Development of Antimicrobial Phototreatment Tolerance: Why the Methodology Matters

Bacterial resistance to antimicrobial photodynamic therapy: A critical update

Bacterial antibiotic resistance is one of the most significant challenges for public health. The increase in bacterial resistance, mainly due to microorganisms harmful to health, and the need to search for alternative treatments to contain infections that cannot be treated by conventional antibiotic therapy has been aroused. An alternative widely studied in recent decades is antimicrobial photodynamic therapy (aPDT), a treatment that can eliminate microorganisms through oxidative stress. Although this therapy has shown satisfactory results in infection control, it is still controversial in the scientific community whether bacteria manage to develop resistance after successive applications of aPDT. Thus, this work provides an overview of the articles that performed successive aPDT applications in models using bacteria published since 2010, focusing on sublethal dose cycles, highlighting the main PSs tested, and addressing the possible mechanisms for developing tolerance or resistance to aPDT, such as efflux pumps, biofilm formation, OxyR and SoxRS systems, catalase and superoxide dismutase enzymes and quorum sensing.

Comparative assessment of UV-C radiation and non-thermal plasma for inactivation of foodborne fungal spores suspension in vitro

Fungal contamination poses a persistent challenge to industries, particularly in food, healthcare, and clinical sectors, due to the remarkable resilience of fungi in withstanding conventional control methods. In this context, our research delves into the comparative efficacy of UV radiation and non-thermal plasma (NTP) on key foodborne fungal contaminants - Alternaria alternata, Aspergillus niger, Fusarium culmorum, and Fusarium graminearum. The study examined the impact of varying doses of UV radiation on the asexual spores of all mentioned fungal strains. Simultaneously, the study compared the effects of UV radiation and NTP on the metabolic activity of cells after spore germination and their subsequent germination ability. The results revealed that UV-C radiation (254 nm) did not significantly suppress the metabolic activity of cells after spore germination. In contrast, NTP exhibited almost 100% effectiveness on both selected spores and their subsequent germination, except for A. niger. In the case of A. niger, the effectiveness of UV-C and NTP was nearly comparable, showing only a 35% decrease in metabolic activity after 48 hours of germination, while the other strains (A. alternata, F. culmorum, F. graminearum) exhibited a reduction of more than 95%. SEM images illustrate the morphological changes in structure of all tested spores after both treatments. This study addresses a crucial gap in existing literature, offering insights into the adaptation possibilities of treated cells and emphasizing the importance of considering exposure duration and nutrient conditions (introduction of fresh medium). The results highlighted the promising antimicrobial potential of NTP, especially for filamentous fungi, paving the way for enhanced sanitation processes with diverse applications.

Evaluation of visible light and natural photosensitizers against Staphylococcus epidermidis and Staphylococcus saprophyticus planktonic cells and biofilm

Antimicrobial photoinactivation (API) has shown some promise in potentially treating different nosocomial bacterial infections, however, its application on staphylococci, especially other than Staphylococcus aureus or methicillin-resistant S. aureus (MRSA) species is still limited. Although S. aureus is a well-known and important nosocomial pathogen, several other species of the genus, particularly coagulase-negative Staphylococcus (CNS) species such as Staphylococcus epidermidis and Staphylococcus saprophyticus, can also cause healthcare-associated infections and foodborne intoxications. CNS are often involved in resilient biofilm formation on medical devices and can cause infections in patients with compromised immune systems or those undergoing invasive procedures. In this study, the effects of chlorophyllin and riboflavin-mediated API on S. epidermidis and S. saprophyticus planktonic cells and biofilm are demonstrated for the first time. Based on the residual growth determination and metabolic reduction ability changes, higher inactivating efficiency of chlorophyllin-mediated API was determined against the planktonic cells of both tested species of bacteria and against S. saprophyticus biofilm. Some insights on whether aqueous solutions of riboflavin and chlorophyllin, when illuminated with optimal exciting wavelength (440 nm and 402 nm, respectively) generate O2-•, are also provided in this work.

