A new acridine-based photosensitizer with ultra-low light requirement efficiently inactivates carbapenem-resistant Acinetobacter baumannii and methicillin-resistant Staphylococcus aureus and degrades their antibiotic resistance genes

The spread of antibiotic resistant pathogens and antibiotic resistance genes (ARGs) in the environment poses a serious threat to public health. However, existing methods are difficult to effectively remove antibiotic resistant pathogens and ARGs from the environment. In this study, we synthesized a new acridine-based photosensitizer, 2,7-dibromo-9-mesityl-10-methylacridinium perchlorate (YM-3), by the heavy atom effect, which could photodynamically inactivate antibiotic resistant pathogens and reduce ARGs by generating singlet oxygen (¹O₂) in an aqueous environment. The ¹O₂ yield of YM-3 was 4.9 times that of its modified precursor. YM-3 could reduce the culturable number and even the viable counts of methicillin-resistant Staphylococcus aureus and carbapenem-resistant Acinetobacter baumannii to 0 (inactivation rate > 99.99999%) after 2 and 8 h of low-intensity blue light (15 W/m²) irradiation, respectively. After 20 h of light exposure, the copy numbers of ARGs in both bacteria were reduced by 5.80 and 4.48 log, respectively, which might indicate that ARGs had been degraded. In addition, YM-3 still had an efficient bactericidal effect after five inactivation cycle. These characteristics of ultra-low light intensity requirement and efficient bactericidal ability make YM-3 have good application prospects for disinfection in indoor and sunlight environment.

Min Oscillations as Real-time Reporter of Sublethal Effects in Photodynamic Treatment of Bacteria

The Min protein system is a cell division regulator in Escherichia coli. Under normal growth conditions, MinD is associated with the membrane and undergoes pole-to-pole oscillations. The period of these oscillations has been previously proposed as a reporter for the bacterial physiological state at the single-cell level and has been used to monitor the response to sublethal challenges from antibiotics, temperature, or mechanical fatigue. Using real-time single-cell fluorescence imaging, we explore here the effect of photodynamic treatment on MinD oscillations. Irradiation of bacteria in the presence of the photosensitizer methylene blue disrupts the MinD oscillation pattern depending on its concentration. In contrast to antibiotics, which slow down the oscillation, photodynamic treatment results in an abrupt interruption, reflecting divergent physiological mechanisms leading to bacterial death. We show that MinD oscillations are sensitive to mild photodynamic effects that are overlooked by traditional methods, expanding the toolbox for mechanistic studies in antimicrobial photodynamic therapy.

Effect of Photosensitization Mediated by Curcumin on Carotenoid and Aflatoxin Content in Different Maize Varieties

Mycotoxins are naturally occurring toxins produced by certain types of fungi that contaminate food and feed, posing serious health risks to human and livestock. This study evaluated the combination of blue light with curcumin to inactivate Aspergillus flavus spores, its effect on aflatoxin B1 (AFB1) production and maintaining carotenoid content in three maize varieties. The study was first conducted in vitro, and the spore suspensions (104 CFU·mL−1) were treated with four curcumin concentrations (25 and 50 µM in ethanol, 1000 and 1250 µM in propylene glycol) and illuminated at different light doses from 0 to 130.3 J·cm−2. The photoinactivation efficiency was light-dose dependent with the highest photoinactivation of 2.3 log CFU·mL−1 achieved using 1000 µM curcumin at 104.2 J·cm−2. Scanning electron microscopy revealed cell wall deformations as well as less density in photosensitized cells. Photosensitization of maize kernels gave rise to a complete reduction in the viability of A. flavus and therefore inhibition of AFB1 production, while no significant (p > 0.05) effect was observed using either light or curcumin. Moreover, photosensitization did not affect the carotenoids in all the studied maize varieties. The results suggest that photosensitization is a green alternative preservation technique to decontaminate maize kernels and reduce consumer exposure to AFB1 without any effect on carotenoid content.

Light-Activated Rhenium Complexes with Dual Mode of Action against Bacteria

New antibiotics and innovative approaches to kill drug-resistant bacteria are urgently needed. Metal complexes offer access to alternative modes of action but have only sparingly been investigated in antibacterial drug discovery. We have developed a light-activated rhenium complex with activity against drug-resistant S. aureus and E. coli. The activity profile against mutant strains combined with assessments of cellular uptake and synergy suggest two distinct modes of action.

