Stable synthetic mono-substituted cationic bacteriochlorins mediate selective broad-spectrum photoinactivation of drug-resistant pathogens at nanomolar concentrations

SIM imaging of bacterial membrane dynamics and lipid peroxidation during photodynamic inactivation with a dual-functional activatable probe

Photodynamic inactivation (PDI) has emerged as a promising antibacterial strategy that mitigates the risk of bacterial resistance. However, the precise morphological dynamics and mechanisms underlying bacterial cell death during PDI remain insufficiently understood. In this study, we developed a dual-functional activatable probe, RDP, which integrated rhodamine B as a fluorophore with moderate brightness and fatty chains for selective bacterial membrane localization. The probe employed an aggregation-disaggregation mechanism to achieve both fluorescence activation and PDI functionality. Using super-resolution fluorescence imaging, we unveiled the selective rupture of bacterial membranes at specific sites during PDI, followed by membrane contraction and internalization, ultimately leading to the formation of lipid-enriched droplets within the bacteria. Further mechanistic investigations confirmed that this membrane rupture was driven by lipid peroxidation. Statistical analysis of bacterial morphological changes during PDI revealed that membrane rupture predominantly occurred at the septum during cell division, whereas in other growth phases, rupture sites were mainly localized at the poles. These findings provide critical insights into the role of selective membrane rupture in bacterial growth and viability, paving the way for the rational design of targeted and highly efficient antibacterial agents.

Near-IR nanolignin sensitizers based on pyrene-conjugated chlorin and bacteriochlorin for ROS generation, DNA intercalation and bioimaging

Near-infrared (NIR) fluorescent agents are extensively used for biomedical imaging due to their ability for deep tissue penetration. Tetrapyrrole-based photosensitizers are promising candidates in this regard. Further, the extended conjugation of such macromolecules with chromophores can enhance their fluorescence efficiency and DNA intercalation ability. Herein, pyrene-conjugated NIR photosensitizers, such as chlorin (PyChl) and bacteriochlorin (PyBac), were synthesized from the corresponding pyrene-porphyrin (PyP). The correlation between the theoretical and experimental optical properties (absorption and fluorescence spectroscopy results) was determined using the DFT/TD-DFT computational approach. Next, studies on the photophysical properties, reactive oxygen species (ROS) production, and DNA binding were conducted on these macrocycles to study the effect of pyrene conjugation on the pyrrolic ring. Furthermore, each photosensitizer was loaded into lignin nanoparticles (LNPs) using the solvent-antisolvent method to accomplish fluorescence-guided imaging. The developed near-IR chlorin- and bacteriochlorin-doped lignin nanocarriers (PyChl-LNCs and PyBac-LNCs) exhibited significant in vitro singlet oxygen generation upon red LED light exposure. Moreover, these macrocycle-loaded nanolignin sensitizers showed good fluorescence-guided bioimaging with fungal cells (Candida albicans). Further, the nanoprobes exhibited pH-dependent release profiles for biological applications. These nanolignin sensitizers demonstrated promising potential to be utilized in near-IR image-guided photodynamic therapy.

Efficient and Selective, In Vitro and In Vivo, Antimicrobial Photodynamic Therapy with a Dicationic Chlorin in Combination with KI

Various cationic photosensitizers employed in antimicrobial photodynamic therapy (aPDT) have the ability to photoinactivate planktonic bacteria under conditions of low phototoxicity to mammalian cells and without generating antimicrobial resistance (AMR). However, the photoinactivation of biofilms requires orders-of-magnitude higher photosensitizer concentrations, which become toxic to host cells. Remarkably, the bactericidal effect of a dicationic di-imidazolyl chlorin toward planktonic S. aureus and E. coli was observed in this work for concentrations below 400 nM under illumination at 660 nm and below 50 μM for the corresponding biofilms. At the latter concentrations, the chlorin is phototoxic toward human keratinocyte cells. However, in the presence of 50 mM KI, bactericidal concentrations are reduced to less than 50 nM for planktonic bacteria and to less than 1 μM for biofilms. It is shown that the potentiation with KI involves the triiodide anion. This potentiation elicits a bactericidal effect without appreciable cytotoxicity to keratinocytes. It becomes possible to selectively inactivate biofilms with aPDT. An exploratory study treating mice with wounds infected with E. coli expressing GFP with 20 μM chlorin and 120 J cm-2 at 652 nm confirmed the potential of this chlorin to control localized infections.

Photoinactivation of microorganisms using bacteriochlorins as photosensitizers

In recent years, some microorganisms have shown resistance to conventional treatments. Considering this increase in resistant pathogens, treatment alternatives are needed to promote greater treatment efficiency. In this sense, antimicrobial photodynamic therapy (aPDT) has been an alternative treatment. This technique uses a photosensitizer that is activated by light with a specific wavelength producing reactive species, leading to the death of pathogenic microorganisms. In this study, bacteriochlorophyll derivatives such as bacteriochlorin metoxi (Bchl-M) and bacteriochlorin trizma (Bchl-T) obtained from purple bacterium (Rhodopseudomonas faecalis), were evaluated as photosensitizers in the aPDT. Photodynamic inactivation (PDI) of the microorganisms Staphylococcus aureus, Micrococcus luteus, Candida albicans and Pseudomonas aeruginosa was investigated with both bacteriochlorins (Bchl-M and Bchl-T) at different concentrations (1, 15 and 30 µM for S. aureus; 1, 15, 30, 45, 60 and 75 µM for M. luteus; 30, 60, 90, 105, 120 and 150 µM for C. albicans; and 200 µM for P. aeruginosa) and different doses of light (20 and 30 J/cm2 for S. aureus and M. luteus; 30 and 45 J/cm2 for C. albicans; and 45 J/cm2 for P. aeruginosa) to inactivate them. Both photosensitizers showed good activation against S. aureus and for M. luteus, we observed the inactivation of these microorganisms at approximately 3 log, showing to be a good photosensitizers for these microorganisms.

