positive Enterococcus faecalis and Gram-negative Actinobacillus actinomycetemcomitans was examined. The possible roles of multidrug efflux pumps and protein transporters in photosensitizer uptake were assessed in E. faecalis cells by studies using an efflux pump inhibitor (verapamil) and trypsin treatment respectively. Divalent cations enhanced the uptake and photodynamic inactivation potential of both RB and ICG in E. faecalis and A. actinomycetemcomitans, while they decreased the uptake and bacterial killing by MB. Verapamil increased the uptake of RB (possibly due to efflux pump inhibition), whereas trypsin treatment resulted in significant decrease in RB and ICG uptake. The results suggested that the uptake of anionic PSs by bacterial cells may be mediated through a combination of electrostatic charge interaction and by protein transporters, while the uptake of cationic PSs, as previously reported, is mediated by electrostatic interactions and self promoted uptake pathways.

Lethal photosensitization of wound-associated microbes using indocyanine green and near-infrared light

Background

The increase in resistance to antibiotics among disease-causing bacteria necessitates the development of alternative antimicrobial approaches such as the use of light-activated antimicrobial agents (LAAAs). Light of an appropriate wavelength activates the LAAA to produce cytotoxic species which can then cause bacterial cell death via loss of membrane integrity, lipid peroxidation, the inactivation of essential enzymes, and/or exertion of mutagenic effects due to DNA modification. In this study, the effect of the LAAA indocyanine green excited with high or low intensity light (808 nm) from a near-infrared laser (NIR) on the viability of Staphylococcus aureus, Streptococcus pyogenes and Pseudomonas aeruginosa was investigated.

Results

All species were susceptible to killing by the LAAA, the bactericidal effect being dependent on both the concentration of indocyanine green and the light dose. Indocyanine green photosensitization using both high (1.37 W cm^-2) and low (0.048 W cm^-2) intensity NIR laser light was able to achieve reductions of 5.6 log₁₀ (>99.99%) and 6.8 log₁₀ (>99.99%) in the viable counts of Staph. aureus and Strep. pyogenes (using starting concentrations of 10⁶–10⁷ CFU ml^-1). Kills of 99.99% were obtained for P. aeruginosa (initial concentration 10⁸–10⁹ CFU ml^-1) photosensitized by the high intensity light (1.37 W cm^-2); while a kill of 80% was achieved using low intensity irradiation (0.07 W cm^-2). The effects of L-tryptophan (a singlet oxygen scavenger) and deuterium oxide (as an enhancer of the life span of singlet oxygen) on the survival of Staph. aureus was also studied. L-tryptophan reduced the proportion of Staph. aureus killed; whereas deuterium oxide increased the proportion killed suggesting that singlet oxygen was involved in the killing of the bacteria.

Conclusion

These findings imply that indocyanine green in combination with light from a near-infrared laser may be an effective means of eradicating bacteria from wounds and burns.

Inhibitors of bacterial multidrug efflux pumps potentiate antimicrobial photoinactivation

Antimicrobial photodynamic inactivation (APDI) combines a nontoxic photoactivatable dye or photosensitizer (PS) with harmless visible light to generate singlet oxygen and reactive oxygen species that kill microbial cells. Cationic phenothiazinium dyes, such as toluidine blue O (TBO), are the only PS used clinically for APDI, and we recently reported that this class of PS are substrates of multidrug efflux pumps in both gram-positive and gram-negative bacteria. We now report that APDI can be significantly potentiated by combining the PS with an efflux pump inhibitor (EPI). Killing of Staphylococcus aureus mediated by TBO and red light is greatly increased by coincubation with known inhibitors of the major facilitator pump (NorA): the diphenyl urea INF271, reserpine, 5'-methoxyhydnocarpin, and the polyacylated neohesperidoside, ADH7. The potentiation effect is greatest in the case of S. aureus mutants that overexpress NorA and least in NorA null cells. Addition of the EPI before TBO has a bigger effect than addition of the EPI after TBO. Cellular uptake of TBO is increased by EPI. EPI increased photodynamic inactivation killing mediated by other phenothiazinium dyes, such as methylene blue and dimethylmethylene blue, but not that mediated by nonphenothiazinium PS, such as Rose Bengal and benzoporphyrin derivative. Killing of Pseudomonas aeruginosa mediated by TBO and light was also potentiated by the resistance nodulation division pump (MexAB-OprM) inhibitor phenylalanine-arginine beta-naphthylamide but to a lesser extent than for S. aureus. These data suggest that EPI could be used in combination with phenothiazinium salts and light to enhance their antimicrobial effect against localized infections.

