This protocol describes the application of antimicrobial Photodynamic Therapy (aPDT) in a murine model of oral candidiasis. aPDT was performed using a water-soluble mixture of curcuminoids and blue LED light.
Antimicrobial Photodynamic Therapy (aPDT) has been extensively investigated in vitro, and preclinical animal models of infections are suitable for evaluating alternative treatments prior to clinical trials. This study describes the efficacy of aPDT in a murine model of oral candidiasis. Forty mice were immunosuppressed with subcutaneous injections of prednisolone, and their tongues were inoculated using an oral swab previously soaked in a C. albicans cell suspension. Tetracycline was administered via drinking water during the course of the experiment. Five days after fungal inoculation, mice were randomly distributed into eight groups; a ninth group of untreated uninfected mice was included as a negative control (n = 5). Three concentrations (20 µM, 40 µM, and 80 µM) of a mixture of curcuminoids were tested with a blue LED light (89.2 mW/cm2; ~455 nm) and without light (C+L+ and C+L- groups, respectively). Light alone (C-L+), no treatment (C-L-), and animals without infection were evaluated as controls. Data were analyzed using Welch's ANOVA and Games-Howell tests (α = 0.05). Oral candidiasis was established in all infected animals and visualized macroscopically through the presence of characteristic white patches or pseudomembranes on the dorsum of the tongues. Histopathological sections confirmed a large presence of yeast and filaments limited to the keratinized layer of the epithelium in the C-L- group, and the presence of fungal cells was visually decreased in the images obtained from mice subjected to aPDT with either 40 µM or 80 µM curcuminoids. aPDT mediated by 80 µM curcuminoids promoted a 2.47 log10 reduction in colony counts in comparison to those in the C-L- group (p = 0.008). All other groups showed no statistically significant reduction in the number of colonies, including photosensitizer (C+L-) or light alone (C-L+) groups. Curcuminoid-mediated aPDT reduced the fungal load from the tongues of mice.
Oral candidiasis (OC) is the main fungal infection of the oral cavity; it is caused by the overgrowth of Candida spp. Predisposing factors for OC include endocrine dysfunction, the use of broad-spectrum antibiotics, radio- and chemotherapy, nutritional deficiencies, xerostomia (low salivary flow), denture use, poor hygiene, and, especially, immunosuppression1. Among Candida species, Candida albicans is the most prevalent and virulent one; it is found as a commensal species in the human body and as an opportunistic pathogen. C. albicans has the ability to change its morphology from commensal yeasts (blastopores) to pathogenic filaments (hyphae and pseudohyphae)2. The filamentous forms, especially the hyphae, can invade the host epithelium by endocytosis or active penetration, causing infection3. Other virulence factors of C. albicans include adhesion, biofilm formation, and secretion of lipolytic and hydrolytic enzymes and toxins, such as lipases, phospholipases, proteinases, and candidalysin4.
OC treatments involve the use of antifungal agents, especially topical polyenes and azoles (nystatin and miconazole)5. However, they only show short-term efficacy, and recurrence is frequent. In addition, the overuse of antifungals has given rise to the problem of antifungal resistance development and spread6. Therefore, alternative therapies are needed, such as antimicrobial Photodynamic Therapy (aPDT), which combines a photosensitizer (PS) and light at an appropriate wavelength (the same as that of the PS absorption) in the presence of oxygen. PSs are bound to or taken up by cells and, when activated by light, produce reactive oxygen species (ROS) that are toxic to sensitized cells7.
In aPDT, one of the photosensitizers (PSs) employed is curcumin (CUR), a naturally occurring compound extracted from the rhizomes of the turmeric plant (Curcuma longa L.). Curcumin possesses numerous therapeutic attributes, including anti-inflammatory, antioxidant, anticancer, and antimicrobial capabilities8,9. A prior investigation found that aPDT utilizing CUR effectively diminished C. albicans in a murine model of oral candidiasis without causing any harm to the host's tissues10. CUR is the main curcuminoid extracted from turmeric, but other polyphenols, such as demethoxycurcumin and bis-demethoxycurcumin, are also found in this plant. Curcuminoid-mediated aPDT demonstrated antibacterial activity against biofilms of Staphylococcus aureus grown in catheters11. However, to the best of our knowledge, its antifungal activity against C. albicans remains unclear. Therefore, in this study, we evaluated aPDT mediated by a curcuminoid salt against C. albicans in a murine model of OC.
The research protocol for the use of mice was approved by the Ethics Committee for Animal Use (case numbers 05/2008 and 09/2020) at the School of Dentistry, Araraquara, UNESP. C. albicans (ATCC 90028) was used as the reference strain. Six-week-old female Swiss mice (n = 45), with a body mass range of 20-30 g, were used for the present study. The animals were provided by São Paulo State University, UNESP, Botucatu.
