Improved l-threonine production of Escherichia coli mutant by optimization of culture conditions

l-Threonine production was investigated in a minimal salt medium using l-threonine-overproducing Escherichia coli MT201, derived from E. coli K-12. It was observed that dry cell weight reached 12.5 g/l with 15.9 g/ll-threonine. To increase dry cell weight and l-threonine production, the fermentation process was optimized. When biotin was added as growth factor, l-threonine production reached 52.0 g/l from 15.9 g/l without biotin. Dry cell weight and l-threonine production were further increased by continuous feeding of the feed media with an optimized l-methionine concentration (5.0 g/l). However, high-cell-density culture caused oxygen-limited condition, which resulted in the accumulation of organic acids. To overcome this problem, oxygen-enriched air was supplied to the fermentor with the minimal salt medium. Under these optimal conditions, we achieved an l-threonine production of 80.2 g/l in the minimal salt medium.