Combined nano-hypericin antimicrobial photodynamic therapy and postbiotic on Streptococcus mutans

Dental caries is a prevalent microbial disease, with Streptococcus mutans as a major pathogen. Increasing antibiotic resistance highlights the need for alternative, non-antibiotic strategies. This study evaluated the antimicrobial and anti-virulence potential of nano-hypericin (nHyp)-mediated antimicrobial photodynamic therapy (aPDT) combined with Lactobacillus casei -derived postbiotic (PS LC ). nHyp was synthesized and characterized by transmission electron microscopy, UV–Vis spectroscopy, and dynamic light scattering, confirming quasi-spherical nanoparticles (~ 47 nm). PS LC was purified via acid precipitation and chemically profiled by gas chromatography–mass spectrometry, revealing diketopiperazines, fatty acids, hydrocarbons, and phenolic compounds. Planktonic S. mutans were treated with nHyp (1.9–1000 µg/mL) ± blue laser irradiation (450 ± 5 nm, 1–5 min), alone or combined with PS LC at 1/2 × MIC (32 µg/mL). Bacterial viability was measured by colony forming unit (CFU)/mL, and gtfB expression quantified by qRT-PCR. nHyp-mediated aPDT induced dose- and time-dependent reductions in bacterial viability (up to 81%), further enhanced by PS LC (up to 90.5%). Sub-lethal aPDT (15.6 µg/mL + 2 min) downregulated gtfB 4.25-fold, whereas combination with PS LC achieved 7.86-fold suppression, indicating inhibition of glucan synthesis relevant to biofilm formation. These findings demonstrate that nHyp-mediated aPDT combined with PS LC enhances antimicrobial and anti-virulence effects under in vitro conditions, representing a promising dual-mechanism, antibiotic-free approach to modulate cariogenic bacterial pathogenicity.