Hydrogen Peroxide Enhances the Antibacterial Effect of Methylene Blue‐based Photodynamic Therapy on Biofilm‐forming Bacteria

Photodynamic treatment of Staphylococcus aureus with non-iron hemin analogs in the presence of hydrogen peroxide

Bacteria subjected to antiseptic or antibiotic stress often develop tolerance, a trait that can lead to permanent resistance. To determine whether photodynamic agents could be used to counter tolerance, we evaluated three non-iron hemin analogs (M-PpIX; M = Al, Ga, In) as targeted photosensitizers for antimicrobial photodynamic inactivation (aPDI) following exposure to sublethal H2O2. Al-PpIX is an active producer of ROS whereas Ga- and In-PpIX are more efficient at generating singlet oxygen. Al- and Ga-PpIX are highly potent aPDI agents against S. aureus and methicillin-resistant strains (MRSA) with antimicrobial activity (3 log reduction in colony-forming units) at nanomolar concentrations. The aPDI activities of Al- and Ga-PpIX against S. aureus were tested in the presence of 1 mM H2O2 added at different stages of growth. Bacteria exposed to H2O2 during log-phase growth were less susceptible to aPDI but bacteria treated with H2O2 in their postgrowth phase exhibited aPDI hypersensitivity, with no detectable colony growth after treatment with 15 nM Ga-PpIX.

In vitro study: methylene blue-based antibacterial photodynamic inactivation of Pseudomonas aeruginosa

Pseudomonas aeruginosa is one of the most antibiotic-resistant and opportunistic pathogens in immunocompromised and debilitated patients. It is considered the cause of most severe skin infections and is frequently found in hospital burn units. Due to its high antibiotic resistance, eliminating P. aeruginosa from skin infections is quite challenging. Therefore, this study aims to assess the novel in vitro antibacterial activity of methylene blue using a 635-nm diode laser to determine the effective power and energy densities for inhibition of P. aeruginosa. The strain was treated with various concentrations of methylene blue and 635-nm diode laser at powers of 300 mW/cm2 and 250 mW/cm2. The diode laser's potency in the photo-destruction of methylene blue and its degradation through P. aeruginosa were also evaluated. Colony-forming unit (CFU)/ml, fluorescence spectroscopy, optical density, and confocal microscopy were used to measure the bacterial killing effect. As a result, the significant decrease of P. aeruginosa was 2.15-log10, 2.71-log10, and 3.48-log10 at 60, 75, and 90 J/cm2 after excitation of MB for 240, 300, and 360 s at a power of 250 mW/cm2, respectively. However, a maximum decrease in CFU was observed by 2.54-log10 at 72 J/cm2 and 4.32-log10 at 90 and 108 J/cm2 after 300 mW/cm2 of irradiation. Fluorescence images confirmed the elimination of bacteria and showed a high degree of photo-destruction compared to treatment with methylene blue and light alone. In conclusion, MB-induced aPDT demonstrated high efficacy, which could be a potential approach against drug-resistant pathogenic bacteria. KEY POINTS: • Combination of methylene blue with 635-nm diode laser for antibacterial activity. • Methylene blue photosensitizer is employed as an alternative to antibiotics. • aPDT showed promising antibacterial activity against Pseudomonas aeruginosa.

Hydrogen peroxide preoxidation as a strategy for enhanced antimicrobial photodynamic action against methicillin-resistant Staphylococcus aureus

Antimicrobial photodynamic treatment (aPDT) is a photooxidative process based on the excitation of a photosensitizer (PS) in the presence of molecular oxygen, under specific wavelengths of light. It is a promising method for advanced treatment of water and wastewater, particularly targeting disinfection challenges, such as antibiotic-resistant bacteria (ARB). Research in improved aPDT has been exploring new PS materials, and additives in general. Hydrogen peroxide (H2O2) a widely applied disinfectant, mostly in the food industry and clinical settings, present environmentally negligible residuals at the usually applied concentrations, making it friendly for the water and wastewater sectors. Here, we explored the effects of preoxidation with H2O2 followed by blue light-mediated (450 nm) aPDT using curcumin (a natural-based PS) against methicillin-resistant Staphylococcus aureus (MRSA). Results of the sequential treatment pointed to a slight hampering in aPDT efficiency at very low H2O2 concentrations, followed by an increasing cooperative effect up to a deleterious point (≥7 log10 inactivation in CFU mL-1), suggesting a synergistic interaction of preoxidation and aPDT. The increased performance in H2O2-pretreated aPDT encourages studies of optimal operational conditions for the assisted technology and describes potentials for using the described strategy to tackle the issue of ARB spread.

