In vitro evaluation of the antimicrobial activity of kiwi and stevia extracts against oral pathogenic microorganisms

Objective: Dental caries is the most common chronic disease in childhood, and interest in natural methods in preventive dentistry is increasing. This study aims to evaluate the antimicrobial activity of kiwi and stevia extracts on cariogenic microorganisms. Methods: Ethanolic extracts of kiwi and stevia were prepared, and their sugar, fat, and polyphenolic content were analyzed. Antimicrobial activity against S. mutans and L. acidophilus was assessed using broth microdilution, minimum bactericidal concentration, time-kill, and biofilm susceptibility studies. Statistical analysis was performed with SPSS software (SPSS Statistics v23.0 for Windows, Chicago, IL, USA). Results: The antioxidant capacity and total phenolic and flavonoid content of stevia extract were found to be 6-17 times higher than those of kiwi extract relatively. In the broth microdilution test, the MIC and MBC values of stevia extract for S. mutans were lower than those of kiwi extract, indicating greater effectiveness. For L. acidophilus, no significant differences were observed between the extracts. Time-kill study results showed that at high doses, kiwi extract exhibited bactericidal activity against S. mutans within 3 hours, but this was not observed at lower doses. Stevia extract demonstrated bactericidal activity at all doses within 24 hours, suggesting it is a more potent antimicrobial agent than kiwi extract. Both extracts were less effective against L. acidophilus compared to their solvents. The evaluation of killing kinetics test results using optical density is thought to potentially lead to procedural errors. Biofilm susceptibility testing revealed that while both extracts were effective against S. mutans, stevia showed higher efficacy. Against L. acidophilus biofilms, only stevia extract was effective. Chlorhexidine outperformed both extracts in all tests. Conclusion: Stevia extract demonstrated significant antimicrobial activity against S. mutans and shows potential in preventive dentistry. In contrast, the efficacy of kiwi extract against cariogenic microorganisms was limited. Further studies are needed to explore the clinical applications of plant extracts.