The pyruvate decarboxylase activity of IpdC is a limitation for isobutanol production by Klebsiella pneumoniae

Targeted pretreatment and inoculation strategies for horse manure fermentation: Impact on metabolites and microbial community composition

Horse manure is a lignocellulosic biomass found in significant quantities with a vast indigenous flora, not yet fully valorized apart from anaerobic digestion. Its use in the fermentation process can lead to the production of higher-value metabolites. This study investigates three inoculation strategies coupled with five pretreatment conditions for horse manure fermentation. Two microwave pretreatments (200W and 1000W) were compared with a conventional thermal pretreatment, a thermo-acid pretreatment, and an unpretreated condition. The sole horse manure indigenous microorganisms were used in fermentation and compared with two inoculation strategies using external inoculum, which was i) thermally treated or ii) pretreated simultaneously with manure. A statistically similar total metabolite production (0.088 ± 0.010 gCOD/gVS) was observed, with more than 50 % of acetate produced for all the pretreated conditions. When no pretreatment was performed (Ctrl), methane was produced as a major metabolite. The metabolic profile of the thermo-acid pretreatment condition using solely indigenous microorganisms was different from the other conditions, with ethanol (0.015 ± 0.004 gCOD/gVS) and hydrogen (0.009 ± 0.002 gCOD/gVS) production. This was related to the Klebsiella genus abundance increase recorded for this condition. Both microwave pretreatments shared similar metabolite results and microbial composition with the conventional thermal pretreatment. However, a heat shock is needed to inhibit methane production from archaea and can be performed by microwave or conventional thermal pretreatment. To conclude, indigenous horse manure microorganisms are suitable for fermentation with equivalent yields compared to an external inoculum from a wastewater treatment plant whatever the pretreatment applied. However, a heat shock is needed to inhibit methane production from archaea and can be performed via microwave or conventional thermal pretreatment.

Mechanism of metabolites distribution between 2,3-butanediol and branched-chain amino acid synthesis pathways in Klebsiella pneumoniae

Klebsiella pneumoniae is a commonly known 2,3-butanediol producer. 2,3-Butanediol synthesis and branched-chain amino acid (BCAA) synthesis pathways share the same step of α-acetolactate synthesis from pyruvate. Those two pathways do not interfere with each other in the wild-type strain. Knocking out budA (encoding α-acetolactate decarboxylase) blocks the 2,3-butanediol synthesis pathway. Meanwhile, metabolites of the BCAA synthesis pathway (valine, 2-ketoisovalerate, 2,3-dihydroxyisovalerate and 2-hydroxyisovalerate) are accumulated. However, the mechanism underlying the metabolite changes resulting from the inactivation of budA remains unclear. In this study, both ex vivo and in vitro experiments were conducted to elucidate this mechanism. Kinetic parameters of BudA and acetohydroxy acid isomeroreductase (IlvC) were determined. BudA has a higher affinity toward α-acetolactate and has a higher catalytic constant (Km = 3.66 mM, kcat = 7.8 s-1) compared to IlvC (Km = 17.98 mM, kcat = 0.68 s-1). ex vivo experiments showed that IlvC activities were not influenced by knocking out budA and vice versa. IlvC activities were improved in the cells in which ilvC was overexpressed, but this did not lead to the accumulation of metabolites of the BCAA synthesis pathway. The activities of IlvC in the cell were not affected by the accumulation of 2,3-dihydroxyisovalerate, 2-ketoisovalerate, or valine in the broth. These results indicated that the competitiveness of BudA and IlvC in the cell determines the metabolites distribution between those two pathways. The inactivation of BudA and intact IlvC led to the exceeded α-acetolactate flow into the BCAA synthesis pathway, which caused the accumulation of metabolites of the BCAA synthesis pathway.

Microbial host engineering for sustainable isobutanol production from renewable resources

Due to the limited resources and environmental problems associated with fossil fuels, there is a growing interest in utilizing renewable resources for the production of biofuels through microbial fermentation. Isobutanol is a promising biofuel that could potentially replace gasoline. However, its production efficiency is currently limited by the use of naturally isolated microorganisms. These naturally isolated microorganisms often encounter problems such as a limited range of substrates, low tolerance to solvents or inhibitors, feedback inhibition, and an imbalanced redox state. This makes it difficult to improve their production efficiency through traditional process optimization methods. Fortunately, recent advancements in genetic engineering technologies have made it possible to enhance microbial hosts for the increased production of isobutanol from renewable resources. This review provides a summary of the strategies and synthetic biology approaches that have been employed in the past few years to improve naturally isolated or non-natural microbial hosts for the enhanced production of isobutanol by utilizing different renewable resources. Furthermore, it also discusses the challenges that are faced by engineered microbial hosts and presents future perspectives to enhancing isobutanol production. KEY POINTS: • Promising potential of isobutanol to replace gasoline • Engineering of native and non-native microbial host for isobutanol production • Challenges and opportunities for enhanced isobutanol production.