Effects of <i>Mentha piperita</i> L. and <i>Stevia</i> rebaudiana extracts on the growth of lactic acid bacteria and their potential for functional food development

Background: A wide variety of prebiotic and probiotic formulations are currently employed in modern medicine to manage gastrointestinal disorders. Mentha piperita L. (peppermint) is well known for its anti-inflammatory, antipyretic, antihypertensive, and anti-allergenic properties. Due to its distinctive aroma and bioactive profile, it is widely used in traditional medicine, the food industry, and cosmetics/perfumery. Stevia rebaudiana is another valuable plant that contains steviol glycosides, widely recognized as natural, non-caloric sweeteners. These compounds not only enhance food palatability but also support the nutritional needs of individuals with dietary restrictions, including those managing diabetes and/or obesity. Objective: The research aimed to evaluate the effects of these plant extracts on the growth of Lactobacillus acidophilus Er 317/402 and Lactobacillus delbrueckii subsp. bulgaricus B7—two probiotic strains with well-established health benefits. Results: Both aqueous and ethanolic M. piperita extracts showed no inhibitory effects on LAB, indicating compatibility with plant bioactives. The two strains exhibited mutualistic interaction in co-culture, supporting their combined use. The optimal formulation included 10% M. piperita, 2.5% S. rebaudiana, and 5% inoculum of both strains, with fermentation optimized at 37 °C for 4.5 hours in 1% fat raw milk. Novelty: This study is among the first to explore the combined impact of M. piperita and S. rebaudiana extracts on probiotic lactic acid bacteria (LAB) within a dairy matrix. Furthermore, it uniquely integrates a honeybee gut-derived Lactobacillus acidophilus strain with a standard probiotic, demonstrating their synergistic potential in developing functional dairy products enriched with bioactive plant compounds. This dual innovation—using both underexplored plant-microbe interactions and a novel probiotic source—provides new perspectives for designing nutritionally enhanced and shelf-stable fermented foods. Conclusion: The final product exhibited antibacterial, anti-yeast, and antifungal activity, suggesting improved shelf life and safety— key traits for functional foods. This is among the first studies to use LAB from the honeybee gut with plant extracts in dairy fermentation. The synergy between a novel bee-derived strain and a standard probiotic not only produced a nutrient- and bioactive-rich product but also demonstrates the commercial potential in enhancing shelf life and health benefits. Keywords: lactic acid bacteria, pre- and probiotics, Mentha piperita L., Stevia rebaudiana, functional food