1. Preparation of PS and selection of the light source for aPDT
2. Preparation of C. albicans inoculum
3. Induction of OC in mice
NOTE: The following methodology was previously described by Takakura et al.13 and reproduced by our group10,14, with some modifications.
4. Antimicrobial Photodynamic Therapy and recovery of C. albicans from oral lesions
5. Histopathological analyses
The murine model of OC showed typical white patches and pseudomembranes on the tongue of all infected mice (Figure 4A). C. albicans recovered from C-L- animals confirmed tissue colonization by this microorganism (values ranged from 1.62 x 104 to 4.80 x 105 CFU/mL). As expected, animals from the NCtr group did not show any tissue alterations or colony growth after sampling (Figure 4B).
aPDT decreased the viability of C. albicans when curcuminoids were used at 80 µM for photosensitization (Figure 5). The mean log10 reduction achieved with 80 µM PS-mediated aPDT was 2.47, compared to that in the C-L- group (p = 0.008).
The number of C. albicans colonies recovered from mice tongues was not significantly different among mice treated with curcuminoids without illumination (C+L- groups), mice treated with light but not previously photosensitized (C-L+ groups), and untreated mice (C-L- group) (p ≥ 0.210).
The histological features of the tongues of uninfected animals (NCtr) showed normal/healthy tissues, including intact lamina propria, basal membrane, and filiform papillae (Figure 6A). In contrast, when examining the histopathological images of the mice's tongues from the C-L- group, it was evident that yeast and filaments were present within the keratinized layer of the epithelium, although there was no infiltration of fungi. In the underlying connective tissue, a mild inflammatory response was observed, primarily mediated by mononuclear cells, and filiform papillae were notably absent (Figure 6B). The histological analysis of the tongues of mice in both the C+L- and C-L+ groups displayed similar characteristics. In contrast, the tongue sections of mice treated with 80 µM curcuminoid-mediated aPDT revealed a reduced number of fungal cells, mainly limited to the keratinized layer of the epithelium (Figure 6C).
Figure 1: Chemical structure of the photosensitizer. The chemical structure of the water-soluble salt mixture used as a photosensitizer, comprising 53.4% natural curcumin and 46.6% other curcuminoids (demethoxycurcumin and bis-demethoxycurcumin). The final average molecular weight is 730.32 g/mol. Please click here to view a larger version of this figure.
Figure 2: Protocol timeline for the murine model of oral candidiasis and aPDT. A timeline outlining the protocol for the murine model of oral candidiasis and antimicrobial photodynamic therapy (aPDT). Please click here to view a larger version of this figure.
Figure 3: Illumination of mice tongues after photosensitization. After photosensitization, (incubation of the infected tissue with the photosensitizer), tongues were illuminated at 37.5 J/cm2 using a blue (~455 nm) LED light. Please click here to view a larger version of this figure.
Figure 4: White lesions in murine oral candidiasis model. (A) Representative images depicting the white lesions observed in the murine model of oral candidiasis. (B) Negative (uninfected) control. Please click here to view a larger version of this figure.
Figure 5: Candida albicans recovered from the tongues of mice. Data represent mean values ± standard deviation of log10(CFU/mL) from the treatment groups. Welch's one-way ANOVA indicated that the effects of the treatments were statistically significantly different among the groups (p < 0.001). Different lowercase letters (a, b, c) next to the means indicate statistically significant differences according to the Games-Howell test (p≤ 0.030). Please click here to view a larger version of this figure.
Figure 6: Representative images of histological sections of mouse tongues. Histological sections of mouse tongues were stained with PAS-H, and the images were captured at 200x. (A) Negative control mice without induced oral candidiasis, photosensitization, and illumination (NCtr group). (B) Mice with induced oral candidiasis, neither photosensitized nor exposed to LED illumination (C-L- group). (C) Animals with induced oral candidiasis, exposed to 80 µM curcuminoids and 37.5 J/cm2 LED illumination (C+L+ 80 group). Scale bars = 100 µm. Please click here to view a larger version of this figure.
Supplementary File 1: Experimental groups. A listing of the experimental groups utilized in the present study. Please click here to download this File.
C. albicans has been associated with oral and esophageal infections in individuals with an immunocompromised state, diabetes mellitus, prolonged use of antibiotics, and poor oral hygiene1,3. The study of human infectious diseases requires both in vitro and in vivo investigations before clinical trials can be safely and accurately designed. The present study describes a method for establishing a murine model of OC, which can be used to evaluate the pathogenesis of oral infections by C. albicans and the efficacy of antifungal approaches15,16,17,18,19.