On the Possibility of Using 5-Aminolevulinic Acid in the Light-Induced Destruction of Microorganisms

Antimicrobial photodynamic inactivation (aPDI) is a method that specifically kills target cells by combining a photosensitizer and irradiation with light at the appropriate wavelength. The natural amino acid, 5-aminolevulinic acid (5-ALA), is the precursor of endogenous porphyrins in the heme biosynthesis pathway. This review summarizes the recent progress in understanding the biosynthetic pathways and regulatory mechanisms of 5-ALA synthesis in biological hosts. The effectiveness of 5-ALA-aPDI in destroying various groups of pathogens (viruses, fungi, yeasts, parasites) was presented, but greater attention was focused on the antibacterial activity of this technique. Finally, the clinical applications of 5-ALA in therapies using 5-ALA and visible light (treatment of ulcers and disinfection of dental canals) were described.

Photodynamic therapy for ESKAPE pathogens: An emerging approach to combat antimicrobial resistance (AMR)

The increase in antimicrobial resistance, particularly in ESKAPE pathogens, has resulted in the dire need for new therapeutic approaches. ESKAPE is an acronym for a group of bacteria that are responsible for a majority of nosocomial and community acquired infections. The acronym stands for Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species. These pathogens are known for their ability to develop resistance to multiple antibiotics, making them difficult to treat thus posing a significant threat to public health. In light of the alarming consequences of antimicrobial resistance, it has been estimated that, in the absence of a substantial increase in the rate of development of new effective drugs, the number of casualties related to these infections will increase from about 700,000 in 2016 up to nearly 10,000,000 in 2050 [1]. One potential strategy to treat these pathogens is photodynamic therapy (PDT). In the early 20th century, Oscar Raab observed the phototoxicity of acridine red against Paramecium caudatum, while Tappenier and Jesionek demonstrated the photodynamic effects of eosin for treating cutaneous diseases. These discoveries laid the foundation for Photodynamic Therapy (PDT), which utilizes a non-toxic photosensitizer (PS) followed by targeted light irradiation for treatment [2]. PDT involves the use of a photosensitizer, a light source, and oxygen to eliminate highly active infectious pathogens such as bacteria, viruses, and fungi. PDT is known to possess several advantages including localized treatment and fewer side effects. Various photosensitizers and light sources have been assessed in different strains, showing promising results suggesting PDT to be a promising potential treatment option. PDT utilizes PS compounds with suitable light absorption that showcase effective results against the pathogens in vitro and in vivo, including BODIPY derivatives, Methylene Blue, and other dyes like porphyrin derivatives, phthalocyanines, indole derivatives, Photophrin, etc., inhibiting the growth of infections, for both in planktonic cells and in biofilms. Combination of PDT with other therapies like efflux pump inhibitors or quorum sensing inhibitors has also proven to be efficacious. However, this domain further needs to be assessed before it reaches the society.

Effect of photodynamic therapy on multidrug-resistant Acinetobacter baumannii: A scoping review

BACKGROUND

Acinetobacter baumannii is a Gram-negative, non-fermenting coccobacillus of the Moraxellaceae family. It is an opportunistic pathogen responsible for several hospital-acquired infections (HAIs) associated with skin and tissue infections at surgical sites, catheter-associated urinary tract infections, and central line catheters. Multidrug-resistant (MDR) A. baumannii has caused hospital outbreaks that are difficult to eradicate and represent one of the leading producers of HAIs. MDR-A. baumannii presents a broad range of resistance to different antimicrobials, including carbapenems. Due to the low sensitivity to conventional antibiotic therapies, it is necessary to identify other therapeutic options. Antimicrobial photodynamic therapy (aPDT) is a promising alternative and complementary approach to address the shortage of antimicrobials in MDR-A. baumannii. APDT combines a photosensitizer agent, light, and oxygen to achieve a bactericidal/bacteriostatic effect. The effect is given by producing reactive oxygen species (ROS) that produce photooxidative stress over bacterial structures, such as the envelope and the DNA.

METHODS

This study aims to systematically collect bibliographic information from databases such as PubMed, Scopus, and google scholar to analyze the relevant articles critically.

RESULTS

An increasing body of evidence demonstrates the efficacy of photodynamic inactivation in eliminating A. baumannii strains, both in vitro and in vivo.

CONCLUSIONS

The evidence supports that photodynamic inactivation is an alternative capable of eliminating strains of Acinetobacter baumannii and may considerably improve the treatment of MDR strains. Although they do exist, aPDT studies on MDR strains of A. baumannii are scarce and should increase since it is on these strains that photodynamic therapy becomes attractive.

The first evidence of antibiofilm action of Proteus mirabilis with tetra-cationic porphyrins containing cisplatin by antimicrobial photodynamic therapy

Biofilms are responsible for up to 80% of antimicrobial-resistant nosocomial infections. Most of these infections are associated with medical devices such as urinary catheters, and in this context, it is estimated that 90-100% of patients who undergo long-term catheterization develop infections. Proteus mirabilis, the most prevalent microorganism, is responsible for 20-45% of these infections. Thus, this study aimed to evaluate, for the first time, the antimicrobial and antibiofilm effects of cationic porphyrins on P. mirabilis. Neutral porphyrins 3-H₂TPyP and 4-H₂TpyP and tetra-cationic derivatives 3-cis-PtTPyP and 4-cis-PtTPyP were evaluated in broth microdilution tests to determine the minimum inhibitory and bactericidal concentrations. Time-kill curves, checkerboard test, reactive oxygen species (ROS) scavenger assays, conventional biofilm formation, and biofilm assay with catheters were also performed. The microdilution tests showed greater efficacy against P. mirabilis when 3-cis-PtTPyP was exposed to white-light conditions; this also occurred when the microbial time-kill curve was performed at 0, 2, 6, and 12 h. The radical superoxide species was possibly responsible for photoinactivation in the ROS scavenger assays. In biofilm assays (conventional and catheter), 3-cis-PtTPyP obtained better results when irradiated with a white-light source. In the checkerboard assay, the same compound showed no differences when tested in association with ciprofloxacin hydrochloride. Our findings lead us to conclude that antimicrobial photodynamic therapy and cationic porphyrins obtained positive results and are promising alternatives to treat P. mirabilis biofilms.

