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Microbial drug-resistance demands immediate implementation of novel therapeutic strategies. Antimicrobial photodynamic therapy (aPDT) combines the administration of a photosensitizer (PS) compound with low-irradiance light to induce photochemical reactions that yield reactive oxygen species (ROS).Since ROS react with nearly all biomolecules, aPDT offers a powerful multitarget method to avoid selection of drug-resistant strains. In this study, we assayed photodynamic inactivation under a standardized method, combining methylene blue (MB) as PS and red light, against global priority pathogens. The species tested include Acinetobacter baumannii, Klebsiella aerogenes, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Enterococcus faecium, Enterococcus faecalis, Staphylococcus aureus, Candida albicans and Cryptococcus neoformans. Our strain collection presents resistance to all tested antimicrobials (>50). All drug-resistant strains were compared to their drug-sensitive counterparts. Regardless of resistance phenotype, MB-aPDT presented species-specific dose-response kinetics.More than 5log10 reduction was observed within less than 75 seconds of illumination for A. baumannii, E. coli, E. faecium, E. faecalis and S. aureus and within less than 7 minutes for K. aerogenes, K. pneumoniae, P. aeruginosa, C. albicans and C. neoformans. No signs of cross-resistance or -tolerance in between aPDT and standard chemotherapy was observed. Therefore, MB-aPDT can provide effective therapeutic protocols for a very broad spectrum of pathogens. Hence, we believe that this study represents a very important step to bring aPDT closer to implementation into mainstream medical practices.

Background: Photodynamic therapy (PDT) has been long used to treat localized tumors and infections. Currently, microbial inactivation data is reported presenting survival fraction averages and standard errors as discrete points instead of a continuous curve of inactivation kinetics. Standardization of this approach would allow clinical protocols to be introduced globally, instead of the piecemeal situation which currently applies. Methods: To this end, we used a power-law function to fit inactivation kinetics and directly report values of lethal doses (LD) and a tolerance factor (T) that informs if inactivation rate varies along the irradiation procedure. A deduced formula was also tested to predict LD for any given survival fraction value. We analyzed the photoantimicrobial effect caused by red light activation of methylene blue (MB-APDT) and by blue light (BL) activation of endogenous microbial pigments against 5 clinically relevant pathogens. Results: Following MB-APDT, E. coli and S. aureus cells become increasingly more tolerant to inactivation along the irradiation process (T<1). K. pneumoniae presents opposite behavior, i.e., more inactivation is observed towards the end of the process (T>1). P. aeruginosa and C. albicans present constant inactivation rate (T~1). In contrast, all bacterial species presented similar behavior during inactivation caused by BL, i.e., continuously becoming more sensitive to blue light exposure (T>1). Conclusion: The power-law function successfully fit all experimental data. The analytical model precisely predicted LD and T values. We expect that these analytical models may contribute to more standardized methods for comparisons of photodynamic inactivation efficiencies.

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Biophotonics is defined as the combination of biology and photonics (the physical science of the light). It is a general term for all techniques that deal with the interaction between biological tissues/cells and photons (light). Biophotonics offers a great variety of techniques that can facilitate the early detection of diseases and promote innovative theragnostic approaches. As the COVID-19 infection can be transmitted due to the face-to-face communication, droplets and aerosol inhalation and the exposure to saliva, blood, and other body fluids, as well as the handling of sharp instruments, dental practices are at increased risk of infection. In this paper, a literature review was performed to explore the application of Biophotonics approaches in Dentistry focusing on the COVID-19 pandemic and how they can contribute to avoid or minimize the risks of infection in a dental setting. For this, search-related papers were retrieved from PubMED, Scielo, Google Schoolar, and American Dental Association and Centers for Disease Control and Prevention databases. The body of evidence currently available showed that Biophotonics approaches can reduce microorganism load, decontaminate surfaces, air, tissues, and minimize the generation of aerosol and virus spreading by minimally invasive, time-saving, and alternative techniques in general. However, each clinical situation must be individually evaluated regarding the benefits and drawbacks of these approaches, but always pursuing less-invasive and less aerosol-generating procedures, especially during the COVID-19 pandemic.