Exploring Non-Thermal Plasma and UV Radiation as Biofilm Control Strategies against Foodborne Filamentous Fungal Contaminants

In recent years, non-thermal plasma (NTP) has emerged as a promising tool for decontamination and disinfection within the food industry. Given the increasing resistance of microbial biofilms to conventional disinfectants and their adverse environmental effects, this method has significant potential for eliminating biofilm formation or mitigating the metabolic activity of grown biofilms. A comparative study was conducted evaluating the efficacy of UV radiation and NTP in eradicating mature biofilms of four common foodborne filamentous fungal contaminants: Alternaria alternata, Aspergillus niger, Fusarium culmorum, and Fusarium graminearum. The findings reveal that while UV radiation exhibits variable efficacy depending on the duration of exposure and fungal species, NTP induces substantial morphological alterations in biofilms, disrupting hyphae, and reducing extracellular polymeric substance production, particularly in A. alternata and F. culmorum. Notably, scanning electron microscopy analysis demonstrates significant disruption of the hyphae in NTP-treated biofilms, indicating its ability to penetrate the biofilm matrix, which is a promising outcome for biofilm eradication strategies. The use of NTP could offer a more environmentally friendly and potentially more effective alternative to traditional disinfection methods.

Thermal-Accelerated Urease-Driven Bowl-Like Polydopamine Nanorobot for Targeted Photothermal/Photodynamic Antibiotic-Free Antibacterial Therapy

The problem of antibiotic resistance seriously affects the treatment of bacterial infections, so there is an urgent need to develop novel antibiotic-independent antimicrobial strategies. Herein, a urease-driven bowl-like mesoporous polydopamine nanorobot (MPDA@ICG@Ur@Man) based on single-wavelength near-infrared (NIR) remote photothermal acceleration to achieve antibiotic-free phototherapy(photothermal therapy, PTT, plus photodynamic therapy, PDT) is first reported. The smart nanorobots can perform active movement by decomposing urea to produce carbon dioxide and ammonia. Particularly, the elevated local temperature during PTT can increase urease activity to enhance the autonomous movement and thus increase the contact between the antimicrobial substance and bacteria. Compared with a nanomotor propelled by urea only, the diffusion coefficient (De) of photothermal-accelerated nanorobots is increased from 1.10 to 1.26 µm2 s-1. More importantly, urease-driven bowl-like nanorobots with photothermal enhancement can specifically identify Escherichia coli (E. coli) and achieve simultaneous PTT/PDT at a single wavelength with 99% antibactericidal activity in vitro. In a word, the urease-driven bowl-like nanorobots guided by photothermal-accelerated strategy could provide a novel perspective for increasing PTT/PDT antibacterial therapeutic efficacy and be promising for various antibiotic-free sterilization applications.

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 Activity of Chlorophyllin and Polyethylenimine on Pseudomonas aeruginosa Planktonic, Biofilm and Persister Cells

Antimicrobial photodynamic inactivation is considered a promising antimicrobial approach that may not develop resistance in the near future. Here, we investigate the influence of the photosensitizer chlorophyllin (CHL) and the cationic permeabilizer polyethylenimine (PEI), exposed to a red light-emitting diode, on the human pathogen Pseudomonas aeruginosa free-living planktonic cells, the sessile biofilm and persister cells. The broth microdilution checkerboard method was used to test antimicrobial susceptibility. As a substrate for biofilms, the Calgary biofilm device was used, and the quantification of the biofilm biomass was carried out using a crystal violet assay. Serine hydroxamate was used for the induction of persisters. Our findings reveal that PEI ameliorates the antimicrobial activity of CHL against P. aeruginosa planktonic and biofilm states, and the concentration required to eradicate the bacteria in the biofilm is more than fourfold that is required to eradicate planktonic cells. Interestingly, the persister cells are more susceptible to CHL/PEI (31.25/100 µg mL-1) than the growing cells by 1.7 ± 0.12 and 0.4 ± 0.1 log10 reduction, respectively, after 15 min of illumination. These data demonstrate that CHL excited with red light together with PEI is promising for the eradication of P. aeruginosa, and the susceptibility of P. aeruginosa to CHL/PEI is influenced by the concentrations and the exposure time.

