Enhanced β‐galactosidase production by high cell‐density culture of recombinant Bacillus subtilis with glucose concentration control

Cell concentration, recombinant protein (β‐galactosidase) level, and the specific enzyme expression level were increased from 19 to 184 g/L, 18.3 to 129 U/mL, and 3.2 to 5.7 U/mg protein, respectively, in fed‐batch culture of recombinant Bacillus subtilis when glucose concentration was controlled at 1 g/L as compared with those of conventional fed‐batch culture. Glucose concentration of the culture broth was monitored by an automatic on‐line glucose analyzer and controlled with a moving identification combined with optimal control (MICOC) strategy. When glucose concentrations were controlled at 10, 1, and 0.2 g/L, accumulated propionic acid concentrations and specific enzyme activities were 18.5, 4.4, and 0.6 g/L and 2.9, 5.7, and 7.1 U/mg protein, respectively. The addition of various concentrations of sodium propionate to the growth medium in batch cultures resulted in a drastic decrease in the growth rate with respect to propionate concentration. The propionic acid was shown to be responsible for cell growth inhibition and enzyme activity reduction in fed‐batch culture. © 1992 John Wiley & Sons, Inc.