Iranian Journal of Medical Microbiology

Background and Aim: The growing prevalence of antibiotic-resistant oral pathogens has increased interest in medicinal plants as alternative sources of antimicrobial agents. Melissa (M.) officinalis L. (lemon balm) is rich in bioactive phytochemicals, particularly terpenoids, known for their antimicrobial potential. This study aimed to evaluate the antibacterial activity of M. officinalis extracts and isolated terpenoids against dental plaque–associated bacteria and to investigate their inhibitory mechanisms against penicillin-binding protein (PBP) and β-lactamase using in vitro and in silico approaches.
Materials and Methods: M. officinalis extracts and terpenoids were tested against Streptococcus viridans, Porphyromonas gingivalis, Prevotella intermedia, and Aggregatibacter actinomycetemcomitans. Antibacterial efficacy was assessed using inhibition zone assay, minimum inhibitory concentration (MIC), and minimum bactericidal concentration (MBC), with amoxicillin as a reference drug. Molecular docking was conducted to analyze the binding interactions of ursolic acid and amoxicillin with consensus-modeled PBP and β-lactamase proteins.
Results: Both crude extracts and terpenoids exhibited strong antibacterial activity, producing large inhibition zones and low MIC and MBC values as compared to amoxicillin. S. viridans displayed greater resistance, with MIC and MBC values approximately twice those of other bacteria. Docking analysis demonstrated strong binding affinities and high specificity for terpenoids toward β-lactamase and PBP, forming stable protein–ligand complexes.
Conclusion: M. officinalis terpenoids represented promising natural inhibitors of PBP and β-lactamase and may serve as alternative antimicrobial agents for controlling dental plaque–associated infections.