Synergistic Antibacterial Activity and Mechanistic Insights of Clove ( <i>Syzygium aromaticum</i> ) Combined With Cinnamon ( <i>Cinnamomum spp.</i> ) Essential Oils Against <i>Streptococcus mutans</i> In Vitro

ABSTRACT Streptococcus mutans is considered to be an important cariogenic organism. Cinnamon and clove essential oil (EOs) have excellent antimicrobial activity against S. mutans , and whether the combination of these EOs can enhance antimicrobial activity against S. mutans and improve cell biocompatibility has not yet been studied. This study optimized the ratio and concentration of the mixed EOs, which are composed of Cinnamomum burmanii essential oil (CBEO), Cinnamomum zeylanicum essential oil (CZEO) and Syzygium aromaticum essential oil (SEO), based on an analysis of their constituent compounds and antimicrobial properties. The antimicrobial activity and underlying mechanisms of the combination EO were subsequently investigated. The antimicrobial activity of the EOs was ranked as follows: CBEO &gt; SEO &gt; CZEO, compared to the positive control of 1% Chlorhexidine gluconate (CHX). The inhibitory effects of SEO and CBEO blended at volume ratios of 1:1, 2:3 and 1:4 were significantly superior to other blends and exhibited synergistic potentiation. The mixed EO (SEO:CBEO = 1:4) demonstrated the greatest inhibitory efficacy against biofilm formation and acid production, as well as the highest cell bio‐compatibility. The mixed EO (SEO:CBEO = 1:4), at concentrations of 1/8 MIC and above, significantly suppressed the expression of virulence factors associated with sugar metabolism ( gtfB , gtfC , gtfD ), biofilm formation ( gbpB , vicR/vicK , spaP ) and acid production ( brpA , relA ) in S. mutans . Cinnamaldehyde and eugenol are the main active constituents of the combination EOs. This study demonstrates that the mixed EOs of S. aromaticum EOand C. burmanii EO exhibit synergistic antimicrobial effects against S. mutans , presenting them as potential antimicrobial agents for the prevention of dental caries.