Streptococcus mutans (S. mutans) is the main cariogenic bacterium with acidophilic properties, in part due to its acid-producing and -resistant properties. As a result of this activity, hard tooth structures may demineralize and form caries. Trans-cinnamaldehyde (TC) is a phytochemical from the cinnamon plant that has established antibacterial properties for Gram-positive and -negative bacteria. This research sought to assess the antibacterial and antibiofilm effects of trans-cinnamaldehyde on S. mutans. TC was diluted to a concentration range of 156.25–5000 μg/mL in dimethyl sulfoxide (DMSO) 0.03–1%, an organic solvent. Antibacterial activity was monitored by testing the range of TC concentrations on 24 h planktonic growth compared with untreated S. mutans. The subminimal bactericidal concentrations (MBCs) were used to evaluate the bacterial distribution and morphology in the biofilms. Our in vitro data established a TC MBC of 2500 μg/mL against planktonic S. mutans using a microplate spectrophotometer. Furthermore, the DMSO-only controls showed no antibacterial effect against planktonic S. mutans. Next, the sub-MBC doses exhibited antibiofilm action at TC doses of ≥625 μg/mL on hydroxyapatite discs, as demonstrated through biofilm analysis using spinning-disk confocal microscopy (SDCM) and high-resolution scanning electron microscopy (HR-SEM). Our findings show that TC possesses potent antibacterial and antibiofilm properties against S. mutans. Our data insinuate that the most effective sub-MBC of TC to bestow these activities is 625 μg/mL.

Background: Dental caries, primarily caused by Streptococcus mutans (S. mutans), is a prevalent condition with significant global impact. Trans-cinnamaldehyde (TC), a phytochemical derived from the cinnamon plant, has shown promising antibacterial and antibiofilm activity against S. mutans. This study aimed to evaluate the anti-cariogenic effects of TC on S. mutans using an innovative mouse jaw explant model. Methods: TC was diluted in an organic solvent across various concentrations. Initially, cytotoxicity assays were performed at all tested TC concentrations. Sub-minimum bactericidal concentrations were then used to examine the distribution and morphology of S. mutans biofilms. Hemi-mandibles were dissected from euthanized, healthy, seven-week-old female mice to study the impact of TC on the cariogenic activity of S. mutans using stereoscopic analysis. Finally, pH changes during exposure to cariogenic conditions and post-treatment bacterial viability were measured. Results: In vitro data demonstrate that TC doses of ≤625 µg/mL were non-cytotoxic. Treatment groups exposed to TC exhibited altered bacterial morphology, including abnormal and incomplete cell division. In the mouse jaw explant model, TC doses of ≥625 µg/mL showed anti-cariogenic effects, evidenced by the absence of visible carious lesions. Additionally, pH changes and post-treatment viable bacterial counts corresponded with the observed anti-cariogenic activity. TC doses ≤625 µg/mL led to a pH drop over time and the presence of bacterial colonies. Conclusions: TC exhibits significant anti-cariogenic activity against S. mutans in the mouse model. Our findings suggest that 625 µg/mL is the lowest non-toxic concentration of TC that effectively inhibits cariogenic activity.