Oxygen Availability on the Application of Antimicrobial Photodynamic Therapy against Multi-Species Biofilms

Methylene blue (MB)- or Curcumin (Cur)-based photodynamic therapy (PDT) has been used as an adjunctive treatment for periodontitis. Its actual clinical efficacy is still in question because the lack of oxygen in a deep periodontal pocket might reduce the PDT efficacy. We aim to investigate the effect of oxygen on PDT efficacy and to examine if the addition of hydrogen peroxide (HP) could improve PDT performance anaerobically. To this end, we cultured 48 h saliva-derived multi-species biofilms and treated the biofilms with 25 µM MB or 40 µM Cur, HP (0.001%, 0.01% and 0.1%), light (L-450 nm or L-660 nm), or combinations thereof under ambient air or strictly anaerobic conditions. MB- and Cur-PDTs significantly reduced biofilm viability in air but not under anaerobic conditions. HP at 0.1% significantly enhanced the killing efficacies of both MB- and Cur-PDTs anaerobically. The killing efficacy of Cur-PDT combined with 0.1% HP was higher anaerobically than in air. However, this was not the case for MB-PDT combined with 0.1% HP. In conclusion, this study demonstrated that the biofilm killing efficacies of MB- and Cur-PDTs diminished when there was no oxygen. HP at 0.1% can enhance the efficacy of PDT performed anaerobically, but the level of enhancement is photosensitizer-dependent.

Hydrogen peroxide enhanced photoinactivation of Candida albicans by a novel boron-dipyrromethene (BODIPY) derivative

Photodynamic inactivation (PDI) has received increasing attention as a promising approach to combat Candida albicans infections. This study aimed to evaluate the synergistic effect of a new BODIPY (4,4-difluoro-boradiazaindacene) derivative and hydrogen peroxide on C. albicans. BDP-4L in combination with H₂O₂ demonstrated enhanced photokilling efficacy. In suspended cultures of C. albicans, the maximum decrease was 6.20 log and 2.56 log for PDI using BDP-4L (2.5 μM) with or without H₂O₂, respectively. For mature C. albicans biofilms, 20 μM BDP-4L plus H₂O₂ eradicated C. albicans, causing an over 6.7 log count reduction in biofilm-associated cells, while only a reduction of ~ 1 log count was observed when H₂O₂ was omitted. Scanning electron microscopy analysis and LIVE/DEAD assays suggested that PDI using BDP-4L plus H₂O₂ induced more damage to the cell membrane. Correspondingly, amplification of nucleic acids release was observed in biofilms treated with the combined PDI. Additionally, we also discovered that the addition of hydrogen peroxide potentiated the generation of ¹O₂ in PDI using the singlet oxygen sensor green probe. Collectively, BDP-4L combined with H₂O₂ presents a promising approach in the treatment of C. albicans infections.

The role of the light source in antimicrobial photodynamic therapy

Antimicrobial photodynamic therapy (APDT) is a promising approach to fight the growing problem of antimicrobial resistance that threatens health care, food security and agriculture. APDT uses light to excite a light-activated chemical (photosensitiser), leading to the generation of reactive oxygen species (ROS). Many APDT studies confirm its efficacy in vitro and in vivo against bacteria, fungi, viruses and parasites. However, the development of the field is focused on exploring potential targets and developing new photosensitisers. The role of light, a crucial element for ROS production, has been neglected. What are the main parameters essential for effective photosensitiser activation? Does an optimal light radiant exposure exist? And finally, which light source is best? Many reports have described the promising antibacterial effects of APDT in vitro, however, its application in vivo, especially in clinical settings remains very limited. The restricted availability may partially be due to a lack of standard conditions or protocols, arising from the diversity of selected photosensitising agents (PS), variable testing conditions including light sources used for PS activation and methods of measuring anti-bacterial activity and their effectiveness in treating bacterial infections. We thus sought to systematically review and examine the evidence from existing studies on APDT associated with the light source used. We show how the reduction of pathogens depends on the light source applied, radiant exposure and irradiance of light used, and type of pathogen, and so critically appraise the current state of development of APDT and areas to be addressed in future studies. We anticipate that further standardisation of the experimental conditions will help the field advance, and suggest key optical and biological parameters that should be reported in all APDT studies. More in vivo and clinical studies are needed and are expected to be facilitated by advances in light sources, leading to APDT becoming a sustainable, alternative therapeutic option for bacterial and other microbial infections in the future.