The Parameters Affecting Antimicrobial Efficiency of Antimicrobial Blue Light Therapy: A Review and Prospect

Antimicrobial blue light (aBL) therapy is a novel non-antibiotic antimicrobial approach which works by generating reactive oxygen species. It has shown excellent antimicrobial ability to various microbial pathogens in many studies. However, due to the variability of aBL parameters (e.g., wavelength, dose), there are differences in the antimicrobial effect across different studies, which makes it difficult to form treatment plans for clinical and industrial application. In this review, we summarize research on aBL from the last six years to provide suggestions for clinical and industrial settings. Furthermore, we discuss the damage mechanism and protection mechanism of aBL therapy, and provide a prospect about valuable research fields related to aBL therapy.

Antimicrobial photodynamic inactivation as an alternative approach to inhibit the growth of Cronobacter sakazakii by fine-tuning the activity of CpxRA two-component system

Cronobacter sakazakii is an opportunistic foodborne pathogen primarily found in powdered infant formula (PIF). To date, it remains challenging to control the growth of this ubiquitous bacterium. Herein, antimicrobial photodynamic inactivation (aPDI) was first employed to inactivate C. sakazakii. Through 460 nm light irradiation coupled with hypocrellin B, the survival rate of C. sakazakii was diminished by 3~4 log. The photokilling effect was mediated by the attenuated membrane integrity, as evidenced by PI staining. Besides, scanning electron microscopy showed the deformed and aggregated cell cluster, and intracellular ROS was augmented by 2~3 folds when light doses increase. In addition to planktonic cells, the biofilm formation of C. sakazakii was also affected, showing an OD_590nm decline from 0.85 to 0.25. In terms of molecular aspects, a two-component system called CpxRA, along with their target genes, was deregulated during illumination. Using the knock-out strain of ΔCpxA, the bacterial viability was reduced by 2 log under aPDI, a wider gap than the wildtype strain. Based on the promoted expression of CpxR and OmpC, aPDI is likely to play its part through attenuating the function of CpxRA-OmpC pathway. Finally, the aPDI system was applied to PIF, and C. sakazakii was inactivated under various desiccated or heated storage conditions. Collectively, aPDI serves as an alternative approach to decontaminate C. sakazakii, providing a new strategy to reduce the health risks caused by this prevalent foodborne pathogen.

The Effect of Sub-Lethal Successive Applications of Photodynamic Therapy on Candida albicans Biofilm Depends on the Photosensitizer

This study aimed to evaluate the potential of successive applications of sub-lethal doses of the antimicrobial photodynamic therapy (aPDT) mediated by Photodithazine^® (PDZ) and curcumin (CUR) associated with LED in the viability, reactive oxygen species (ROS) production, and gene expression of Candida albicans. The microbial assays were performed using planktonic cultures and biofilms. Ten successive applications (Apl#) were performed: aPDT (P+L+; C+L+), photosensitizer (P+L-; C+L-), and LED (P-L+; C-L+). Control groups were used (P-L-; C-L-). The viability of C. albicans was determined by cultivating treated cultures on agar plates with or without fluconazole (FLU). In addition, the ROS detection and expression of SOD1, CAP1, and ERG11 genes were determined. For planktonic cultures, no viable colonies were observed after Apl#3 (without FLU) and Apl#2 (with FLU) for either photosensitizer. Biofilm treated with P+L+ resulted in the absence of cell viability after Apl#7, while C+L+ showed ~1.40 log₁₀ increase in cell viability after Apl#2, regardless of FLU. For both photosensitizers, after the last application with viable colonies, the production of ROS was higher in the biofilms than in the planktonic cultures, and SOD1 expression was the highest in P+L+. A reduction of CAP1 and ERG11 expression occurred after P+L+, regardless of FLU. C+L+ had a higher level of ROS, and the treatments were non-significant for gene expression. Sub-lethal doses of aPDT mediated by CUR could induce C. albicans resistance in biofilms, while C. albicans cells in biofilms were susceptible to aPDT mediated by PDZ.