Photoantimicrobials in agriculture

Classical approaches for controlling plant pathogens may be impaired by the development of pathogen resistance to chemical pesticides and by limited availability of effective antimicrobial agents. Recent increases in consumer awareness of and/or legislation regarding environmental and human health, and the urgent need to improve food security, are driving increased demand for safer antimicrobial strategies. Therefore, there is a need for a step change in the approaches used for controlling pre- and post-harvest diseases and foodborne human pathogens. The use of light-activated antimicrobial substances for the so-called antimicrobial photodynamic treatment is known to be effective not only in a clinical context, but also for use in agriculture to control plant-pathogenic fungi and bacteria, and to eliminate foodborne human pathogens from seeds, sprouted seeds, fruits, and vegetables. Here, we take a holistic approach to review and re-evaluate recent findings on: (i) the ecology of naturally-occurring photoantimicrobials, (ii) photodynamic processes including the light-activated antimicrobial activities of some plant metabolites, and (iii) fungus-induced photosensitization of plants. The inhibitory mechanisms of both natural and synthetic light-activated substances, known as photosensitizers, are discussed in the contexts of microbial stress biology and agricultural biotechnology. Their modes-of-antimicrobial action make them neither stressors nor toxins/toxicants (with specific modes of poisonous activity), but a hybrid/combination of both. We highlight the use of photoantimicrobials for the control of plant-pathogenic fungi and quantify their potential contribution to global food security.

Antibacterial Photodynamic Therapy in the Near-Infrared Region with a Targeting Antimicrobial Peptide Connected to a π-Extended Porphyrin

The increase of antimicrobial resistance to conventional antibiotics is worldwide a major health problem that requires the development of new bactericidal strategies. Antimicrobial photodynamic therapy (a-PDT) that generates reactive oxygen species acting on multiple cellular targets is unlikely to induce bacterial resistance. This localized treatment requires, for safe and efficient treatment of nonsuperficial infections, a targeting photosensitizer excited in the near IR. To this end, a new conjugate consisting of an antimicrobial peptide linked to a π-extended porphyrin photosensitizer was designed for a-PDT. Upon irradiation at 720 nm, the conjugate has shown at micromolar concentration strong bactericidal action on both Gram-positive and Gram-negative bacteria. Moreover, this conjugate allows one to reach a low minimum bactericidal concentration with near IR excitation without inducing toxicity to skin cells.

The Meta-Substituted Isomer of TMPyP Enables More Effective Photodynamic Bacterial Inactivation than Para-TMPyP In Vitro

Porphyrinoid-based photodynamic inactivation (PDI) provides a promising approach to treating multidrug-resistant infections. However, available agents for PDI still have optimization potential with regard to effectiveness, toxicology, chemical stability, and solubility. The currently available photosensitizer TMPyP is provided with a para substitution pattern (para-TMPyP) of the pyridinium groups and has been demonstrated to be effective for PDI of multidrug-resistant bacteria. To further improve its properties, we synthetized a structural variant of TMPyP with an isomeric substitution pattern in a meta configuration (meta-TMPyP), confirmed the correct structure by crystallographic analysis and performed a characterization with NMR-, UV/Vis-, and IR spectroscopy, photostability, and singlet oxygen generation assay. Meta-TMPyP had a hypochromic shift in absorbance (4 nm) with a 55% higher extinction coefficient and slightly improved photostability (+6.9%) compared to para-TMPyP. Despite these superior molecular properties, singlet oxygen generation was increased by only 5.4%. In contrast, PDI, based on meta-TMPyP, reduced the density of extended spectrum β-lactamase-producing and fluoroquinolone-resistant Escherichia coli by several orders of magnitude, whereby a sterilizing effect was observed after 48 min of illumination, while para-TMPyP was less effective (p < 0.01). These findings demonstrate that structural modification with meta substitution increases antibacterial properties of TMPyP in PDI.

Photodynamic therapy for treatment of bacterial keratitis

Microbial keratitis is the main cause of corneal opacification and the fourth leading cause of blindness worldwide, with bacteria the major infectious agent. Recently, bacterial keratitis has become a serious threat due to routine use of antibiotics leading to selection of resistant and multidrug-resistant bacteria strains. New approaches for treatment of bacterial keratitis are necessary to outcome the increasing antibiotic resistance. Antimicrobial photodynamic therapy is based on three agents: photosensitizer, oxygen, and light radiation. This therapy has been successful for treatment of infections in different tissues and organs as well as against different type of infectious agents and no resistance development. Also, new photosensitizers are being developed that has increased the spectrum of therapeutic protocols for treatment of a number of infectious diseases. Thus, antimicrobial photodynamic therapy has an extraordinary potential for treatment of those bacterial keratitis cases that actually are not solved by traditional antibiotic therapy.

Photodynamic treatment of pathogens

The current viral pandemic has highlighted the compelling need for effective and versatile treatments, that can be quickly tuned to tackle new threats, and are robust against mutations. Development of such treatments is made even more urgent in view of the decreasing effectiveness of current antibiotics, that makes microbial infections the next emerging global threat. Photodynamic effect is one such method. It relies on physical processes proceeding from excited states of particular organic molecules, called photosensitizers, generated upon absorption of visible or near infrared light. The excited states of these molecules, tailored to undergo efficient intersystem crossing, interact with molecular oxygen and generate short lived reactive oxygen species (ROS), mostly singlet oxygen. These species are highly cytotoxic through non-specific oxidation reactions and constitute the basis of the treatment. In spite of the apparent simplicity of the principle, the method still has to face important challenges. For instance, the short lifetime of ROS means that the photosensitizer must reach the target within a few tens nanometers, which requires proper molecular engineering at the nanoscale level. Photoactive nanostructures thus engineered should ideally comprise a functionality that turns the system into a theranostic means, for instance, through introduction of fluorophores suitable for nanoscopy. We discuss the principles of the method and the current molecular strategies that have been and still are being explored in antimicrobial and antiviral photodynamic treatment.

