Lack of Protective Osmolytes Limits Final Cell Density and Volumetric Productivity of Ethanologenic Escherichia coli KO11 during Xylose Fermentation

Limited cell growth and the resulting low volumetric productivity of ethanologenic Escherichia coli KO11 in mineral salts medium containing xylose have been attributed to inadequate partitioning of carbon skeletons into the synthesis of glutamate and other products derived from the citrate arm of the anaerobic tricarboxylic acid pathway. The results of nuclear magnetic resonance investigations of intracellular osmolytes under different growth conditions coupled with those of studies using genetically modified strains have confirmed and extended this hypothesis. During anaerobic growth in mineral salts medium containing 9% xylose (600 mM) and 1% corn steep liquor, proline was the only abundant osmolyte (71.9 nmol ml⁻¹ optical density at 550 nm OD₅₅₀] unit⁻¹), and growth was limited. Under aerobic conditions in the same medium, twice the cell mass was produced, and cells contained a mixture of osmolytes: glutamate (17.0 nmol ml⁻¹ OD₅₅₀ unit⁻¹), trehalose (9.9 nmol ml⁻¹ OD₅₅₀ unit⁻¹), and betaine (19.8 nmol ml⁻¹ OD₅₅₀ unit⁻¹). Two independent genetic modifications of E. coli KO11 (functional expression of Bacillus subtilis citZ encoding NADH-insensitive citrate synthase; deletion of ackA encoding acetate kinase) and the addition of a metabolite, such as glutamate (11 mM) or acetate (24 mM), as a supplement each increased the intracellular glutamate pool during fermentation, doubled cell growth, and increased volumetric productivity. This apparent requirement for a larger glutamate pool for increased growth and volumetric productivity was completely eliminated by the addition of a protective osmolyte (2 mM betaine or 0.25 mM dimethylsulfoniopropionate), consistent with adaptation to osmotic stress rather than relief of a specific biosynthetic requirement.