Protein Expression Modifications in Phage-Resistant Mutants of Aeromonas salmonicida after AS-A Phage Treatment

The occurrence of infections by pathogenic bacteria is one of the main sources of financial loss for the aquaculture industry. This problem often cannot be solved with antibiotic treatment or vaccination. Phage therapy seems to be an alternative environmentally-friendly strategy to control infections. Recognizing the cellular modifications that bacteriophage therapy may cause to the host is essential in order to confirm microbial inactivation, while understanding the mechanisms that drive the development of phage-resistant strains. The aim of this work was to detect cellular modifications that occur after phage AS-A treatment in A. salmonicida, an important fish pathogen. Phage-resistant and susceptible cells were subjected to five successive streak-plating steps and analysed with infrared spectroscopy, a fast and powerful tool for cell study. The spectral differences of both populations were investigated and compared with a phage sensitivity profile, obtained through the spot test and efficiency of plating. Changes in protein associated peaks were found, and these results were corroborated by 1-D electrophoresis of intracellular proteins analysis and by phage sensitivity profiles. Phage AS-A treatment before the first streaking-plate step clearly affected the intracellular proteins expression levels of phage-resistant clones, altering the expression of distinct proteins during the subsequent five successive streak-plating steps, making these clones recover and be phenotypically more similar to the sensitive cells.

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

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

A Perspective on the Trends and Challenges Facing Porphyrin-Based Anti-Microbial Materials

The emergence of multidrug resistant bacterium threatens to unravel global healthcare systems, built up over centuries of medical research and development. Current antibiotics have little resistance against this onslaught as bacterium strains can quickly evolve effective defense mechanisms. Fortunately, alternative therapies exist and, at the forefront of research lays the photodynamic inhibition approach mediated by porphyrin based photosensitizers. This review will focus on the development of various porphyrins compounds and their incorporation as small molecules, into polymers, fibers and thin films as practical therapeutic agents, utilizing photodynamic therapy to inhibit a wide spectrum of bacterium. The use of photodynamic therapy of these porphyrin molecules are discussed and evaluated according to their electronic and bulk material effect on different bacterium strains. This review also provides an insight into the general direction and challenges facing porphyrins and derivatives as full-fledged therapeutic agents and what needs to be further done in order to be bestowed their rightful and equal status in modern medicine, similar to the very first antibiotic; penicillin itself. It is hoped that, with this perspective, new paradigms and strategies in the application of porphyrins and derivatives will progressively flourish and lead to advances against disease.

Overall biochemical changes in bacteria photosensitized with cationic porphyrins monitored by infrared spectroscopy

BACKGROUND

Photodynamic inactivation of micro-organisms is a promising nonantibiotic multitarget approach to treat localized and superficial infections through oxidative stress. Herein, the changes occurring on major cellular components of Escherichia coli and Staphylococcus warneri, induced by photosensitization with cationic porphyrins (Tri-Py(+)-Me-PF and Tetra-Py(+)-Me) and white light, were monitored by infrared spectroscopy.

RESULTS

In E. coli, most of the changes occurred on proteins and lipids, suggesting a key effect on lipopolysaccharides in the first irradiation times. In S. warneri, proteins were the major molecular targets of oxidative damage but phospholipids and polysaccharides were also affected.

CONCLUSION

Infrared spectroscopy is a very interesting tool to monitor biochemical changes induced by photosensitization in bacteria and also to infer on its mechanism of action.