Photodynamic disinfection and its role in controlling infectious diseases

Photodynamic therapy is witnessing a revival of its origins as a response to the rise of multi-drug resistant infections and the shortage of new classes of antibiotics. Photodynamic disinfection (PDDI) of microorganisms is making progresses in preclinical models and in clinical cases, and the perception of its role in the clinical armamentarium for the management of infectious diseases is changing. We review the positioning of PDDI from the perspective of its ability to respond to clinical needs. Emphasis is placed on the pipeline of photosensitizers that proved effective to inactivate biofilms, showed efficacy in animal models of infectious diseases or reached clinical trials. Novel opportunities resulting from the COVID-19 pandemic are briefly discussed. The molecular features of promising photosensitizers are emphasized and contrasted with those of photosensitizers used in the treatment of solid tumors. The development of photosensitizers has been accompanied by the fabrication of a variety of affordable and customizable light sources. We critically discuss the combination between photosensitizer and light source properties that may leverage PDDI and expand its applications to wider markets. The success of PDDI in the management of infectious diseases will ultimately depend on the efficacy of photosensitizers, affordability of the light sources, simplicity of the procedures, and availability of fast and efficient treatments.

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.

Pentamidine enhances photosensitization of Acinetobacter baumannii using diode lasers with emission of light at wavelength of ʎ = 405 nm and ʎ = 635 nm

Antimicrobial photodynamic inactivation is currently one of the most promising trends in the modern bactericidal protocols. Under the conditions defined in our studies, we found that in vitro photosensitization of A. baumannii with 5-ALA as a precursor of protoporphyrin IX (photosensitizer) reduces the concentration of viable cells in planktonic cultures, and this process can be strongly enhanced by pentamidine. Diode lasers with the peak-power wavelength of ʎ = 405 nm (radiation intensity of 26 mW cm^-2) and ʎ = 635 nm (radiation intensity of 55 mW cm^-2) were used in this study. It was found that a blue laser light (energy fluence of 64 J cm^-2; no external photosensitizer) in the presence of pentamidine resulted in a reduction of CFU of 99.992 % compared to 99.97 % killing without pentamidine. When a red laser light was used in the experiments (energy fluence of 136 J cm^-2; no external photosensitizer), the mortality rate was 99.98 % in the presence of pentamidine compared to 99.93 % of those killed without the addition of this drug. The lethal effect with 5-ALA was achieved under blue light fluence of 16 J cm^-2 (in the presence of pentamidine) and 32 J cm^-2 (without pentamidine). In the case of laser light of 635 nm, the lethal effect with 5-ALA was attained with energy fluence of 51 J cm^-2 (with pentamidine) and 102 J cm^-2 (without pentamidine). The possible roles of pentamidine in enhancing photodynamic inactivation of A. baumannii have been discussed.

Exploiting a Neutral BODIPY Copolymer as an Effective Agent for Photodynamic Antimicrobial Inactivation

We report the synthesis and photophysical properties of a neutral BODIPY photosensitizing copolymer (poly-8-(4-hydroxymethylphenyl)-4,4-difluoro-2,6-diethynyl-4-bora-3a,4a-diaza-s-indacene) containing ethynylbenzene links between the BODIPY units. The copolymer absorbs further towards the red in the UV-vis spectrum compared to the BODIPY precursor. Photolysis of the polymer produces a singlet excited state which crosses to the triplet surface in less than 300 ps. This triplet state was used to form singlet oxygen with a quantum yield of 0.34. The steps leading to population of the triplet state were studied using time-resolved spectroscopic techniques spanning the pico- to nanosecond timescales. The ability of the BODIPY polymer to generate a biocidal species for bactericidal activity in both solution- and coating-based studies was assessed. When the BODIPY copolymer was dropcast onto a surface, 4 log and 6 log reductions in colony forming units/ml representative of Gram-positive and Gram-negative bacteria, respectively, under illumination at 525 nm were observed. The potent broad-spectrum antimicrobial activity of a neutral metal-free copolymer when exposed to visible light conditions may have potential clinical applications in infection management.

Antibacterial Photodynamic Inactivation of Antibiotic-Resistant Bacteria and Biofilms with Nanomolar Photosensitizer Concentrations

Gram-negative bacteria and bacteria in biofilms are very difficult to eradicate and are the most antibiotic-resistant bacteria. Therapeutic alternatives less susceptible to mechanisms of resistance are urgently needed to respond to an alarming increase of resistant nosocomial infections. Antibacterial photodynamic inactivation (PDI) generates oxidative stress that triggers multiple cell death mechanisms that are more difficult to counteract by bacteria. We explore PDI of multidrug-resistant bacterial strains collected from patients and show how positive charge distribution in the photosensitizer drug impacts the efficacy of inactivation. We demonstrate the relevance of size for drug diffusion in biofilms. The designed meso-imidazolyl porphyrins of small size with positive charges surrounding the macrocycle enabled the inactivation of bacteria in biofilms by 6.9 log units at 5 nM photosensitizer concentration and 5 J cm^-2, which offers new opportunities to treat biofilm infections.

Tetrahydroporphyrin-tetratosylate (THPTS)-based photodynamic inactivation of critical multidrug-resistant bacteria in vitro

BACKGROUND

Photodynamic inactivation (PDI) is a promising approach to treat multidrug-resistant infections. However, effectiveness of PDI is limited, particularly in Gram-negative bacteria. The use of photosensitizer (PS) 3,3',3'',3'''-(7,8,17,18-tetrahydro-21H,23H-porphyrine-5,10,15,20-tetrayl)tetrakis[1-methyl-pyridinium]tetratosylate (THPTS) and laser light has led to very promising results. This study focuses on the effects of THPTS in various critical multidrug-resistant bacterial strains and explores the possibility of light-emitting diode (LED)-based activation as a clinically more feasible alternative to laser light.