Specific-wavelength visible light irradiation inhibits bacterial growth of Porphyromonas gingivalis

BACKGROUND AND OBJECTIVE

The effects of laser irradiation on Porphyromonas gingivalis have been reported, but the results are still controversial regarding the efficiency because of the differences of the light sources and irradiation conditions. The aim of this study was to determine the wavelength and irradiation conditions under which the most effective inhibitory effect on P. gingivalis growth was seen without any photosensitizers.

MATERIAL AND METHODS

Using an Okazaki large spectrograph, monochromatic light spectra ranging from 400 to 700 nm were evaluated to determine which spectra effectively inhibited bacterial growth. Moreover, using a monochromatic 405-nm irradiating device, the effects of various irradiating conditions on P. gingivalis growth were examined.

RESULTS

Growth of bacteria irradiated at 400 nm and 410 nm was significantly suppressed compared with a nonirradiated control, whereas wavelengths of 430 nm and longer produced no significant inhibition. A constant energy density of 15 J/cm2 was found to be enough to show an inhibitory effect. Significant inhibition of bacterial growth was found after only 1 min at 50 mW/cm2 irradiation.

CONCLUSION

These results indicate that P. gingivalis growth is specifically suppressed by 405-nm light irradiation, suggesting that visible blue light irradiation is a promising means for eradicating periodontopathogenic bacteria from periodontal lesions.

Effects on gram-negative and gram-positive bacteria mediated by 5-aminolevulinic Acid and 5-aminolevulinic acid derivatives

Due mainly to the extensive use of antibiotics, the spread of multiresistant bacterial strains is one of the most worrying threats to public health. One strategy that can be used to overcome potential shortcomings might be the inactivation of these microorganisms by 5-aminolevulinic acid (5-ALA) or 5-ALA derivative-mediated photodynamic therapy (PDT). 5-ALA has no photoactive properties, but when it is given exogenously, it acts as a precursor of photosensitive porphyrins predominantly in tissues or organisms that are characterized by a high metabolic turnover, such as tumors, macrophages, and bacteria. However, the weak ability of 5-ALA to cross biological barriers has led to the introduction of more lipophilic derivatives, such as methyl aminolevulinate or hexyl aminolevulinate, which display improved capacities to reach the cytoplasm. Starting from the hypothesis that more lipophilic compounds carrying only a permanent positive charge under physiological conditions may more easily cross the bacterial multilayer barrier, we have tested the efficacies of some 5-ALA n-alkyl esters for the inactivation of bacteria. For this purpose, different bacterial strains were incubated with 5-ALA or its corresponding esters of different lipophilicities. Then, the bacteria were irradiated with light and the numbers of CFU post-PDT were counted and compared to those for the controls, which were kept in the dark. Furthermore, the total amount of accumulated porphyrins was quantified by high-pressure liquid chromatography analysis. In our studies, analysis of the bacterial extracts revealed the presence of all the porphyrins involved in heme biosynthesis, from uroporphyrin to protoporphyin IX. The efficacy of bacterial inactivation was a function of the total amount of porphyrins produced, independently of their nature. The 5-ALA methyl and butyl esters were the most effective compounds with respect to the photodynamic inactivation of bacteria. We observed significant differences in terms of the optimal drug concentration, bactericidal activities, and porphyrin production.

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.

The role of singlet oxygen and oxygen concentration in photodynamic inactivation of bacteria

New antibacterial strategies are required in view of the increasing resistance of bacteria to antibiotics. One promising technique involves the photodynamic inactivation of bacteria. Upon exposure to light, a photosensitizer in bacteria can generate singlet oxygen, which oxidizes proteins or lipids, leading to bacteria death. To elucidate the oxidative processes that occur during killing of bacteria, Staphylococcus aureus was incubated with a standard photosensitizer, and the generation and decay of singlet oxygen was detected directly by its luminescence at 1,270 nm. At low bacterial concentrations, the time-resolved luminescence of singlet oxygen showed a decay time of 6 +/- 2 micros, which is an intermediate time for singlet oxygen decay in phospholipids of membranes (14 +/- 2 micros) and in the surrounding water (3.5 +/- 0.5 micros). Obviously, at low bacterial concentrations, singlet oxygen had sufficient access to water outside of S. aureus by diffusion. Thus, singlet oxygen seems to be generated in the outer cell wall areas or in adjacent cytoplasmic membranes of S. aureus. In addition, the detection of singlet oxygen luminescence can be used as a sensor of intracellular oxygen concentration. When singlet oxygen luminescence was measured at higher bacterial concentrations, the decay time increased significantly, up to approximately 40 micros, because of oxygen depletion at these concentrations. This observation is an important indicator that oxygen supply is a crucial factor in the efficacy of photodynamic inactivation of bacteria, and will be of particular significance should this approach be used against multiresistant bacteria.