Visible light-regulated cationic polymer coupled with photodynamic inactivation as an effective tool for pathogen and biofilm elimination

Background

Pathogenic microorganism pollution has been a challenging public safety issue, attracting considerable scientific interest. A more problematic aspect of this phenomenon is that planktonic bacteria exacerbate biofilm formation. There is an overwhelming demand for developing ultra-efficient, anti-drug resistance, and biocompatibility alternatives to eliminate stubborn pathogenic strains and biofilms.

Results

The present work aims to construct a visible light-induced anti-pathogen agents to ablate biofilms using the complementary merits of ROS and cationic polymers. The photosensitizer chlorin e6-loaded polyethyleneimine-based micelle (Ce6-TPP-PEI) was constructed by an amphiphilic dendritic polymer (TPP-PEI) and physically loaded with photosensitizer chlorin e6. Cationic polymers can promote the interaction between photosensitizer and Gram-negative bacteria, resulting in enhanced targeting of PS and lethality of photodynamic therapy, and remain active for a longer duration to prevent bacterial re-growth when the light is turned off. As expected, an eminent antibacterial effect was observed on the Gram-negative Escherichia coli, which is usually insensitive to photosensitizers. Surprisingly, the cationic polymer and photodynamic combination also exert significant inhibitory and ablative effects on fungi and biofilms. Subsequently, cell hemolysis assessments suggested its good biocompatibility.

Conclusions

Given the above results, the platform developed in this work is an efficient and safe tool for public healthcare and environmental remediation.

Sweet light o' mine: Photothermal and photodynamic inactivation of tenacious pathogens using conjugated polymers

Each year a rising number of infections can not be successfully treated owing to the increasing pandemic of antibiotic resistant pathogens. The global shortage of innovative antibiotics fuels the emergence and spread of drug resistant microbes. Basic research, development, and applications of alternative therapies are urgently needed. Since the 90´s, light-mediated therapies have promised to be the next frontier combating multidrug-resistance microbes. These platforms have demonstrated to be a reliable, rapid, and efficient alternative to eliminate tenacious pathogens while avoiding the emergence of resistance mechanisms. Among the materials showing antimicrobial activity triggered by light, conjugated polymers (CPs) have risen as the most promising option to tackle this complex situation. These materials present outstanding characteristics such as high absorption coefficients, great photostability, easy processability, low cytotoxicity, among others, turning them into a powerful class of photosensitizer (PS)/photothermal agent (PTA) materials. Herein, we summarize and discuss the advances in the field of CPs with applications in photodynamic inactivation and photothermal therapy towards bacteria elimination. Additionally, a section of current challenges and needs in terms of well-defined benchmark experiments and conditions to evaluate the efficiency of phototherapies is presented.

Application of Nanomaterials in the Prevention, Detection, and Treatment of Methicillin-Resistant Staphylococcus aureus (MRSA)

Due to differences in geographic surveillance systems, chemical sanitization practices, and antibiotic stewardship (AS) implementation employed during the COVID-19 pandemic, many experts have expressed concerns regarding a future surge in global antimicrobial resistance (AMR). A potential beneficiary of these differences is the Gram-positive bacteria MRSA. MRSA is a bacterial pathogen with a high potential for mutational resistance, allowing it to engage various AMR mechanisms circumventing conventional antibiotic therapies and the host’s immune response. Coupled with a lack of novel FDA-approved antibiotics reaching the clinic, the onus is on researchers to develop alternative treatment tools to mitigate against an increase in pathogenic resistance. Mitigation strategies can take the form of synthetic or biomimetic nanomaterials/vesicles employed in vaccines, rapid diagnostics, antibiotic delivery, and nanotherapeutics. This review seeks to discuss the current potential of the aforementioned nanomaterials in detecting and treating MRSA.

Antimicrobial blue light: A 'Magic Bullet' for the 21st century and beyond?