Carboranyl-Chlorin e6 as a Potent Antimicrobial Photosensitizer

Antimicrobial photodynamic inactivation is currently being widely considered as alternative to antibiotic chemotherapy of infective diseases, attracting much attention to design of novel effective photosensitizers. Carboranyl-chlorin-e₆ (the conjugate of chlorin e₆ with carborane), applied here for the first time for antimicrobial photodynamic inactivation, appeared to be much stronger than chlorin e₆ against Gram-positive bacteria, such as Bacillus subtilis, Staphyllococcus aureus and Mycobacterium sp. Confocal fluorescence spectroscopy and membrane leakage experiments indicated that bacteria cell death upon photodynamic treatment with carboranyl-chlorin-e₆ is caused by loss of cell membrane integrity. The enhanced photobactericidal activity was attributed to the increased accumulation of the conjugate by bacterial cells, as evaluated both by centrifugation and fluorescence correlation spectroscopy. Gram-negative bacteria were rather resistant to antimicrobial photodynamic inactivation mediated by carboranyl-chlorin-e₆. Unlike chlorin e₆, the conjugate showed higher (compared to the wild-type strain) dark toxicity with Escherichia coli ΔtolC mutant, deficient in TolC-requiring multidrug efflux transporters.

In vitro photodynamic inactivation effects of Ru(II) complexes on clinical methicillin-resistant Staphylococcus aureus planktonic and biofilm cultures

Photosensitizers (PSs) combined with light are able to generate antimicrobial effects. Ru(II) complexes have been recognized as a novel class of PSs. In this study, we investigated the effectiveness of photodynamic inactivation (PDI) mediated by three Ru(II) polypyridine complexes, 1-3, against four isolates of clinical methicillin-resistant Staphylococcus aureus (MRSA-1, MRSA-2, MRSA-3 and MRSA-4). In PDI of a planktonic culture of MRSA-1, compound 3 showed the highest efficacy, likely owing to its advantageous light absorption, (1) O2 quantum yield and bacterial cellular binding. The PDI efficacy of 3 was further evaluated against all other strains and MRSA-1 biofilms. At appropriate PS concentrations, viability reduction of 100% or 96.83% was observed in planktonic or biofilm forms of MRSA, respectively. The mechanisms of action were investigated using negative staining transmission electron microscopy (TEM), confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). It was demonstrated that PDI of planktonic bacteria was achieved primarily through damage to the cell envelope. Biofilms were eliminated through both the destruction of their structure and inactivation of the individual bacterial cells. In conclusion, Ru(II) complexes, especially 3, are potential candidates for the effective photodynamic control of MRSA infections.

An insight on bacterial cellular targets of photodynamic inactivation

The emergence of microbial resistance is becoming a global problem in clinical and environmental areas. As such, the development of drugs with novel modes of action will be vital to meet the threats created by the rise in microbial resistance. Microbial photodynamic inactivation is receiving considerable attention for its potentialities as a new antimicrobial treatment. This review addresses the interactions between photosensitizers and bacterial cells (binding site and cellular localization), the ultrastructural, morphological and functional changes observed at initial stages and during the course of photodynamic inactivation, the oxidative alterations in specific molecular targets, and a possible development of resistance.

Antimicrobial and anti-biofilm effect of a novel BODIPY photosensitizer against Pseudomonas aeruginosa PAO1

Photodynamic therapy (PDT) combines the use of organic dyes (photosensitizers, PSs) and visible light in order to elicit a photo-oxidative stress which causes bacterial death. GD11, a recently synthesized PS belonging to the boron-dipyrromethene (BODIPY) class, was demonstrated to be efficient against planktonic cultures of Pseudomonas aeruginosa, causing a 7 log unit reduction of viable cells when administered at 2.5 μM. The effectiveness of GD11 against P. aeruginosa biofilms grown in flow-cells and microtiter trays was also demonstrated. Confocal laser scanning microscopy of flow-cell-grown biofilms suggests that the treatment has a biocidal effect against bacterial biofilm cells.

Photodynamic oxidation of Escherichia coli membrane phospholipids: new insights based on lipidomics

RATIONALE

The irreversible oxidation of biological molecules, such as lipids, can be achieved with a photosensitizing agent and subsequent exposure to light, in the presence of molecular oxygen. Although lipid peroxidation is an important toxicity mechanism in bacteria, the alterations caused by the photodynamic therapy on bacterial phospholipids are still unknown. In this work, we studied the photodynamic oxidation of Escherichia coli membrane phospholipids using a lipidomic approach.