An in vitro study on the efficacy of hydrogen peroxide mediated high-power photodynamic therapy affecting Enterococcus faecalis biofilm formation and dispersal

OBJECTIVE

Biofilms are involved in failure of root canal treatment due to their high resistance to antimicrobial agents, which make their removal as a big challenge. The present study aims at utilizing hydrogen peroxide (HP) plus high frequency laser reinforced antimicrobial photodynamic therapy (a-PDT) as a complementary therapy against Enterococcus faecalis (E. faecalis) at planktonic and biofilm stages.

MATERIALS AND METHODS

E. faecalis at planktonic and biofilm stages was treated with the photosensitizer HP, followed by no irradiation or irradiation with a power of 2.5 W (ʎ = 980 nm). The cell viability, anti-biofilm, anti-metabolic potential, and temperature changes were evaluated.

RESULTS

The combination of HP and 980 nm diode laser intensely boosted antibacterial and anti-biofilm efficacy compared with either component alone, affirming HP reinforcement as a bacteriostatic agent. The maximum effect on biofilm occurs in 5.25% sodium hypochlorite (NaOCl) group. During laser irradiations, the mean of temperature changes remains below 5.6 °C.

CONCLUSIONS

It could be concluded that the HP could improve anti-biofilm efficacy as a photosensitizer in a-PDT.

Clinical applications of antimicrobial photodynamic therapy in dentistry

Given the emergence of resistant bacterial strains and novel microorganisms that globally threaten human life, moving toward new treatment modalities for microbial infections has become a priority more than ever. Antimicrobial photodynamic therapy (aPDT) has been introduced as a promising and non-invasive local and adjuvant treatment in several oral infectious diseases. Its efficacy for elimination of bacterial, fungal, and viral infections and key pathogens such as Streptococcus mutans, Porphyromonas gingivalis, Candida albicans, and Enterococcus faecalis have been investigated by many invitro and clinical studies. Researchers have also investigated methods of increasing the efficacy of such treatment modalities by amazing developments in the production of natural, nano based, and targeted photosensitizers. As clinical studies have an important role in paving the way towards evidence-based applications in oral infection treatment by this method, the current review aimed to provide an overall view of potential clinical applications in this field and summarize the data of available randomized controlled clinical studies conducted on the applications of aPDT in dentistry and investigate its future horizons in the dental practice. Four databases including PubMed (Medline), Web of Science, Scopus and Embase were searched up to September 2022 to retrieve related clinical studies. There are several clinical studies reporting aPDT as an effective adjunctive treatment modality capable of reducing pathogenic bacterial loads in periodontal and peri-implant, and persistent endodontic infections. Clinical evidence also reveals a therapeutic potential for aPDT in prevention and reduction of cariogenic organisms and treatment of infections with fungal or viral origins, however, the number of randomized clinical studies in these groups are much less. Altogether, various photosensitizers have been used and it is still not possible to recommend specific irradiation parameters due to heterogenicity among studies. Reaching effective clinical protocols and parameters of this treatment is difficult and requires further high quality randomized controlled trials focusing on specific PS and irradiation parameters that have shown to have clinical efficacy and are able to reduce pathogenic bacterial loads with sufficient follow-up periods.

Evaluation of photodynamic therapy against Leishmania tropica promastigotes using different photosensitizers

BACKGROUND

Photodynamic therapy is a two-step procedure, involving the use of photosensitizing agents followed by selective illumination of the target lesion with visible light. Photodynamic therapy has been described recently as a promising strategy for treatment of leishmaniasis. This study aims to evaluate the in vitro phototoxic, morphological, and apoptotic effect of methylene blue, toluidine blue, chloro-aluminum phthalocyanine, and pheophorbide a-mediated photodynamic therapy on the viability of Leishmania tropica promastigotes.

METHODS

Parasites were treated with methylene blue, toluidine blue, chloro-aluminum phthalocyanine, and pheophorbide a or/and methylene blue, toluidine blue, chloro-aluminum phthalocyanine, and pheophorbide a-mediated photodynamic therapy, and cell proliferation, morphological changes, and apoptosis were evaluated by XTT, giemsa staining, DAPI staining, and DNA fragmentation, respectively.

