Antifungal Activity of Essential Oil and Ethanolic Propolis Extract Mixtures against BiofilmProducing Pichia manshurica

Propolis is a bee product produced by honeybees (Apis mellifera L.) using plant-based resin mixed with beeswax and salivary enzymes (β-glucosidase). It has extensive biological activity. Essential oils are fragrant oily liquids with various biological activities obtained from different parts of plants. Treating diseases is becoming increasingly challenging due to the rise of antibiotic-resistant microorganisms. Biofilm structure is an important virulence factor that contributes to antibiotic resistance. In this study, we investigated the in vitro antifungal activity of 10 essential oils and mixtures of ethanolic propolis extract and essential oil (1:1, v/v) against Pichia manshurica, a strong biofilm producer isolated from the mouths of young individuals, using the agar well diffusion method. The results showed that orange, lemon, garlic, black grape seed, grape seed, pomegranate seed, and black cumin oils did not exhibit antifungal activity. Thyme and rosemary essential oils gave inhibition zones of 23.81±1.45 mm and 35.93±2.00 mm, respectively. It was found that when mixed with propolis at a 1:1 (v/v) ratio, all essential oils except thyme and rosemary exhibited a synergistic effect, leading to a 216% to 297.83% increase in antifungal activity. On the other hand, combining thyme and rosemary essential oil with propolis at the same ratio resulted in an antagonistic effect, causing a reduction in antifungal activity by 24.40% and 13.36%, respectively. Consequently, thyme and rosemary essential oils displayed the highest antifungal activity among the essential oils tested. Furthermore, the study revealed that the impact of propolis on antifungal activity varied depending on the essential oil used, demonstrating both synergistic and antagonistic effects. Therefore, while the benefits of propolis have been scientifically established, it should not be assumed that it consistently enhances the desired activity when interpreting the study's data.