Strain Selection and Optimization of Mixed Culture Conditions for Lactobacillus pentosus K1-23 with Antibacterial Activity and Aureobasidium pullulans NRRL 58012 Producing Immune-Enhancing ��-Glucan

Lactobacillus pentosus K1-23 was selected from among 25 lactic acid bacterial strains owing to its high inhibitory activity against several pathogenic bacteria, including Escherichia coli , Salmonella typhimurium , S. gallinarum , Staphylococcus aureus , Pseudomonas aeruginosa , Clostridium perfringens , and Listeria monocytogenes . Additionally, among 13 strains of Aureobasidium spp., A. pullulans NRRL 58012 was shown to produce the highest amount of β-glucan (15.45 ± 0.07%) and was selected. Next, the optimal conditions for a solid-phase mixed culture with these two different microorganisms (one bacterium and one yeast) were determined. The optimal inoculum sizes for L. pentosus and A. pullulans were 1% and 5%, respectively. The appropriate inoculation time for L. pentosus K1-23 was 3 days after the inoculation of A. pullulans to initiate fermentation. The addition of 0.5% corn steep powder and 0.1% FeSO₄ to the basal medium resulted in the increased production of lactic acid bacterial cells and β-glucan. The following optimal conditions for solid-phase mixed culture were also statistically determined by using the response surface method: 37.84°C, pH 5.25, moisture content of 60.82%, and culture time of 6.08 days for L. pentosus ; and 24.11°C, pH 5.65, moisture content of 60.08%, and culture time of 5.71 days for A. pullulans. Using the predicted optimal conditions, the experimental production values of L. pentosus cells and β-glucan were 3.15 ± 0.10 × 10⁸ CFU/g and 13.41 ± 0.04%, respectively. This mixed culture may function as a highly efficient antibiotic substitute based on the combined action of its anti-pathogenic bacterial and immune-enhancing activities.