RESULTS

Parasite viability was significantly different in between the groups treated with methylene blue, toluidine blue, and pheophorbide a, with or without irradiation. chloro-aluminum phthalocyanine treatment did not lead to any alterations in cell viability in Leishmania tropica promastigotes with or without irradiation. DAPI staining results indicated that apoptotic bodies and nucleus fragmentation started to be visible in methylene blue, chloro-aluminum phthalocyanine, and pheophorbide a-mediated photodynamic therapy groups. DNA ladder pattern which is used to define apoptosis was observed in irradiated methylene blue, chloro-aluminum phthalocyanine, and pheophorbide a groups.

CONCLUSIONS

The results revealed that apoptosis-induced cell death was observed in Leishmania tropica promastigotes after the application of photosensitizers in combination with light irradiation.

A narrative review on current duodenoscope reprocessing techniques and novel developments

Duodenoscopy-associated infections occur worldwide despite strict adherence to reprocessing standards. The exact scope of the problem remains unknown because a standardized sampling protocol and uniform sampling techniques are lacking. The currently available multi-society protocol for microbial culturing by the Centers for Disease Control and Prevention, the United States Food and Drug Administration (FDA) and the American Society for Microbiology, published in 2018 is too laborious for broad clinical implementation. A more practical sampling protocol would result in increased accessibility and widespread implementation. This will aid to reduce the prevalence of duodenoscope contamination. To reduce the risk of duodenoscopy-associated pathogen transmission the FDA advised four supplemental reprocessing measures. These measures include double high-level disinfection, microbiological culturing and quarantine, ethylene oxide gas sterilization and liquid chemical sterilization. When the supplemental measures were advised in 2015 data evaluating their efficacy were sparse. Over the past five years data regarding the supplemental measures have become available that place the efficacy of the supplemental measures into context. As expected the advised supplemental measures have resulted in increased costs and reprocessing time. Unfortunately, it has also become clear that the efficacy of the supplemental measures falls short and that duodenoscope contamination remains a problem. There is a lot of research into new reprocessing methods and technical applications trying to solve the problem of duodenoscope contamination. Several promising developments such as single-use duodenoscopes, electrolyzed acidic water, and vaporized hydrogen peroxide plasma are already applied in a clinical setting.

Synergetic antimicrobial effect of chlorin e6 and hydrogen peroxide on multi-species biofilms

Antimicrobial photodynamic therapy (aPDT) has been considered as a potential alternative to antibiotics for the treatment of biofilm infections. There is evidence that an additional H₂O₂ enhances the antimicrobial efficacy of aPDT. However, the minimum H₂O₂ concentration to achieve this synergistic effect is unclear. A saliva-derived multi-species biofilm was treated with the photosensitizer chlorin e6 (Ce6, 50 µM), H₂O₂ (0.3, 3.3, 33.3 mM), or their combination for 5 min, followed by no irradiation or irradiation at 15 J (cm²)^-1 (λ = 450 nm or 660 nm), with or without oxygen. Biofilm viability and metabolic activity were evaluated. The combination of 33.3 mM H₂O₂ and Ce6-aPDT strongly enhanced antimicrobial efficacy compared with either component alone, irrespective of oxygen availability and irradiation wavelength. In particular, the combination resulted in a 6.6-log colony forming unit (CFU) reduction anaerobically under blue irradiation. This combination is a promising treatment for biofilm infections, especially those thriving in an anaerobic microenvironment.

Antimicrobial Photodynamic Approach in the Inactivation of Viruses in Wastewater: Influence of Alternative Adjuvants

Pathogenic viruses are frequently present in marine and estuarine waters, due to poor wastewater (WW) treatments, which consequently affect water quality and human health. Chlorination, one of the most common methods used to ensure microbiological safety in tertiarily treated effluents, may lead to the formation of toxic chemical disinfection by-products on reaction with organic matter present in the effluents. Antimicrobial photodynamic therapy (aPDT) can be a promising disinfecting approach for the inactivation of pathogens, without the formation of known toxic by-products. Additionally, some studies have reported the potentiator effect on aPDT of some compounds, such as potassium iodide (KI) and hydrogen peroxide (H2O2). In the present study, the aPDT efficiency of a PS formulation constituted of five cationic porphyrins (Form) in the inactivation of E. coli T4-like bacteriophage, a model of mammalian viruses, in different aqueous matrices with different organic matter content, was evaluated. Photoinactivation studies were performed at different concentrations of Form and in the presence of the adjuvants KI and H2O2. The results showed that the efficiency of bacteriophage photoinactivation is correlated with the Form concentration, the amount of the organic matter in WW, and the adjuvant type. Form can be an effective alternative to controlling viruses in WW, particularly if combined with H2O2, allowing to significantly reduce PS concentration and treatment time. When combined with KI, the Form is less effective in inactivating T4-like bacteriophage in WW.

