Effect of glycine on the cell yield and growth rate of Escherichia coli: evidence for cell-density-dependent glycine degradation as determined by 13C NMR spectroscopy

Abstract

Addition of selected amino acids could be a means to improve production of recombinant proteins in industrial processes. We found that glycine increased the maximum specific growth rate of Escherichia coli from 0.67 to 0.78 h −1, and the cell yield from 0.57 to 0.98 g dry weight per g substrate, when supplemented to batch cultures in a glucose-mineral medium. Maximum effect occurred at pH 6.8, at a glycine concentration of 6–12 mmol l −1, and at cell densities below 1.15 g dry weight l −1 (0D 610·3). When glycine was added to a culture at a cell density of 1.15 g l −1 or above, no growth promoting effect of glycine was seen. The ‘glycine effect’ was not due to CO 2 produced by the glycine cleavage system (GCV), and the lack of effect at higher cell densities was not masked by acetate accumulation, but coincided with increased acetate production. The metabolism of glycine was further investigated in cultures supplied with [2- 13C] labelled glycine, and the redistribution of label in the [1- 13C], [2- 13C], and [1,2- 13C] isotopomeres of excreted acetate was analysed by 13C NMR. The NMR data revealed that very little degradation of glycine occurred at cell densities below 1.15 g l −1. Simultaneously the biosynthesis of serine and glycine was repressed as judged by the absence of [2- 13C] acetate, implying that added glycine was used as a source of glycine, serine, one-carbon units, and threonine. At cell densities above 1.15 g l −1, 53% of the consumed glycine carbon was excreted as acetate. Degradation of glycine was associated with an increased uptake rate, cleavage by GCV, and degradation of both glycine-derived serine, and glucose-derived serine to pyruvate. This switch in metabolism appears to be regulated by quorum sensing.