The murine model of OC employed here was successfully established, as evidenced by the substantial fungal load recovery from lesions as well as by the characteristic infection features observed in the macroscopic and histopathological analyses of the tongues of infected mice. Many studies have used similar murine models of OC. In such models, female mice are immunosuppressed and inoculated with C. albicans, resulting in lesions on the tongue10,13,14,15,20,21. Immunosuppression with prednisolone, a glucocorticoid, inhibits the activity of neutrophils against C. albicans22. In this study, female mice were immunosuppressed with two subcutaneous injections of prednisolone, one day prior to and three days after infection with C. albicans13. In addition, the administration of tetracycline in drinking water during the course of the experiment caused oral dysbiosis by disturbing oral bacteria and helping C. albicans to thrive23,24. Furthermore, the sedation caused by the intramuscular injection of chlorpromazine chloride prevented the animals from drinking water and eating immediately after inoculation. Thus, fungal cells stayed in contact with the dorsum of the tongue for a longer time, enabling the development of germ tubes and the transition from yeasts to filaments (hyphae and pseudohyphae), which are the pathogenic morphologies of C. albicans that can invade the human epithelium. Teichert et al.25 used an immunodeficiency protocol to induce OC in mice and recovered only 2 x 102 CFU/mL of C. albicans.
While Totti et al.26 utilized sialoadenectomized mice and conducted four separate inoculations with a C. albicans suspension, it's noteworthy that, in their case, the infection was not sustained in most animals over the course of the experiment. In contrast, in the present study, the inoculation with fungal cells was carried out only once, and it resulted in the recovery of 104 CFU/mL of C. albicans from the oral cavity. This study employed the murine model of candidiasis described by Takakura et al.13, who performed oral inoculation with a clinical strain isolated from a patient with cutaneous candidiasis (106 CFU/mL). Three to seven days post-inoculation, 105-106 CFU/mL of C. albicans were recovered from the oral cavity of mice13. The differences between the method of Takakura et al.13 and this study include the use of different fungal concentrations in the inoculum (this study used 107 CFU/mL of C. albicans) and different C. albicans strains for oral inoculation (the reference strain ATCC 90028 was used here). Carmello et al.20 employed a similar protocol, which involved using immunosuppressed animals. However, they administered two additional subcutaneous injections of prednisolone to the animals on days 1, 5, 9, and 13 of the experiment. Their study revealed a positive correlation between the scores assigned to the oral lesions of the infected animals and the number of CFU/mL over a period ranging from 5 to 16 days post-infection. It has been well-established in previous research that it is crucial to closely monitor animals under anesthesia to prevent hypothermia. Additional maintenance doses of ketamine should be administered with discretion, only when necessary27.
Regarding the efficacy of aPDT application, the results showed that the irradiation of tongues previously treated with an 80 µM curcuminoid salt mixture caused a significant reduction (2.47 log10) in the viability of C. albicans. Histological analyses revealed that sections from tongues treated with 80 µM curcuminoid-mediated aPDT showed a reduced number of fungal cells, which were limited to the keratinized layer, and a low inflammatory response. It is worth emphasizing that an inflammatory response was detected in all mice that were infected with C. albicans. This observation implies that the inflammation observed in all the aPDT groups might be linked to Candida infection rather than being attributed to aPDT, a consistent finding in line with our prior investigations10,13,14,15.
Previous investigations used CUR, methylene blue, and photodithazine (PDZ) as PSs and obtained promising results10,20,24,25,27. In a similar study10, a combined exposure to CUR and LED light caused a significant reduction in the viability of C. albicans; however, the use of 80 µM CUR and light reduced fungal viability by 4.0 log10. Dovigo et al.10 used only CUR as PS, whereas we used a salt containing the three main curcuminoids from C. longa. When CUR (260 µM) and LED light were used for five consecutive days in the treatment of oral candidiasis in mice, the authors observed a reduction of 1.11 log10 in fungal viability21. When methylene blue was used as PS at 450 µg/mL and 500 µg/mL, aPDT totally eradicated C. albicans from the oral cavity of mice25. Moreover, when aPDT was mediated by PDZ (100 mg/L), a 3.0 log10 reduction and complete remission of oral lesions were observed20. In addition, aPDT increased TNF-α expression in comparison with that in the untreated group20. In a study where a fluconazole-resistant strain was used, aPDT mediated by PDZ (200 mg/L) promoted a reduction equivalent to 1.3 log1027. Moreover, the combination of aPDT with nystatin resulted in a substantial reduction in fungal viability, amounting to a decrease of 2.6 log10, along with notable improvements in oral lesions and a reduction in the inflammatory response27. Collectively, these studies provide compelling evidence for the effectiveness of aPDT in reducing fungal burden within the murine model of oral candidiasis, underscoring its potential as a clinical treatment option due to its antimicrobial efficacy without causing harm to host tissues.