METHODS

THPTS was further chemically characterized and in vitro testing of PDI of different multidrug-resistant bacterial strains was performed under various experimental conditions, including varying drug concentration, incubation time, light source (laser and LED) and light intensity, by determination of viable bacteria after treatment. The effect of hyaluronic acid as an adjuvant for medical applications was also evaluated.

RESULTS

Bacterial density of all investigated bacterial strains was reduced by several orders of magnitude, irrespective of multidrug-resistance or hyaluronic acid addition. The effect was less intense in Gram-negative strains (disinfection), and more pronounced in Gram-positive strains (sterilization), even at reduced THPTS concentrations or decreased light treatment intensity. Controls without THPTS or without light treatment did not indicate reduced bacterial density.

CONCLUSIONS

PDI with THPTS and laser light was effective in all investigated bacterial strains. Gram-negative strains were less, but sufficiently, susceptible to PDI. Adding hyaluronic acid did not reduce the antibacterial treatment effect. LED-based PDI is equally effective when illumination duration is increased to compensate for reduced light intensity.

Bacteriochlorin-bis(spermine) conjugate affords an effective photodynamic action to eradicate microorganisms

A novel bacteriochlorin bearing two spermine units (BCS) was synthesized from 3,13-dibromo-8,8,18,18-tetramethylbacteriochlorin (BC-Br ^3,13 ). The synthesis involved the Suzuki coupling of BC-Br ^3,13 to obtain a bacteriochlorin-dibenzaldehyde (BCA), which was subjected to reductive amination with spermine. The resulting bacteriochlorin BCS presents a strong near-infrared absorption band at 747 nm, emits at 750 nm with fluorescence quantum yield of 0.14, and generates singlet molecular oxygen, O₂ (¹ Δ_g ), with a quantum yield of 0.27. Photokilling capacities mediated by BCS were evaluated in microbial cells. The viability of Staphylococcus aureus decreased 7 logs when cells were incubated with 1 μM BCS and irradiated for 15 minutes. Comparable photocytotoxic effect was obtained with Escherichia coli, when cells were treated for 30 minutes with visible light. BCS was also an effective photosensitizer to inactivate Candida albicans. In addition, this bacteriochlorin was able to eradicate bacteria at short incubation times. The structure of BCS contains eight basic amino groups that, when protonated in water, increase the binding to the cell envelope. In summary, the readily accessible bacteriochlorin BCS was highly effective at low concentrations as a broad-spectrum antimicrobial photosensitizer.

Oxygen-Independent Antimicrobial Photoinactivation: Type III Photochemical Mechanism?

Since the early work of the 1900s it has been axiomatic that photodynamic action requires the presence of sufficient ambient oxygen. The Type I photochemical pathway involves electron transfer reactions leading to the production of reactive oxygen species (superoxide, hydrogen peroxide, and hydroxyl radicals), while the Type II pathway involves energy transfer from the PS (photosensitizer) triplet state, leading to production of reactive singlet oxygen. The purpose of the present review is to highlight the possibility of oxygen-independent photoinactivation leading to the killing of pathogenic bacteria, which may be termed the "Type III photochemical pathway". Psoralens can be photoactivated by ultraviolet A (UVA) light to produce DNA monoadducts and inter-strand cross-links that kill bacteria and may actually be more effective in the absence of oxygen. Tetracyclines can function as light-activated antibiotics, working by a mixture of oxygen-dependent and oxygen independent pathways. Again, covalent adducts may be formed in bacterial ribosomes. Antimicrobial photodynamic inactivation can be potentiated by addition of several different inorganic salts, and in the case of potassium iodide and sodium azide, bacterial killing can be achieved in the absence of oxygen. The proposed mechanism involves photoinduced electron transfer that produces reactive inorganic radicals. These new approaches might be useful to treat anaerobic infections or infections in hypoxic tissue.

Tetracyclines: light-activated antibiotics?

Tetracyclines are well established antibiotics but show phototoxicity as a side effect. Antimicrobial photodynamic inactivation uses nontoxic dyes combined with harmless light to destroy microbial cells by reactive oxygen species. Tetracyclines (demeclocycline and doxycycline) can act as light-activated antibiotics by binding to bacterial cells and killing them only upon illumination. The remaining tetracyclines can prevent bacterial regrowth after illumination has ceased. Antimicrobial photodynamic inactivation can be potentiated by potassium iodide. Azide quenched the formation of iodine, but not hydrogen peroxide. Demeclotetracycline (but not doxycycline) iodinated tyrosine after light activation in the presence of potassium iodide. Bacteria are killed by photoactivation of tetracyclines in the absence of oxygen. Since topical tetracyclines are already used clinically, blue light activation may increase the bactericidal effect.

Amphiphilic tetracationic porphyrins are exceptionally active antimicrobial photosensitizers: In vitro and in vivo studies with the free-base and Pd-chelate

Antimicrobial photodynamic inactivation (aPDI) employs the combination of nontoxic photosensitizing dyes and visible light to kill pathogenic microorganisms regardless of drug-resistance, and can be used to treat localized infections. A meso-substituted tetra-methylpyridinium porphyrin with one methyl group replaced by a C12 alkyl chain (FS111) and its Pd-derivative (FS111-Pd) were synthesized and tested as broad-spectrum antimicrobial photosensitizers when excited by blue light (5 or 10 J/cm² ). Both compounds showed unprecedented activity, with the superior FS111-Pd giving 3 logs of killing at 1 nM, and eradication at 10 nM for Gram-positive methicillin-resistant Staphylococcus aureus. For the Gram-negative Escherichia coli, both compounds produced eradication at 100 nM, while against the fungal yeast Candida albicans, both compounds produced eradication at 500 nM. Both compounds could be categorized as generators of singlet oxygen (Φ_Δ = 0.62 for FS111 and 0.71 for FS111-Pd). An in vivo study was carried out using a mouse model of localized infection in a partial thickness skin abrasion caused by bioluminescent Gram-negative uropathogenic E. coli. Both compounds were effective in reducing bioluminescent signal in a dose-dependent manner when excited by blue light (405 nm), but aPDI with FS111-Pd was somewhat superior both during light and in preventing recurrence during the 6 days following PDT.