Effect of long-term methylene blue treatment on the composition of mouse gut microbiome and its relationship with the cognitive abilities of mice

In recent years, methylene blue (MB) has attracted considerable interest as a potential drug for the treatment of methemoglobinemia and neurodegenerative diseases. MB is active against microorganisms from various taxonomic groups. However, no studies have yet been conducted on the effect of MB on the intestinal microbiome of model animals. The aim of this work was to study the effect of different concentrations of MB on the mouse gut microbiome and its relationship with the cognitive abilities of mice. We showed that a low MB concentration (15 mg/kg/day) did not cause significant changes in the microbiome composition. The Bacteroidetes/Firmicutes ratio decreased relative to the control on the 2nd and 3rd weeks. A slight decrease in the levels Actinobacteria was detected on the 3rd week of the experiment. Changes in the content of Delta, Gamma, and Epsilonproteobacteria have been also observed. We did not find significant alterations in the composition of intestinal microbiome, which could be an indication of the development of dysbiosis or other gut dysfunction. At the same time, a high concentration of MB (50 mg/kg/day) led to pronounced changes, primarily an increase in the levels of Delta, Gamma and Epsilonproteobacteria. Over 4 weeks of therapy, the treatment with high MB concentration has led to an increase in the median content of Proteobacteria to 7.49% vs. 1.61% in the control group. Finally, we found that MB at a concentration of 15 mg/kg/day improved the cognitive abilities of mice, while negative correlation between the content of Deferribacteres and cognitive parameters was revealed. Our data expand the understanding of the relationship between MB, cognitive abilities, and gut microbiome in respect to the antibacterial properties of MB.

Fast and High-Throughput Evaluation of Photodynamic Effect by Monitoring Specific Protein Oxidation with MALDI-TOF Mass Spectrometry

In antibacterial practices by photodynamic treatment, bacteria are incubated with photosensitizers and then oxidized to death by generating reactive oxygen species (ROS) under light irradiation. Generally, Luria-Bertani (LB) agar colony is a conventional method to evaluate the photodynamic effect. However, this method is time consuming, easily disturbed by pollutants, and limited to the analysis of a pure bacteria sample. Herein, we introduce a novel method of photodynamic effect evaluation through in situ detection of specific protein oxidation by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) with only 1 μL of sample in a fast (less than 1 min per sample) and high-throughput (up to 384 samples per run) way. The oxidation rates of specific proteins stayed highly consistent with bactericidal rates and thus MALDI-TOF MS might be able to replace the LB agar colony to evaluate the photodynamic effect. With the present method, several experimental conditions including different photosensitizer types, dosage controls, and different illumination times were easily screened to optimize photodynamic effect. Photodynamic effects of various bacteria species, cancer cells, and even mixture samples were also evaluated. The results demonstrate the promising application of MALDI-TOF MS in evaluating the photodynamic effect of each component in a mixture sample without any separation or purification, which could not be achieved by the traditional LB agar colony method.

Photodynamic antimicrobial chemotherapy (PACT) using riboflavin inhibits the mono and dual species biofilm produced by antibiotic resistant Staphylococcus aureus and Escherichia coli

PURPOSE

Multispecies biofilms play a significant role in persistent infections. Furthermore, by interspecies transfer of antibiotic resistance genes, multispecies biofilms spread antibiotic resistance. The purpose of this study was to investigate the effect of Photodynamic Antimicrobial Chemotherapy (PACT) using riboflavin on mono and multi species biofilms.

METHODS

For this we used two clinically relevant opportunistic pathogens species E. coli and S. aureus as mono-species and multispecies biofilms. We did broth dilution assay for antibacterial, crystal violet assay for biofilms and fluorometric study for reactive oxygen species (ROS) and extracellular polymeric substance (EPS) production by phenol-HCl method.

RESULTS

Antibacterial study revealed that photo-illuminated riboflavin shows bactericidal effect against each bacteria and their mix culture. E. coli was found to be little more resistant than S. aureus. Crystal violet assay revealed photo-illuminated riboflavin shows anti-biofilms activity against both mono and mix species biofilms. But mix species biofilms were more resistant to PACT than mono species biofilms. Further study revealed this may be due to the interaction between different EPS production, hence in mix species biofilms EPS production is less affected after PACT than mono species biofilms. We found photo-illuminated riboflavin increased the intracellular ROS production.

