We recently engineered Corynebacterium glutamicum for aerobic production of 2-ketoisovalerate by inactivation of the pyruvate dehydrogenase complex, pyruvate:quinone oxidoreductase, transaminase B, and additional overexpression of the ilvBNCD genes, encoding acetohydroxyacid synthase, acetohydroxyacid isomeroreductase, and dihydroxyacid dehydratase. Based on this strain, we engineered C. glutamicum for the production of isobutanol from glucose under oxygen deprivation conditions by inactivation of l -lactate and malate dehydrogenases, implementation of ketoacid decarboxylase from Lactococcus lactis , alcohol dehydrogenase 2 (ADH2) from Saccharomyces cerevisiae , and expression of the pntAB transhydrogenase genes from Escherichia coli . The resulting strain produced isobutanol with a substrate-specific yield (Y _P/S ) of 0.60 ± 0.02 mol per mol of glucose. Interestingly, a chromosomally encoded alcohol dehydrogenase rather than the plasmid-encoded ADH2 from S. cerevisiae was involved in isobutanol formation with C. glutamicum , and overexpression of the corresponding adhA gene increased the Y _P/S to 0.77 ± 0.01 mol of isobutanol per mol of glucose. Inactivation of the malic enzyme significantly reduced the Y _P/S , indicating that the metabolic cycle consisting of pyruvate and/or phosphoenolpyruvate carboxylase, malate dehydrogenase, and malic enzyme is responsible for the conversion of NADH+H ⁺ to NADPH+H ⁺ . In fed-batch fermentations with an aerobic growth phase and an oxygen-depleted production phase, the most promising strain, C. glutamicum Δ aceE Δ pqo Δ ilvE Δ ldhA Δ mdh (pJC4 ilvBNCD-pntAB )(pBB1 kivd-adhA ), produced about 175 mM isobutanol, with a volumetric productivity of 4.4 mM h ⁻¹ , and showed an overall Y _P/S of about 0.48 mol per mol of glucose in the production phase.

Summary: The shortage of oil resources, the steadily rising oil prices and the impact of its use on the environment evokes an increasing political, industrial and technical interest for development of safe and efficient processes for the production of chemicals from renewable biomass. Thus, microbial fermentation of renewable feedstocks found its way in white biotechnology, complementing more and more traditional crude oil‐based chemical processes. Rational strain design of appropriate microorganisms has become possible due to steadily increasing knowledge on metabolism and pathway regulation of industrially relevant organisms and, aside from process engineering and optimization, has an outstanding impact on improving the performance of such hosts. C orynebacterium glutamicum is well known as workhorse for the industrial production of numerous amino acids. However, recent studies also explored the usefulness of this organism for the production of several organic acids and great efforts have been made for improvement of the performance. This review summarizes the current knowledge and recent achievements on metabolic engineering approaches to tailor C . glutamicum for the bio‐based production of organic acids. We focus here on the fermentative production of pyruvate, l ‐ and d ‐lactate, 2‐ketoisovalerate, 2‐ketoglutarate, and succinate. These organic acids represent a class of compounds with manifold application ranges, e.g. in pharmaceutical and cosmetics industry, as food additives, and economically very interesting, as precursors for a variety of bulk chemicals and commercially important polymers.

Summary: The reduction of CO ₂ emissions is a global effort which is not only supported by the society and politicians but also by the industry. Chemical producers worldwide follow the strategic goal to reduce CO ₂ emissions by replacing existing fossil‐based production routes with sustainable alternatives. The smart use of CO and CO ₂ /H ₂ mixtures even allows to produce important chemical building blocks consuming the said gases as substrates in carboxydotrophic fermentations with acetogenic bacteria. However, existing industrial infrastructure and market demands impose constraints on microbes, bioprocesses and products that require careful consideration to ensure technical and economic success. The mini review provides scientific and industrial facets finally to enable the successful implementation of gas fermentation technologies in the industrial scale.

The productivity of industrial fermentation processes is essentially limited by the biomass-specific substrate consumption rate ( q _S ) of the applied microbial production system. Since q _S depends on the growth rate (μ), we highlight the potential of the fastest-growing nonpathogenic bacterium, Vibrio natriegens , as a novel candidate for future biotechnological processes. V. natriegens grows rapidly in BHIN complex medium with a μ of up to 4.43 h ⁻¹ (doubling time of 9.4 min) as well as in minimal medium supplemented with various industrially relevant substrates. Bioreactor cultivations in minimal medium with glucose showed that V. natriegens possesses an exceptionally high q _S under aerobic (3.90 ± 0.08 g g ⁻¹ h ⁻¹ ) and anaerobic (7.81 ± 0.71 g g ⁻¹ h ⁻¹ ) conditions. Fermentations with resting cells of genetically engineered V. natriegens under anaerobic conditions yielded an overall volumetric productivity of 0.56 ± 0.10 g alanine liter ⁻¹ min ⁻¹ (i.e., 34 g liter ⁻¹ h ⁻¹ ). These inherent properties render V. natriegens a promising new microbial platform for future industrial fermentation processes operating with high productivity. IMPORTANCE Low conversion rates are one major challenge to realizing microbial fermentation processes for the production of commodities operating competitively with existing petrochemical approaches. For this reason, we screened for a novel platform organism possessing characteristics superior to those of traditionally employed microbial systems. We identified the fast-growing V. natriegens , which exhibits a versatile metabolism and shows striking growth and conversion rates, as a solid candidate to reach outstanding productivities. Due to these inherent characteristics, V. natriegens can speed up common laboratory routines, is suitable for already existing production procedures, and forms an excellent foundation for engineering next-generation bioprocesses.

Corynebacterium glutamicum was engineered for the production of l -valine from glucose by deletion of the aceE gene encoding the E1p enzyme of the pyruvate dehydrogenase complex and additional overexpression of the ilvBNCE genes encoding the l -valine biosynthetic enzymes acetohydroxyacid synthase, isomeroreductase, and transaminase B. In the absence of cellular growth, C. glutamicum Δ aceE showed a relatively high intracellular concentration of pyruvate (25.9 mM) and produced significant amounts of pyruvate, l -alanine, and l -valine from glucose as the sole carbon source. Lactate or acetate was not formed. Plasmid-bound overexpression of ilvBNCE in C. glutamicum Δ aceE resulted in an approximately 10-fold-lower intracellular pyruvate concentration (2.3 mM) and a shift of the extracellular product pattern from pyruvate and l -alanine towards l -valine. In fed-batch fermentations at high cell densities and an excess of glucose, C. glutamicum Δ aceE (pJC4ilvBNCE) produced up to 210 mM l -valine with a volumetric productivity of 10.0 mM h ⁻¹ (1.17 g l ⁻¹ h ⁻¹ ) and a maximum yield of about 0.6 mol per mol (0.4 g per g) of glucose.