Cationic diarylporphyrins: In vitro versatile anticancer and antibacterial photosensitizers

The visible light combined with photosensitizers (PSs) is exploited in both antitumoral and antimicrobial fields inducing a photo-oxidative stress within the target cells. Among the different PSs, porphyrins belong to the family of the most promising compounds to be used in clinical photodynamic applications. Although in the last years many porphyrins have been synthesised and tested, only a few reports concern the in vitro effects of the 5,15-diarylporphyrins. In this work, the activity of four 5,15-diarylporphyrins (compounds 7-10), bearing alkoxy-linked pyridinium appendixes, have been tested on cancer cell lines and against bacterial cultures. Among the synthetized PSs, compounds 7 and 9 are not symmetrically substituted porphyrins showing one cationic charge tethered at the end of one 4C or 8C carbon chains, respectively. On the other hand, compounds 8 and 10 are symmetrically substituted and show two chains of C4 and C8 carbons featuring a cationic charge at the end of both chains. The dicationic 8 and 10 were more hydrophilic than monocationic 7 and 9, outlining that the presence of two pyridinium salts have a higher impact on the solubility in the aqueous phase than the lipophilic effect exerted by the length of the alkyl chains. Furthermore, these four PSs showed a similar rate of photobleaching, irrespective of the length and number of chains and the number of positive charges. Among the eukaryotic cell lines, the SKOV3 cells were particularly sensitive to the photodynamic activity of all the tested diarylporphyrins, while the HCT116 cells were found more sensitive to PSs bearing C4 chain (7 and 8), regardless the number of cationic charges. The photo-induced killing effect of these porphyrins was also tested against two different bacterial cultures. As expected, the Gram positive Bacillus subtilis was more sensitive than the Gram negative Escherichia coli, and the dicationic porphyrin 8, bearing two C4 chains, was the most efficient on both microorganisms. In conclusion, the new compound 8 seems to be an optimal candidate to deepen as versatile anticancer and antibacterial photosensitizer.

Photoinactivation of ESKAPE pathogens: overview of novel therapeutic strategy

The emergence of antimicrobial drug resistance requires development of alternative therapeutic options. Multidrug-resistant strains of Enterococcus spp., Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa and Enterobacter spp. are still the most commonly identified antimicrobial-resistant pathogens. These microorganisms are part of the so-called 'ESKAPE' pathogens to emphasize that they currently cause the majority of hospital acquired infections and effectively 'escape' the effects of antibacterial drugs. Thus, alternative, safer and more efficient antimicrobial strategies are urgently needed, especially against 'ESKAPE' superbugs. Antimicrobial photodynamic inactivation is a therapeutic option used in the treatment of infectious diseases. It is based on a combination of a photosensitizer, light and oxygen to remove highly metabolically active cells.

Antimicrobial Photodynamic Inactivation Mediated by Tetracyclines in Vitro and in Vivo: Photochemical Mechanisms and Potentiation by Potassium Iodide

Tetracyclines (including demeclocycline, DMCT, or doxycycline, DOTC) represent a class of dual-action antibacterial compounds, which can act as antibiotics in the dark, and also as photosensitizers under illumination with blue or UVA light. It is known that tetracyclines are taken up inside bacterial cells where they bind to ribosomes. In the present study, we investigated the photochemical mechanism: Type 1 (hydroxyl radicals); Type 2 (singlet oxygen); or Type 3 (oxygen independent). Moreover, we asked whether addition of potassium iodide (KI) could potentiate the aPDI activity of tetracyclines. High concentrations of KI (200–400 mM) strongly potentiated (up to 5 logs of extra killing) light-mediated killing of Gram-negative Escherichia coli or Gram-positive MRSA (although the latter was somewhat less susceptible). KI potentiation was still apparent after a washing step showing that the iodide could penetrate the E. coli cells where the tetracycline had bound. When cells were added to the tetracycline + KI mixture after light, killing was observed in the case of E. coli showing formation of free molecular iodine. Addition of azide quenched the formation of iodine but not hydrogen peroxide. DMCT but not DOTC iodinated tyrosine. Both E. coli and MRSA could be killed by tetracyclines plus light in the absence of oxygen and this killing was not quenched by azide. A mouse model of a superficial wound infection caused by bioluminescent E. coli could be treated by topical application of DMCT and blue light and bacterial regrowth did not occur owing to the continued anti biotic activity of the tetracycline.

Progressive cationic functionalization of chlorin derivatives for antimicrobial photodynamic inactivation and related vancomycin conjugates

It is known that multiple cationic charges are required to produce broad-spectrum antimicrobial photosensitizers (PS) for photodynamic inactivation (aPDI) or photodynamic therapy of bacteria and fungi. In the present study we describe the synthesis and aPDI testing of a set of derivatives prepared from the parent pheophytin molecule with different numbers of attached side arms (1-3) each consisting of five quaternized cationic groups (pentacationic), producing the corresponding [Zn2+]pheophorbide-a-N(C2N+C1C3)5 (Zn-Phe-N5+, 5 charges), [Zn2+]chlorin e6-[N(C2N+C1C3)5]2 (Zn-Chl-N10+, 10 charges) and [Zn2+]mesochlorin e6-[N(C2N+C1C3)5]3 (Zn-mChl-N15+, 15 charges). Moreover, a conjugate between Zn-Phe-N5+ and the antibiotic vancomycin called Van-[Zn2+]-m-pheophorbide-N(C2N+C1C3)5 (Van-Zn-mPhe-N5+) was also prepared. The aPDI activities of all compounds were based on Type-II photochemistry (1O2 generation). We tested these compounds against Gram-positive methicillin-resistant Staphylococcus aureus (MRSA), Gram-negative Escherichia coli, and the fungal yeast Candida albicans. All three compounds were highly active against MRSA, giving eradication (≥6 logs of killing) with <1.0 μM and 10 J cm-2 of 415 nm light. The order of activity was Zn-Phe-N5+ > Zn-Chl-N10+ > Zn-mChl-N15+. In the case of E coli the activity was much lower (eradication was only possible with 50 μM Zn-mChl-N15+ and 20 J cm-2). The order of activity was the reverse of that found with MRSA (Zn-mChl-N15+ > Zn-Chl-N10+ > Zn-Phe-N5+). Activity against C. albicans was similar to E. coli with Zn-mChl-N15+ giving eradication. The activity of Van-Zn-mPhe-N5+ was generally lower than that of Zn-Phe-N5+ (except for E. coli). Red (660 nm) light was also effective as might be expected from the absorption spectra. An initial finding that Van-Zn-mPhe-N5+ might have higher activity against vancomycin resistant Enterococcus fecium (VRE) strains (compared to vancomycin sensitive strains) was disproved when it was found that VRE strains were also more sensitive to aPDI with Zn-Phe-N5+. The minimum inhibitory concentrations of Van-Zn-mPhe-N5+ were higher than those of Van alone, showing that the antibiotic properties of the Van moiety were lessened in the conjugate. In conclusion, Zn-Phe-N5+ is a highly active PS against Gram-positive species and deserves further testing. Increasing the number of cationic charges increased aPDI efficacy on C. albicans and Gram-negative E. coli.