Over the past decade, antimicrobial blue light (aBL) at 400 - 470 nm wavelength has demonstrated immense promise as an alternative approach for the treatment of multidrug-resistant infections. Since our last review was published in 2017, there have been numerous studies that have investigated aBL in terms of its, efficacy, safety, mechanism, and propensity for resistance development. In addition, researchers have looked at combinatorial approaches that exploit aBL and other traditional and non-traditional therapeutics. To that end, this review aims to update the findings from numerous studies that capitalize on the antimicrobial effects of aBL, with a focus on: efficacy of aBL against different microbes, identifying endogenous chromophores and targets of aBL, Resistance development to aBL, Safety of aBL against host cells, and Synergism of aBL with other agents. We will also discuss our perspective on the future of aBL.

Genetic Factors Affect the Survival and Behaviors of Selected Bacteria during Antimicrobial Blue Light Treatment

Antimicrobial resistance is a global, mounting and dynamic issue that poses an immediate threat to human, animal, and environmental health. Among the alternative antimicrobial treatments proposed to reduce the external use of antibiotics is electromagnetic radiation, such as blue light. The prevailing mechanistic model is that blue light can be absorbed by endogenous porphyrins within the bacterial cell, inducing the production of reactive oxygen species, which subsequently inflict oxidative damages upon different cellular components. Nevertheless, it is unclear whether other mechanisms are involved, particularly those that can affect the efficacy of antimicrobial blue light treatments. In this review, we summarize evidence of inherent factors that may confer protection to a selected group of bacteria against blue light-induced oxidative damages or modulate the physiological characteristics of the treated bacteria, such as virulence and motility. These include descriptions of three major photoreceptors in bacteria, chemoreceptors, SOS-dependent DNA repair and non-SOS protective mechanisms. Future directions are also provided to assist with research efforts to increase the efficacy of antimicrobial blue light and to minimize the development of blue light-tolerant phenotypes.

Can microorganisms develop resistance against light based anti-infective agents?

Recently, there have been increasing numbers of publications illustrating the potential of light-based antimicrobial therapies to combat antimicrobial resistance. Several modalities, in particular, which have proven antimicrobial efficacy against a wide range of pathogenic microbes include: photodynamic therapy (PDT), ultraviolet light (UVA, UVB and UVC), and antimicrobial blue light (aBL). Using these techniques, microbial cells can be inactivated rapidly, either by inducing reactive oxygen species that are deleterious to the microbial cells (PDT, aBL and UVA) or by causing irreversible DNA damage via direct absorption (UVB and UVC). Given the multi-targeted nature of light-based antimicrobial modalities, it has been hypothesised that resistance development to these approaches is highly unlikely. Furthermore, with the exception of a small number of studies, it has been found that resistance to light based anti-infective agents appears unlikely, irrespective of the modality in question. The concurrent literature however stipulates, that further studies should incorporate standardised microbial tolerance assessments for light-based therapies to better assess the reproducibility of these observations.

Applications of Cold Atmospheric Pressure Plasma Technology in Medicine, Agriculture and Food Industry

In recent years, cold atmospheric pressure plasma (CAPP) technology has received substantial attention due to its valuable properties including operational simplicity, low running cost, and environmental friendliness. Several different gases (air, nitrogen, helium, argon) and techniques (corona discharge, dielectric barrier discharge, plasma jet) can be used to generate plasma at atmospheric pressure and low temperature. Plasma treatment is routinely used in materials science to modify the surface properties (e.g., wettability, chemical composition, adhesion) of a wide range of materials (e.g., polymers, textiles, metals, glasses). Moreover, CAPP seems to be a powerful tool for the inactivation of various pathogens (e.g., bacteria, fungi, viruses) in the food industry (e.g., food and packing material decontamination, shelf life extension), agriculture (e.g., disinfection of seeds, fertilizer, water, soil) and medicine (e.g., sterilization of medical equipment, implants). Plasma medicine also holds great promise for direct therapeutic treatments in dentistry (tooth bleaching), dermatology (atopic eczema, wound healing) and oncology (melanoma, glioblastoma). Overall, CAPP technology is an innovative, powerful and effective tool offering a broad application potential. However, its limitations and negative impacts need to be determined in order to receive regulatory approval and consumer acceptance.