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.

Photodynamic Therapy for Endodontic Disinfection

The aims of this study were to investigate the effects of photodynamic therapy (PDT) on endodontic pathogens in planktonic phase as well as on Enterococcus faecalis biofilms in experimentally infected root canals of extracted teeth. Strains of microorganisms were sensitized with methylene blue (25 microg/ml) for 5 minutes followed by exposure to red light of 665 nm with an energy fluence of 30 J/cm2. Methylene blue fully eliminated all bacterial species with the exception of E. faecalis (53% killing). The same concentration of methylene blue in combination with red light (222 J/cm2) was able to eliminate 97% of E. faecalis biofilm bacteria in root canals using an optical fiber with multiple cylindrical diffusers that uniformly distributed light at 360 degrees. We conclude that PDT may be developed as an adjunctive procedure to kill residual bacteria in the root canal system after standard endodontic treatment.

Synthesis and properties of benzo[a]phenoxazinium chalcogen analogues as novel broad-spectrum antimicrobial photosensitizers

The goal of this investigation was to develop improved photosensitizers for use as antimicrobial drugs in photodynamic therapy of localized infections. Replacement of the oxygen atom in 5-(ethylamino)-9-diethylaminobenzo[a]phenoxazinium chloride (1) with sulfur and selenium afforded thiazinium and selenazinium analogues 2 and 3, respectively. All three dyes are water soluble, lipophilic, and red light absorbers. The relative photodynamic activities of the chalcogen series were evaluated against a panel of prototypical pathogenic microorganisms: the Gram-positive Enterococcus faecalis, the Gram-negative Escherichia coli, and the fungus Candida albicans. Selenium dye 3 was highly effective as a broad-spectrum antimicrobial photosensitizer with fluences of 4-32 J/cm2 killing 2-5 more logs of all cell types than sulfur dye 2, which was slightly more effective than oxygen analogue 1. These data, taken with the findings of uptake and retention studies, suggest that the superior activity of selenium derivative 3 can be attributed to its much higher triplet quantum yield.

Photosensitized oxidation and inactivation of pyocyanin, a virulence factor of Pseudomonas aeruginosa

Pyocyanin (PyO-) (1-hydroxy-5-methylphenazine) is a cytotoxic compound secreted by Pseudomonas aeruginosa, an omnipresent bacterium and a human pathogen. We report that visible light illumination in the presence of rose bengal, or riboflavin, in aerated solutions (pH 7.0-7.2) induces irreversible loss of the pigment's characteristic absorption band at 690 nm, indicating its oxidation. This photobleaching was paralleled by generation of a multiline Electron Paramagnetic Resonance (EPR) spectrum attributed to a PyO(-)-derived radical. The reaction was dependent on the presence of air, sensitizers and light, was inhibited by sodium azide and was unaffected by ethanol. This suggests that PyO- was oxidized largely via singlet oxygen and that hydroxyl radicals were not involved. The photochemically modified pigment was less efficient in oxidizing NAD(P)H and generated less superoxide (by approximately 50%) than the intact PyO-, indicating its partial inactivation. 1-Methoxy-5-methylphenazine, a PyO- analog in which the -O- moiety was replaced by the methoxy group (-OMe), was resistant to oxidation, suggesting that oxidation of PyO- involves its phenolate moiety. These results also suggest that photosensitization could be a potentially useful method for inactivation of PyO- and, possibly, detoxification of superficial wounds (skin, eye) infected with P. aeruginosa.

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.

Antibacterial activity of tetraaryl-porphyrin photosensitizers: an in vitro study on Gram negative and Gram positive bacteria

BACKGROUND

Photodynamic therapy exploits visible light and photosensitizers to inactivate cells and this methodology is currently used for the treatment of several types of malignancy. Although various tumours are successfully treated with PSs and light, the application on microorganisms (photodynamic antimicrobial chemotherapy) has not yet found specific medical applications and still remains an open field of fundamental research.

PURPOSE

The assessment of the effect of a panel of seven tetraaryl-porphyrins, two commercial (PS 1 and 2) and five synthetic (PS 3-7) in in vitro experiments against Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus.

METHODS

Three of the new photosensitizers (PS 3, 4 and 5) are tetracationic porphyrins and were prepared by N-alkylation of 5,10,15,20-tetra-4-pyridylporphyrin with a large excess of different benzyl chlorides; compound 7 is a dicationic porphyrin and was obtained in a similar way using a lower excess of 4-methoxybenzyl chloride. The neutral porphyrin (PS 6) was previously described. Dose-response curves were obtained titrating the survivors of cell suspensions (10(8)cfu/ml) exposed to the PSs and irradiated with visible light (total fluence rate 266 J/cm2).