In vitro photodynamic inactivation effects of benzylidene cyclopentanone photosensitizers on clinical fluconazole-resistant Candida albicans

BACKGROUND

The incidence of Candida infections has increased for various reasons, including, the more frequent use of immunosuppresants or broad-spectrum antibiotics. Photodynamic inactivation (PDI) is a promising approach for treating localized Candida infections.

METHODS

The PDI efficacies of three benzylidene cyclopentanone-based (BCB) photosensitizers (PSs: P1, P2 and Y1) against three fluconazole-resistant C. albicans (cal-1, cal-2, and cal-3) and one control C. albicans (ATCC 90028), respectively, were evaluated using an established plate dilution method. The binding of PSs to C. albicans was determined by fluorescence spectroscopy. The mechanism of antifungal PDI was investigated using confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM).

RESULTS

Three BCB PSs all bound rapidly to C. albicans. After incubation with PSs for 30 min and irradiation with a 532 nm laser for 10 min (40 mW cm^-2, 24 J cm^-2), the fungicidal activity was achieved as 7.5 μM for P1 and P2, and 25 μM for Y1. CLSM confirmed that P1 and Y1 were located in intracellular components, including mitochondria, while P2 bound to the protoplast exterior and failed to enter the cells. TEM revealed the damage of mitochondria ultrastructures after P1- or Y1-mediated PDI, consistenting with the CLSM results. However, most cells became edematous, enlarged or deformation after P2-mediated PDI.

CONCLUSIONS

The three BCB PSs all have remarkable PDI effects on C. albicans. The best effect is obtained by P1, which has one cationic charge with a proper lipophilicity. The respective subcellular localization of the three PSs led to different PDI mechanisms.

Some activities of PorphyChem illustrated by the applications of porphyrinoids in PDT, PIT and PDI

Photodynamic therapy is an innovative approach to treat diverse cancers and diseases that involves the use of photosensitizing agents along with light of an appropriate wavelength to generate cytotoxic reactive oxygen species.

Properties of halogenated and sulfonated porphyrins relevant for the selection of photosensitizers in anticancer and antimicrobial therapies

The impact of substituents on the photochemical and biological properties of tetraphenylporphyrin-based photosensitizers for photodynamic therapy of cancer (PDT) as well as photodynamic inactivation of microorganisms (PDI) was examined. Spectroscopic and physicochemical properties were related with therapeutic efficacy in PDT of cancer and PDI of microbial cells in vitro. Less polar halogenated, sulfonamide porphyrins were most readily taken up by cells compared to hydrophilic and anionic porphyrins. The uptake and PDT of a hydrophilic porphyrin was significantly enhanced with incorporation in polymeric micelles (Pluronic L121). Photodynamic inactivation studies were performed against Gram-positive (S. aureus, E. faecalis), Gram-negative bacteria (E. coli, P. aeruginosa, S. marcescens) and fungal yeast (C. albicans). We observed a 6 logs reduction of S. aureus after irradiation (10 J/cm²) in the presence of 20 μM of hydrophilic porphyrin, but this was not improved with incorporation in Pluronic L121. A 2–3 logs reduction was obtained for E. coli using similar doses, and a decrease of 3–4 logs was achieved for C. albicans. Rational substitution of tetraphenylporphyrins improves their photodynamic properties and informs on strategies to obtain photosensitizers for efficient PDT and PDI. However, the design of the photosensitizers must be accompanied by the development of tailored drug formulations.

Cholesterol-Assisted Bacterial Cell Surface Engineering for Photodynamic Inactivation of Gram-Positive and Gram-Negative Bacteria

Antibacterial photodynamic therapy (PDT), which enables effective killing of regular and multidrug-resistant (MDR) bacteria, is a promising treatment modality for bacterial infection. However, because most photosensitizer (PS) molecules fail to strongly interact with the surface of Gram-negative bacteria, this technique is suitable for treating only Gram-positive bacterial infection, which largely hampers its practical applications. Herein, we reveal for the first time that cholesterol could significantly facilitate the hydrophobic binding of PSs to the bacterial surface, achieving the hydrophobic interaction-based bacterial cell surface engineering that could effectively photoinactivate both Gram-negative and Gram-positive bacteria. An amphiphilic polymer composed of a polyethylene glycol (PEG) segment terminated with protoporphyrin IX (PpIX, an anionic PS) and cholesterol was constructed (abbreviated Chol-PEG-PpIX), which could self-assemble into micelle-like nanoparticles (NPs) in aqueous solution. When encountering the Gram-negative Escherichia coli cells, the Chol-PEG-PpIX NPs would disassemble and the PpIX moieties could effectively bind to the bacterial surface with the help of the cholesterol moieties, resulting in the significantly enhanced fluorescence emission of the bacterial surface. Under white light irradiation, the light-triggered singlet oxygen (¹O₂) generation of the membrane-bound PpIX could not only severely damage the outer membrane but also facilitate the entry of external Chol-PEG-PpIX into the bacteria, achieving >99.99% bactericidal efficiency. Besides, as expected, the Chol-PEG-PpIX NPs also exhibited excellent antibacterial performance against the Gram-positive Staphylococcus aureus. We also verified that this nanoagent possesses negligible dark cytotoxicity toward mammalian cells and good hemocompatibility. To the best of our knowledge, this study demonstrates for the first time the feasibility of constructing a fully hydrophobic interaction-based and outer membrane-anchored antibacterial PDT nanoagent.