CONCLUSION

Photo-illuminated riboflavin shows bactericidal and anti-biofilms effect against each bacteria and their mix culture. Photo-illuminated increased intracellular ROS production, which may induce the oxidative stress and destroy the respiratory system of bacteria.

Exploring the relationship between structure and activity in BODIPYs designed for antimicrobial phototherapy

A synthetic strategy to BODIPY dyes is presented giving access to a range of new compounds relevant in the context of antimicrobial photodynamic therapy (aPDT). BODIPYs with the 8-(4-fluoro-3-nitrophenyl) and the 8-pentafluorophenyl substituents were used for the synthesis of new mono- and dibrominated BODIPYs. The para-fluorine atoms in these electron-withdrawing groups facilitate functional modification via nucleophilic aromatic substitution (SNAr) with a number of amines and thio-carbohydrates. Subsequently, the antibacterial phototoxic activity of these BODIPYs has been assessed in bacterial assays against the Gram-positive germ S. aureus and also against the Gram-negative germ P. aeruginosa. The bacterial assays allowed to identify substitution patterns which ensured antibacterial activity not only in phosphate-buffered saline (PBS) but also in the presence of serum, hereby more realistically modelling the complex biological environment that is present in clinical applications.

Antimicrobial photodynamic therapy fighting polymicrobial infections – a journey from in vitro to in vivo

The batericidal effects of antimicrobial photodynamic therapy (aPDT), using methylene blue as a photosensitizer and pulsed red LED light for activation, were tested in various environments in vitro and in a wound model in mice infected with a fecal bacterial suspension.

Surface Modification of Nanocrystalline TiO2 Materials with Sulfonated Porphyrins for Visible Light Antimicrobial Therapy

Highly-active, surface-modified anatase TiO2 nanoparticles were successfully synthesized and characterized. The morphological and optical properties of the obtained (metallo)porphyrin@qTiO2 materials were evaluated using absorption and fluorescence spectroscopy, scanning electron microscopy (SEM) imaging, and dynamic light scattering (DLS). These hybrid nanoparticles efficiently generated reactive oxygen species (ROS) under blue-light irradiation (420 ± 20 nm) and possessed a unimodal size distribution of 20–70 nm in diameter. The antimicrobial performance of the synthetized agents was examined against Gram-negative and Gram-positive bacteria. After a short-term incubation of microorganisms with nanomaterials (at 1 g/L) and irradiation with blue-light at a dose of 10 J/cm2, 2–3 logs of Escherichia coli, and 3–4 logs of Staphylococcus aureus were inactivated. A further decrease in bacteria viability was observed after potentiation photodynamic inactivation (PDI), either by H2O2 or KI, resulting in complete microorganism eradication even when using low material concentration (from 0.1 g/L). SEM analysis of bacteria morphology after each mode of PDI suggested different mechanisms of cellular disruption depending on the type of generated oxygen and/or iodide species. These data suggest that TiO2-based materials modified with sulfonated porphyrins are efficient photocatalysts that could be successfully used in biomedical strategies, most notably, photodynamic inactivation of microorganisms.

Chitosan-Based Drug Delivery Systems for Optimization of Photodynamic Therapy: a Review

Drug delivery systems (DDS) can be designed to enrich the pharmacological and therapeutic properties of several drugs. Many of the initial obstacles that impeded the clinical applications of conventional DDS have been overcome with nanotechnology-based DDS, especially those formed by chitosan (CS). CS is a linear polysaccharide obtained by the deacetylation of chitin, which has potential properties such as biocompatibility, hydrophilicity, biodegradability, non-toxicity, high bioavailability, simplicity of modification, aqueous solubility, and excellent chemical resistance. Furthermore, CS can prepare several DDS as films, gels, nanoparticles, and microparticles to improve delivery of drugs, such as photosensitizers (PS). Thus, CS-based DDS are broadly investigated for photodynamic therapy (PDT) of cancer and fungal and bacterial diseases. In PDT, a PS is activated by light of a specific wavelength, which provokes selective damage to the target tissue and its surrounding vasculature, but most PS have low water solubility and cutaneous photosensitivity impairing the clinical use of PDT. Based on this, the application of nanotechnology using chitosan-based DDS in PDT may offer great possibilities in the treatment of diseases. Therefore, this review presents numerous applications of chitosan-based DDS in order to improve the PDT for cancer and fungal and bacterial diseases.