In conclusion, the murine model of OC used in this study is appropriate for mimicking infection and evaluating aPDT efficacy. As a limitation, the OC model used here employed only one reference strain of C. albicans (other strains, clinical isolates, and non-albicans Candida species were not evaluated). In addition, the corticosteroid-induced immunosuppression used in mice to develop oral infection may not mimic other immunodeficiency states, such as that due to HIV infection. There may also be differences in the host conditions for developing OC, such as the oral microbiota of mice and humans. This protocol should be expanded further to evaluate mixed biofilms formed by more than one species or by different strains from the same species. Furthermore, keeping mice adequately sedated and preventing hypothermia while avoiding anesthesia-related mortality are the most difficult steps of the protocol.
The authors thank the financial support from FAPESP (São Paulo Research Foundation, process number FAPESP #2013/07276-1 (CePID CePOF) and 2008/00601-6. We also thank Dr. Ana Paula Silva for providing the information about the CUR-based water-soluble salt.
C. albicans | ATCC (Rockville, Md, USA) | 90028 | Used to prepare the Candida inoculum |
Centrifuge | Eppendorf Centrifuge 5804/5804R,B. Braun, Melsungen, Hesse, Germany | 022628146 (NA) | Used to prepare the Candida inoculum |
Chlorpromazine chloride 2 mg/mL | Compounding pharmacy, Araraquara, SP, Brazil | - | Used to sedate animals during candida inoculation |
Curcumin-based water-soluble salt | PDTPharma, Cravinhos, Brazil | - | Consisting of 53.4% of natural curcumin, and 46.6% of other curcuminoids (demethoxycurcumin and bis-demethoxycurcumin). Prepared in water and N-MethylD-Glucamine (final average molecular weight of 730.32 g.mol−1) |
Digital colony counter | CP 600 Plus, Phoenix Ind Com Equipamentos Científicos Ltda, Araraquara, SP, Brazil | - | Used to count colonies on agar plates |
Extruded mouse chow | Benelab food, Industry Qualy Animal Nutrition and Commerce Ltda., Lindóia, São Paulo State, Brazil. | - | Used for the feeding of the mice |
Ketamine Hydrochloride 10% | Ketamina Agener, União Química Farmacêutica Nacional S/A, Embu-Guaçu, SP, Brazil | - | Used to anesthetize animals before treatments and for euthanasia |
Light-emitting diode handpiece (prototype) | Instituto de Física de São Carlos, University of São Paulo, São Carlos, SP, Brazil | - | Fabricated with LXHL-PR09, Luxeon III Emitter, Lumileds Lighting, San Jose, California, USA |
Methylprednisolone acetate 40 mg | DEPO-MEDROL, Pfizer, New York | - | Used as an immunosuppressant |
Microtome | Leica Microsystems, Bannockburn, IL, USA | SM2500 | Used to cut the serial sections of the tongues |
Propylene boxes (cages housing) H13 x L20 x D30 cm | Bonther Equipaments, Ribeirão Preto, SP, Brazil | - | Used to keep the animals throughout the experimental period |
Sabouraud Dextrose Agar with Chloramphenicol | HiMedia, Mumbai, India | MM1067-500G | Culture medium for yeast growth (agar) |
Spectrophotometer | Spectrophotometer Kasvi K37-VIS , São José dos Pinhais, PR, Brazil | K37-VIS | Used to standardize the inoculum concentration |
Tetracycline hydrochloride | Compounding pharmacy, Araraquara, SP, Brazil | - | Antibiotic given to induce oral dysbiosis |
Wood shavings | J.R. Wood Shavings, Comerce of Sawdust Ltda., Conchal, São Paulo State, Brazil | - | Used for floor covering inside the housing boxes |
Xylazine 2% | Calmiun, União Química Farmacêutica Nacional S/A, Embu-Guaçu, SP, Brazil | - | Used in combination with ketamine for anesthesia |
Yeast Nitrogen Broth | Difco, InterLab, Detroit, MI, USA | DF0919-07-3 | Culture medium for yeast growth (broth) |
Yeast Peptone Dextrose Broth | NutriSelect Basic, Sigma Aldrich | Y1375 | Culture medium for maintaining the strains at -80°C and grow |