RESULTS

The non ionic porphyrin 6 was the least active PS against all the tested bacteria. Cationic PSs 3, 4, 5 and 7 were more active than the commercial 1 and 2. The Gram positive S. aureus was more sensitive to all the PSs than the Gram negative E. coli and P. aeruginosa, the latter being the more resistant one. Compound 7 was found particularly efficient against P. aeruginosa, causing a 7 log units reduction of survivors at a concentration of 8 microM.

CONCLUSIONS

The reported results confirm that the presence of positively charged groups on porphyrin frame is fundamental for PSs antibacterial activity, however our data suggest that a moderate degree of lipophilicity, achievable by the introduction of aromatic hydrocarbon side chains on the pyridyl moieties, may improve PSs efficiency. Furthermore dicationic porphyrin 7 seems to be more efficient than the corresponding tetracationic derivatives thus emphasizing an interesting feature involved in the PSs activity.

Phenothiazinium antimicrobial photosensitizers are substrates of bacterial multidrug resistance pumps

Antimicrobial photodynamic therapy (PDT) combines a nontoxic photoactivatable dye, or photosensitizer (PS), with harmless visible light to generate singlet oxygen and free radicals that kill microbial cells. Although the light can be focused on the diseased area, the best selectivity is achieved by choosing a PS that binds and penetrates microbial cells. Cationic phenothiazinium dyes, such as methylene blue and toluidine blue O, have been studied for many years and are the only PSs used clinically for antimicrobial PDT. Multidrug resistance pumps (MDRs) are membrane-localized proteins that pump drugs out of cells and have been identified for a wide range of organisms. We asked whether phenothiazinium salts with structures that are amphipathic cations could potentially be substrates of MDRs. We used MDR-deficient mutants of Staphylococcus aureus (NorA), Escherichia coli (TolC), and Pseudomonas aeruginosa (MexAB) and found 2 to 4 logs more killing than seen with wild-type strains by use of three different phenothiazinium PSs and red light. Mutants that overexpress MDRs were protected from killing compared to the wild type. Effective antimicrobial PSs of different chemical structures showed no difference in light-mediated killing depending on MDR phenotype. Differences in uptake of phenothiazinium PS by the cells depending on level of MDR expression were found. We propose that specific MDR inhibitors could be used in combination with phenothiazinium salts to enhance their photodestructive efficiency.

Cationic fullerenes are effective and selective antimicrobial photosensitizers

Fullerenes are soccer ball-shaped molecules composed of carbon atoms, and, when derivatized with functional groups, they become soluble and can act as photosensitizers. Antimicrobial photodynamic therapy combines a nontoxic photosensitizer with harmless visible light to generate reactive oxygen species that kill microbial cells. We have compared the antimicrobial activity of six functionalized C(60) compounds with one, two, or three hydrophilic or cationic groups in combination with white light against gram-positive bacteria, gram-negative bacteria, and fungi. After a 10 min incubation, the bis- and tris-cationic fullerenes were highly active in killing all tested microbes (4-6 logs) under conditions in which mammalian cells were comparatively unharmed. These compounds performed significantly better than a widely used antimicrobial photosensitizer, toluidine blue O. The high selectivity and efficacy exhibited by these photosensitizers encourage further testing for antimicrobial applications.

Monitoring photodynamic therapy of localized infections by bioluminescence imaging of genetically engineered bacteria

The increasing occurrence of multi-antibiotic resistant microbes has led to the search for alternative methods of killing pathogens and treating infections. Photodynamic therapy (PDT) uses the combination of non-toxic dyes and harmless visible light to produce reactive oxygen species that can kill mammalian and microbial cells. Although the photodynamic inactivation of bacteria has been known for over a hundred years, its use to treat infections has not been much developed. This may be partly due to the difficulty of monitoring the effectiveness of PDT in animal models of infection. In order to facilitate this monitoring process, we have developed a procedure that uses bioluminescent genetically engineered bacteria and a light sensitive imaging system to allow real-time visualization of infections. When these bacteria are treated with PDT in vitro, the loss of luminescence parallels the loss of colony-forming ability. We have developed several models of infections in wounds and soft-tissue abscesses in mice that can be followed by bioluminescence imaging. The size and intensity of the infection can be sequentially monitored in a non-invasive fashion in individual mice in real-time. When photosensitizers are introduced into the infected tissue followed by illumination with red light, a light-dose dependent loss of luminescence is seen. If the bacterium is invasive, the loss of luminescence correlates with increased survival of the mice, whilst animals in control groups die of sepsis within five days. Healing of the PDT treated wounds is not impaired and may actually be improved. This approach can allow many animal models of localized infections to be accurately monitored for efficacy of treatment by PDT.

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.