METHODS

E. coli ATCC 25922 were irradiated for 90 min with white light (4 mW cm(-2), 21.6 J cm(-2)) in the presence of a tricationic porphyrin [(5,10,15-tris(1-methylpyridinium-4-yl)-20-(pentafluorophenyl)porphyrin triiodide, Tri-Py(+)-Me-PF]. Lipids were extracted and separated by thin-layer chromatography. Phospholipid classes were quantified by phosphorus assay and analyzed by electrospray ionization tandem mass spectrometry. Fatty acids were analyzed by gas chromatography. Quantification of lipid hydroperoxides was performed by FOX2 assay. Analysis of the photodynamic oxidation of a phospholipid standard was also performed.

RESULTS

Our approach allowed us to see that the photodynamic treatment induced the formation of a high amount of lipid hydroperoxides in the E. coli lipid extract. Quantification of fatty acids revealed a decrease in the unsaturated C16:1 and C18:1 species suggesting that oxidative modifications were responsible for their variation. It was also observed that photosensitization induced the oxidation of phosphatidylethanolamines with C16:1, C18:1 and C18:2 fatty acyl chains, with formation of hydroxy and hydroperoxy derivatives.

CONCLUSIONS

Membrane phospholipids of E. coli are molecular targets of the photodynamic effect induced by Tri-Py(+) -Me-PF. The overall change in the relative amount of unsaturated fatty acids and the formation of PE hydroxides and hydroperoxides evidence the damages in bacterial phospholipids caused by this lethal treatment.

Photoinduced antibacterial activity of two dicationic 5,15-diarylporphyrins

Antimicrobial photodynamic treatment combines the use of photosensitizers (PSs) and visible light to kill bacterial cells. Cationic porphyrins are PSs largely used against bacteria and, among them, those featuring one positive charge on each of the 5,10,15,20-tetraaryl substituent (tetracationic) are the most used. The aim of this study was to synthesize two dicationic 5,15-di(N-alkyl-4-pyridyl)porphyrins, bearing methyl (PS 3) and benzyl (PS 4) N-alkylating groups, and to compare the efficiency in antibacterial photodynamic treatment, upon irradiation with a halogen-tungsten white lamp. The killing efficiency of the PS 4 was constantly found higher than that of the PS 3 against both pure and mixed cultures of laboratory model microorganisms as well as against wild wastewater microflora. The two PSs are comparable as regards singlet oxygen generation, but show a different repartition coefficient; the more lipophilic benzylated PS 4 shows a better interaction with the bacterial cells than the methylated one (PS 3). The data support the hypothesis that an efficient PS-cell binding is required to obtain significant effects. A correlation among cell binding, photoinactivation and PS lipophilicity is suggested.

A genetically-encoded photosensitiser demonstrates killing of bacteria by purely endogenous singlet oxygen

TagRFP, a fluorescent protein capable of photosensitizing the production of singlet oxygen, was expressed in E. coli. Subsequent exposure to green light induced bacterial cell death in a light-dose dependent manner. It is demonstrated for the first time that intracellular singlet oxygen is sufficient to kill bacteria.

Antimicrobial peptide-modified liposomes for bacteria targeted delivery of temoporfin in photodynamic antimicrobial chemotherapy

Photodynamic antimicrobial chemotherapy (PACT) and antimicrobial peptides (AMPs) are two promising strategies to combat the increasing prevalence of antibiotic-resistant bacteria. To take advantage of these two strategies, we integrated a novel antimicrobial peptide (WLBU2) and a potent generation II photosensitizer (temoporfin) into liposomes by preparing WLBU2-modified liposomes, aiming at bacteria targeted delivery of temoporfin for PACT. WLBU2 was successfully coupled to temoporfin-loaded liposomes using a functional phospholipid. The delivery of temoporfin to bacteria was confirmed by fluorescence microscopy and flow cytometry, thus demonstrating that more temoporfin was delivered to bacteria by WLBU2-modified liposomes than by unmodified liposomes. Consequently, the WLBU2-modified liposomes eradicated all methicillin-resistant Staphylococcus aureus (MRSA) and induced a 3.3 log(10) reduction of Pseudomonas aeruginosa in the in vitro photodynamic inactivation test. These findings demonstrate that the use of AMP-modified liposomes is promising for bacteria-targeted delivery of photosensitizers and for improving the PACT efficiency against both gram-positive and gram-negative bacteria in the local infections.