Anti-infective dyes in the time of COVID

Light-activated antimicrobial coatings: the great potential of organic photosensitizers

Contamination of inanimate surfaces with microorganisms is considered one of the routes for transmission of pathogens, which is a matter of concern not only in healthcare-related facilities, but also in public areas. Durable antimicrobial coatings have emerged as the one of most promising strategies for reducing the accumulation of microorganisms on high-touch surfaces. Light-activated antimicrobial layers are of particular interest for such a purpose, as they generate singlet oxygen and other reactive oxygen species that are effective against a broad spectrum of bacteria, viruses, and fungi. In this review, the antimicrobial coatings containing organic photosensitizers are discussed, focusing on the recent advances in the strategies for PSs' immobilization on solid surfaces. The review attempts to assess the advantages and limitations of those systems, and the challenges that still need to be overcome.

A Comparative Kinetic and Thermodynamic Adsorption Study of Methylene Blue and Its Analogue Dye on Filter Paper

A comparative adsorption study was carried out for methylene blue (MB) and its 3,7-bis(N,N-(2-hydroxyethyl)amino)-phenothiazinium dye analog (MBI). Batch experiments employed aqueous solutions and commercial filter paper. Out of seven kinetic models tested by means of four quality statistical indicators, the pseudo-second-order, the double-exponential, and the bi-linear Weber-Morris equations were best fits. For both dyes, the process was described as a succession of two diffusion-controlled steps. The Freundlich isotherm was chosen from 11 models describing a variety of mechanism assumptions. Physisorption was considered responsible for the dye removal from liquid. Adsorption of MB is thermodynamically favored, whereas that of MBI is sterically hindered. Both processes are exothermic and exhibit reduced randomness at the S-L interface. The paper was found suitable for retaining MB but served rather filtration/purification purposes for MBI.

Refining antimicrobial photodynamic therapy: effect of charge distribution and central metal ion in fluorinated porphyrins on effective control of planktonic and biofilm bacterial forms

Antibiotic resistance represents a pressing global health challenge, now acknowledged as a critical concern within the framework of One Health. Photodynamic inactivation of microorganisms (PDI) offers an attractive, non-invasive approach known for its flexibility, independence from microbial resistance patterns, broad-spectrum efficacy, and minimal risk of inducing resistance. Various photosensitizers, including porphyrin derivatives have been explored for pathogen eradication. In this context, we present the synthesis, spectroscopic and photophysical characteristics as well as antimicrobial properties of a palladium(II)-porphyrin derivative (PdF2POH), along with its zinc(II)- and free-base counterparts (ZnF2POH and F2POH, respectively). Our findings reveal that the palladium(II)-porphyrin complex can be classified as an excellent generator of reactive oxygen species (ROS), encompassing both singlet oxygen (Φ△ = 0.93) and oxygen-centered radicals. The ability of photosensitizers to generate ROS was assessed using a variety of direct (luminescence measurements) and indirect techniques, including specific fluorescent probes both in solution and in microorganisms during the PDI procedure. We investigated the PDI efficacy of F2POH, ZnF2POH, and PdF2POH against both Gram-negative and Gram-positive bacteria. All tested compounds proved high activity against Gram-positive species, with PdF2POH exhibiting superior efficacy, leading to up to a 6-log reduction in S. aureus viability. Notably, PdF2POH-mediated PDI displayed remarkable effectiveness against S. aureus biofilm, a challenging target due to its complex structure and increased resistance to conventional treatments. Furthermore, our results show that PDI with PdF2POH is more selective for bacterial than for mammalian cells, particularly at lower light doses (up to 5 J/cm2 of blue light illumination). This enhanced efficacy of PdF2POH-mediated PDI as compared to ZnF2POH and F2POH can be attributed to more pronounced ROS generation by palladium derivative via both types of photochemical mechanisms (high yields of singlet oxygen generation as well as oxygen-centered radicals). Additionally, PDI proved effective in eliminating bacteria within S. aureus-infected human keratinocytes, inhibiting infection progression while preserving the viability and integrity of infected HaCaT cells. These findings underscore the potential of metalloporphyrins, particularly the Pd(II)-porphyrin complex, as promising photosensitizers for PDI in various bacterial infections, warranting further investigation in advanced infection models.

Interaction of Methylene Blue with Severe Acute Respiratory Syndrome Coronavirus 2 Envelope Revealed by Molecular Modeling

Methylene blue has multiple antiviral properties against Severe Acute Respiratory Syndrome-related Coronavirus 2 (SARS-CoV-2). The ability of methylene blue to inhibit different stages of the virus life cycle, both in light-independent and photodynamic processes, is used in clinical practice. At the same time, the molecular aspects of the interactions of methylene blue with molecular components of coronaviruses are not fully understood. Here, we use Brownian dynamics to identify methylene blue binding sites on the SARS-CoV-2 envelope. The local lipid and protein composition of the coronavirus envelope plays a crucial role in the binding of this cationic dye. Viral structures targeted by methylene blue include the S and E proteins and negatively charged lipids. We compare the obtained results with known experimental data on the antiviral effects of methylene blue to elucidate the molecular basis of its activity against coronaviruses.

Can machine learning methods accurately predict the molar absorption coefficient of different classes of dyes?

In this article, we provide a convenient tool for all researchers to predict the value of the molar absorption coefficient for a wide number of dyes without any computer costs. The new model is based on RFR method (ALogPS, OEstate + Fragmentor + QNPR) and is able to predict the molar absorption coefficient with an accuracy (5-fold cross-validation RMSE) of 0.26 log unit. This accuracy was achieved due to the fact that the model was trained on data for more than 20,000 unique dye molecules. To our knowledge, this is the first model for predicting the molar absorption coefficient trained on such a large and diverse set of dyes. The model is available at https://ochem.eu/article/145413. We hope that the new model will allow researchers to predict dyes with practically significant spectral characteristics and verify existing experimental data.

