Antibacterial activity of oregano essential oils against Streptococcus mutans in vitro and analysis of active components

Streptococcus mutans (S. mutans) is considered the most relevant bacteria during the transition of the non-pathogenic commensal oral microbial community to plaque biofilms that promote the development of dental caries. Oregano (Origanum vulgare L.), is a universally natural flavoring and its essential oil has been demonstrated to have good antibacterial effects. However, the specific antibacterial mechanism of oregano essential oil (OEO) against S. mutans is still not completely understood.

In this work, the composition of two different OEOs was determined by GC‒MS. Disk-diffusion method, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined to assess their antimicrobial effect on S. mutans. The inhibition of acid production, hydrophobicity, biofilm formation and real-time PCR for gtfB/C/D, spaP, gbpB, vicR, relA and brpA mRNA expression by S. mutans were assessed to preliminarily investigate the mechanisms of action. Molecular docking was performed to simulate the interactions with the virulence proteins and active constituents. MTT test using immortalized human keratinocytes cells was also performed to investigate cytotoxicity.

Compared with the positive drug Penicillin /streptomycin 100X (DIZ: 34.13 ± 0.85 mm, MIC: 0.78125 μL/mL, MBC: 6.25 μL/mL), the essential oils of Origanum vulgare L. (DIZ: 80 mm, MIC: 0.625μL/mL, MBC:2.5μL/mL) and Origanum heracleoticum L. (DIZ: 39.67 ± 0.81 mm, MIC: 0.625μL/mL, MBC: 1.25μL/mL) could also exhibit similar effects to inhibit the acid production and reduce the hydrophobicity and biofilm formation of S. mutans at 1/2-1MIC concentration. And gene expression of gtfB/C/D, spaP, gbpB, vicR and relA were found to be downregulated. Due to the composition of essential oils from different sources being highly variable, through effective network pharmacology analysis, we found that OEOs contained many effective compounds, like carvacrol and its biosynthetic precursors γ-terpinene and p-cymene, which may directly target several virulence proteins of S. mutans. Besides, no toxic effect was instigated by OEOs at 0.1 μL/mL in the immortalized human keratinocytes cells.

The integrated analysis in the present study suggested that OEO might be a potential antibacterial agent for the prevention of dental caries.