Abstract

Background: Fixed orthodontic brackets introduce structural niches that promote the accumulation of cariogenic biofilms. While Streptococcus mutans initiates enamel demineralization, Lactobacilli drive the progression of cavitated lesions. Traditional antimicrobials present significant clinical limitations. Antimicrobial photodynamic therapy (aPDT) offers a compelling, resistance-free alternative, yet the relative, intrinsic susceptibility of these two key pathogens to natural photosensitizer-mediated aPDT around orthodontic hardware remains poorly characterized. Objective: To systematically evaluate and compare the intrinsic susceptibility of Streptococcus mutans and Lactobacilli biofilms formed around stainless steel orthodontic brackets to aPDT mediated by curcumin or riboflavin activated by a 445 nm blue diode laser (BDL) at varying power densities. Methods: One hundred twenty extracted human premolars bonded with stainless steel brackets were inoculated with either S. mutans (n=60) or Lactobacilli (n=60) to establish 72-hour mature single-species biofilms. Within each bacterial division, specimens were randomized into three core experimental blocks (each): Group I (100 µM Riboflavin), Group II (40 µM Curcumin), or Group III (Laser/Compound Controls). These were further subdivided (n=10) to undergo 30-second irradiation with a 445 nm BDL at either a low-power output (200 mW •) or high-power output (500 mW •). Surviving sessile colonies were liberated, serially diluted, cultured, and quantified as CFU/mL. Data were analyzed via Shapiro-Wilk testing, oneway ANOVA with Tukey’s post hoc test, and independent t-tests (α =0.05) Results: Baseline unexposed control loads were equivalent for S. mutans (5.98 ± 0.12log10 CFU/mL) and Lactobacilli (5.97 ± 0.13log10 CFU/mL). Under highefficacy configurations, S. mutans consistently exhibited significantly greater susceptibility to photodynamic eradication than Lactobacilli (p < 0.05). Curcumin aPDT at 500 mW generated the maximum bacterial reduction, yielding a 99.4% clearance (3.75 ± 0.19log10CFU/mL) for S. mutans compared to a 98.9% clearance (4.02 ± 0.22log10 CFU/mL) for Lactobacilli (p < 0.01 relative to baseline controls). boflavinaPDTachievedacceptableclinicalreductiononlyat500mW,resultingin a 92.6% reduction (4.85 ± 0.20log10 CFU/mL) in S. mutans versus an 88.0% reduction (5.05 ± 0.22log10 CFU/mL) in Lactobacilli (p < 0.31). Increasing the BDL output from 200 mW to 500 mW significantly amplified bactericidal kinetics across all protocols (p < 0.01), though the interspecies susceptibility gap was preserved throughout. Conclusions: Streptococcus mutans is intrinsically more susceptible to blue light-mediated photodynamic inactivation than Lactobacilli around orthodontic brackets. Lactobacilli exhibit higher tolerance, requiring elevated photodynamic energy thresholds to achieve comparable logs of reduction. Curcumin-mediated aPDT at 500 mW delivers superior bactericidal efficacy against both target pathogens compared to riboflavin, supporting organism-specific, targeted aPDT regimens in orthodontic patients.

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