Culture Optimization Strategy for 1-Deoxynojirimycin-producing Bacillus methylotrophicus K26 Isolated from Korean Fermented Soybean Paste, Doenjang

1-Deoxynojirimycin (1-DNJ) is an α-glucosidase inhibitor that is used for the treatment of type 2 diabetes. In this study, we isolated Bacillus methylotrophicus K26 with α-glucosidase inhibition (AGI) activity from Korean fermented soybean paste (Doenjang) and confirmed that the genome harbored the DNJ biosynthesis genes including gabT1, yktc1, and gutB1 by PCR screening, while 1-DNJ production was confirmed by ultra-performance liquid chromatography–quadrupole time-of-flight–mass spectrometry. To increase 1-DNJ production by B. methylotrophicus K26, culture conditions were optimized with one-factor-ata- time (OFAT) and response surface methodology (RSM) approaches. Screen of 11 carbon and 9 nitrogen sources by the OFAT method identified sucrose and yeast extract as optimal culture components. Sucrose concentration (X1), yeast extract concentration (X2), and culture temperature (X3) were selected as independent variables for central composite design. The coefficient of determination (R2) for the model was 0.927, and the probability value of the regression model was highly significant. RSM predicted the optimal conditions for 1-DNJ production by B. methylotrophicus K26 as sucrose and yeast extract concentrations of 4.61% and 7.03%, respectively, at a temperature of 34°C. Under these conditions, AGI activity was experimentally measured as 89.3%, which was close to the predicted value of 91.9%.