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286 PROCEEDINGS OF INTERNATIONAL SCIENTIFIC CONFERENCE ON APPLIED BIOTECHNOLOGYvitamin K2 content in animal-based foods, such as 1 mg from 6.25 kg of pork or 7.14 kg of eggs [15], along with the limited consumption of vitamin K2-rich foods (e.g., natto and cheese), results in a low dietary intake of vitamin K2, especially in Vietnam.The biosynthesis of vitamin K2 is a complex process, and the yield produced by microorganisms remains relatively low. Consequently, numerous studies have sought to improve vitamin K2 production. Strategies for enhancing microbial vitamin K2 production include: 1) mutagenesis using structural analogs, such as 1,4-dihydroxy-2-naphthoate and 1-hydroxy-2-naphthoate; 2) genetic modifications to alter metabolic pathways in Bacillusor construct new pathways in other model microorganisms; 3) innovative bioreactor designs; 4) optimization of fermentation conditions, including adjustments to medium composition and cultivation parameters; and 5) enhancement of vitamin K2 secretion through the addition of surfactants or detergents [16].This study represents an initial step toward optimizing medium composition for improved vitamin K2 production. Among the Bacillus subtilis strains analyzed, NTNS1, BST1, and BST2 emerged as promising vitamin K2 producers. Further investigation, focused on strain BST2, showed that this strain produced a higher yield of vitamin K2 (1309.7 %u00b5g/L) in a medium supplemented with 0.5 g/L K2HPO4 compared to media with K2HPO4 concentrations ranging from 1 to 10 g/L. These findings align with previous studies that identified 0.4 g/L [7] and 0.5 g/L [9] as optimal K2HPO4 concentrations for menaquinone production. In contrast, Hu et al. (2017) reported that Bacillus nattoR127 achieved optimal menaquinone-7 synthesis in a medium containing 10 g/L K2HPO4 [17].Soybean peptone is considered an optimal nitrogen source for Bacillus subtilis in vitamin K2 production; however, excessive