Photodynamic inactivation of Staphylococcus aureus in the presence of aggregation-prone photosensitizers based on BODIPY used at submicromolar concentrations

Two new brominated BODIPYs (1 and 2) bearing amino acid-based chains (l-valine for 1, and dimethyl-l-lysine for 2) were synthesized and characterized. In organic solvents, 1 and 2 were fully soluble and showed the photophysical properties expected for brominated BODIPY dyes, including efficient generation of singlet oxygen (¹O₂), upon irradiation. In contrast, in aqueous media, both compounds were prone to aggregation and the photo-induced generation of ¹O₂ was halted. Despite the lack of generation of this reactive species in aqueous media (in cuvette), both 1 and 2 have positive antimicrobial Photodynamic Inactivation (aPDI) effect. The activity against gram-positive Staphylococcus aureus and gram-negative Escherichia coli was determined through the inactivation curves, with a total energy dose of 5.3 J/cm² (white light LED used as an energy source). Compound 2 was highly active against both gram-positive and gram-negative bacteria (3 log CFU/mL reduction was obtained at 0.16 μM for S. aureus and 2.5-5.0 μM for E. coli), whereas 1 was less effective to kill S. aureus (3 log CFU/mL at 0.32 μM) and ineffective for E. coli. The higher efficiency of 2, as compared to 1, to reduce the population of bacteria, can reside in the presence of a protonatable residue in 2, allowing a more effective interaction of this molecule with the cell walls of the microorganisms. In order to explain the lack of reactivity in pure aqueous media (in cuvette) and the contrasting good activity in the presence of bacterial cells it can be hypothesized that upon interaction with the walls of the microorganisms, the aggregated photosensitizers suffer a disaggregation process restoring the ability to generate ¹O₂, and hence leading to efficient photodynamic activity against these pathogenic microorganisms, in agreement with the similar effect observed recently for porphyrinoid photosensitizers.

Development of a Polymeric Film Entrapping Rose Bengal and Iodide Anion for the Light-Induced Generation and Release of Bactericidal Hydrogen Peroxide

A series of poly(2-hydroxyethyl methacrylate) (PHEMA) thin films entrapping photosensitizer Rose Bengal (RB) and tetrabutylammonium iodide (TBAI) have been synthetized. The materials have been characterized by means of Thermogravimetric Analysis (TGA), Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) and UV-vis Absorption spectroscopy. Irradiation of the materials with white light led to the generation of several bactericidal species, including singlet oxygen (¹O₂), triiodide anion (I₃^-) and hydrogen peroxide (H₂O₂). ¹O₂ production was demonstrated spectroscopically by reaction with the chemical trap 2,2'-(anthracene-9,10-diylbis(methylene))dimalonic acid (ABDA). In addition, the reaction of iodide anion with ¹O₂ yielded I₃^- inside the polymeric matrix. This reaction is accompanied by the formation of H₂O₂, which diffuses out the polymeric matrix. Generation of both I₃^- and H₂O₂ was demonstrated spectroscopically (directly in the case of triiodide by the absorption at 360 nm and indirectly for H₂O₂ using the xylenol orange test). A series of photodynamic inactivation assays were conducted with the synthesized polymers against Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa. Complete eradication (7 log₁₀ CFU/mL) of both bacteria occurred after only 5 min of white light irradiation (400-700 nm; total energy dose 24 J/cm²) of the polymer containing both RB and TBAI. The control polymer without embedded iodide (only RB) showed only marginal reductions of ca. 0.5 log₁₀ CFU/mL. The main novelty of the present investigation is the generation of three bactericidal species (¹O₂, I₃^- and H₂O₂) at the same time using a single polymeric material containing all the elements needed to produce such a bactericidal cocktail, although the most relevant antimicrobial activity is shown by H₂O₂. This experimental approach avoids multistep protocols involving a final step of addition of I^-, as described previously for other assays in solution.

Photodynamic Inactivation of SARS-CoV-2 Infectivity and Antiviral Treatment Effects In Vitro

Despite available vaccines, antibodies and antiviral agents, the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) pandemic still continues to cause severe disease and death. Current treatment options are limited, and emerging new mutations are a challenge. Thus, novel treatments and measures for prevention of viral infections are urgently required. Photodynamic inactivation (PDI) is a potential treatment for infections by a broad variety of critical pathogens, including viruses. We explored the infectiousness of clinical SARS-CoV-2 isolates in Vero cell cultures after PDI-treatment, using the photosensitizer Tetrahydroporphyrin-tetratosylate (THPTS) and near-infrared light. Replication of viral RNA (qPCR), viral cytopathic effects (microscopy) and mitochondrial activity were assessed. PDI of virus suspension with 1 µM THPTS before infection resulted in a reduction of detectable viral RNA by 3 log levels at day 3 and 6 after infection to similar levels as in previously heat-inactivated virions (<99.9%; p < 0.05). Mitochondrial activity, which was significantly reduced by viral infection, was markedly increased by PDI to levels similar to uninfected cell cultures. When applying THPTS-based PDI after infection, a single treatment had a virus load-reducing effect only at a higher concentration (3 µM) and reduced cell viability in terms of PDI-induced toxicity. Repeated PDI with 0.3 µM THPTS every 4 h for 3 d after infection reduced the viral load by more than 99.9% (p < 0.05), while cell viability was maintained. Our data demonstrate that THPTS-based antiviral PDI might constitute a promising approach for inactivation of SARS-CoV-2. Further testing will demonstrate if THPTS is also suitable to reduce the viral load in vivo.