Streptococcus mutans (S. mutans) and Candida albicans (C. albicans) are prominent microbes associated with rapid and aggressive caries. In the present study, we investigated the antimicrobial efficacy, cytotoxicity, and mechanism of toluidine blue O (TBO)-mediated antimicrobial photodynamic therapy (aPDT) and potassium iodide (KI). The dependence of KI concentration, TBO concentration and light dose on the antimicrobial effect of aPDT plus KI was determined. The cytotoxicity of TBO-mediated aPDT plus KI was analyzed by cell counting kit-8 (CCK-8) assay. A singlet oxygen (1O2) probe test, time-resolved 1O2 detection, and a 1O2 quencher experiment were performed to evaluate the role of 1O2 during aPDT plus KI. The generation of iodine and hydrogen peroxide (H2O2) were analyzed by an iodine starch test and Amplex red assay. The anti-biofilm effect of TBO-mediated aPDT plus KI was also evaluated by counting forming unit (CFU) assay. KI could potentiate TBO-mediated aPDT against S. mutans and C. albicans in planktonic and biofilm states, which was safe for human dental pulp cells. 1O2 measurement showed that KI could quench 1O2 signals, implicating that 1O2 may act as a principal mediator to oxidize excess iodide ions to form iodine and H2O2. KI could highly potentiate TBO-mediated aPDT in eradicating S. mutans and C. albicans due to the synergistic effect of molecular iodine and H2O2.
Background
The D‐alanylation of lipoteichoic acid (LTA) is essential for the physiological metabolism of Streptococcus mutans (S. mutans). This study was designed to investigate the influence of D‐alanylation of LTA on interspecies competitiveness of S. mutans.
Methods
The process of D‐alanylation was blocked by the inactivation of dltC. Agar competition assays, conditioned medium assays, and qRT‐PCR were used to evaluate the production of antimicrobial compounds in S. mutans mutant. Dual‐species biofilm was formed to investigate the competitiveness of S. mutans mutant cocultured with S. sanguinis or S. gordonii.
Results
S. mutans mutant could not produce antimicrobial compounds efficiently when cocultured with commensal bacteria (*p < 0.05). The mutant showed compromised competitiveness in dual‐species biofilms. The ratio of the mutant in dual‐species biofilms decreased, and the terminal pH of the culture medium in mutant groups (mutant+S. sanguinis/S. gordonii) was higher than that in wild‐type groups (*p < 0.05). Scanning electron microscope (SEM) showed weaker demineralization of enamel treated with dual‐species biofilms consisting of mutant and commensal bacteria.
Conclusion
D‐Alanylation is involved in interspecies competitiveness of S. mutans within oral biofilm by regulating mutacins and lactic acid production, which may modulate the profiles of dental biofilms. Results provide new insights into dental caries prevention and treatment.
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