Potassium Iodide Potentiates Broad-Spectrum Antimicrobial Photodynamic Inactivation Using Photofrin

It is known that noncationic porphyrins such as Photofrin (PF) are effective in mediating antimicrobial photodynamic inactivation (aPDI) of Gram-positive bacteria or fungi. However, the aPDI activity of PF against Gram-negative bacteria is accepted to be extremely low. Here we report that the nontoxic inorganic salt potassium iodide (KI) at a concentration of 100 mM when added to microbial cells (10⁸/mL) + PF (10 μM hematoporphyrin equivalent) + 415 nm light (10 J/cm²) can eradicate (>6 log killing) five different Gram-negative species (Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Proteus mirabilis, and Acinetobacter baumannii), whereas no killing was obtained without KI. The mechanism of action appears to be the generation of microbicidal molecular iodine (I₂/I₃^-) as shown by comparable bacterial killing when cells were added to the mixture after completion of illumination and light-dependent generation of iodine as detected by the formation of the starch complex. Gram-positive methicillin-resistant Staphylococcus aureus is much more sensitive to aPDI (200-500 nM PF), and in this case potentiation by KI may be mediated mainly by short-lived iodine reactive species. The fungal yeast Candida albicans displayed intermediate sensitivity to PF-aPDI, and killing was also potentiated by KI. The reaction mechanism occurs via singlet oxygen (¹O₂). KI quenched ¹O₂ luminescence (1270 nm) at a rate constant of 9.2 × 10⁵ M^-1 s^-1. Oxygen consumption was increased when PF was illuminated in the presence of KI. Hydrogen peroxide but not superoxide was generated from illuminated PF in the presence of KI. Sodium azide completely inhibited the killing of E. coli with PF/blue light + KI.

Antimicrobial photodynamic inactivation: a bright new technique to kill resistant microbes

Photodynamic therapy (PDT) uses photosensitizers (non-toxic dyes) that are activated by absorption of visible light to form reactive oxygen species (including singlet oxygen) that can oxidize biomolecules and destroy cells. Antimicrobial photodynamic inactivation (aPDI) can treat localized infections. aPDI neither causes any resistance to develop in microbes, nor is affected by existing drug resistance status. We discuss some recent developments in aPDI. New photosensitizers including polycationic conjugates, stable synthetic bacteriochlorins and functionalized fullerenes are described. The microbial killing by aPDI can be synergistically potentiated (several logs) by harmless inorganic salts via photochemistry. Genetically engineered bioluminescent microbial cells allow PDT to treat infections in animal models. Photoantimicrobials have a promising future in the face of the unrelenting increase in antibiotic resistance.

New photosensitizers for photodynamic therapy

Photodynamic therapy (PDT) was discovered more than 100 years ago, and has since become a well-studied therapy for cancer and various non-malignant diseases including infections. PDT uses photosensitizers (PSs, non-toxic dyes) that are activated by absorption of visible light to initially form the excited singlet state, followed by transition to the long-lived excited triplet state. This triplet state can undergo photochemical reactions in the presence of oxygen to form reactive oxygen species (including singlet oxygen) that can destroy cancer cells, pathogenic microbes and unwanted tissue. The dual-specificity of PDT relies on accumulation of the PS in diseased tissue and also on localized light delivery. Tetrapyrrole structures such as porphyrins, chlorins, bacteriochlorins and phthalocyanines with appropriate functionalization have been widely investigated in PDT, and several compounds have received clinical approval. Other molecular structures including the synthetic dyes classes as phenothiazinium, squaraine and BODIPY (boron-dipyrromethene), transition metal complexes, and natural products such as hypericin, riboflavin and curcumin have been investigated. Targeted PDT uses PSs conjugated to antibodies, peptides, proteins and other ligands with specific cellular receptors. Nanotechnology has made a significant contribution to PDT, giving rise to approaches such as nanoparticle delivery, fullerene-based PSs, titania photocatalysis, and the use of upconverting nanoparticles to increase light penetration into tissue. Future directions include photochemical internalization, genetically encoded protein PSs, theranostics, two-photon absorption PDT, and sonodynamic therapy using ultrasound.

A purpurin-peptide derivative for selective killing of Gram-positive bacteria via insertion into cell membrane

A purpurin-peptide derivative was used for discriminating and killing bacteria based on the different surface components of bacteria.

Quaternary Ammonium Compounds: An Antimicrobial Mainstay and Platform for Innovation to Address Bacterial Resistance

Quaternary ammonium compounds (QACs) have represented one of the most visible and effective classes of disinfectants for nearly a century. With simple preparation, wide structural variety, and versatile incorporation into consumer products, there have been manifold developments and applications of these structures. Generally operating via disruption of one of the most fundamental structures in bacteria-the cell membrane-leading to cell lysis and bacterial death, the QACs were once thought to be impervious to resistance. Developments over the past decades, however, have shown this to be far from the truth. It is now known that a large family of bacterial genes (generally termed qac genes) encode efflux pumps capable of expelling many QAC structures from bacterial cells, leading to a decrease in susceptibility to QACs; methods of regulation of qac transcription are also understood. Importantly, qac genes can be horizontally transferred via plasmids to other bacteria and are often transmitted alongside other antibiotic-resistant genes; this dual threat represents a significant danger to human health. In this review, both QAC development and QAC resistance are documented, and possible strategies for addressing and overcoming QAC-resistant bacteria are discussed.