Improved 2-methyl-1-propanol production in an engineered Bacillus subtilis by constructing inducible pathways

Pot-Pollen Volatiles, Bioactivity, Synergism with Antibiotics, and Bibliometrics Overview, Including Direct Injection in Food Flavor

Stingless bees (Hymenoptera; Apidae; Meliponini), with a biodiversity of 605 species, harvest and transport corbicula pollen to the nest, like Apis mellifera, but process and store the pollen in cerumen pots instead of beeswax combs. Therefore, the meliponine pollen processed in the nest was named pot-pollen instead of bee bread. Pot-pollen has nutraceutical properties for bees and humans; it is a natural medicinal food supplement with applications in health, food science, and technology, and pharmaceutical developments are promising. Demonstrated synergism between Tetragonisca angustula pot-pollen ethanolic extracts, and antibiotics against extensively drug-resistant (XDR) bacteria revealed potential to combat antimicrobial resistance (AMR). Reviewed pot-pollen VOC richness was compared between Australian Austroplebeia australis (27), Tetragonula carbonaria (31), and Tetragonula hogkingsi (28), as well as the Venezuelan Tetragonisca angustula (95). Bioactivity and olfactory attributes of the most abundant VOCs were revisited. Bibliometric analyses with the Scopus database were planned for two unrelated topics in the literature for potential scientific advances. The top ten most prolific authors, institutions, countries, funding sponsors, and sources engaged to disseminate original research and reviews on pot-pollen (2014-2023) and direct injection food flavor (1976-2023) were ranked. Selected metrics and plots were visualized using the Bibliometrix-R package. A scholarly approach gained scientific insight into the interaction between an ancient fermented medicinal pot-pollen and a powerful bioanalytical technique for fermented products, which should attract interest from research teams for joint projects on direct injection in pot-pollen flavor, and proposals on stingless bee nest materials. Novel anti-antimicrobial-resistant agents and synergism with conventional antibiotics can fill the gap in the emerging potential to overcome antimicrobial resistance.

Microbiota Composition during Fermentation of Broomcorn Millet Huangjiu and Their Effects on Flavor Quality

Broomcorn millet Huangjiu brewing is usually divided into primary fermentation and post-fermentation. Microbial succession is the major factor influencing the development of the typical Huangjiu flavor. Here, we report the changes in flavor substances and microbial community during the primary fermentation of broomcorn millet Huangjiu. Results indicated that a total of 161 volatile flavor compounds were measured during primary fermentation, and estragole was detected for the first time in broomcorn millet Huangjiu. A total of 82 bacteria genera were identified. Pediococcus, Pantoea, and Weissella were the dominant genera. Saccharomyces and Rhizopus were dominant among the 30 fungal genera. Correlation analysis showed that 102 microorganisms were involved in major flavor substance production during primary fermentation, Lactobacillus, Photobacterium, Hyphodontia, Aquicella, Erysipelothrix, Idiomarina, Paraphaeosphaeria, and Sulfuritalea were most associated with flavoring substances. Four bacteria, Lactobacillus (R1), Photobacterium (R2), Idiomarina (R3), and Pediococcus (R4), were isolated and identified from wheat Qu, which were added to wine Qu to prepare four kinds of fortified Qu (QR1, QR2, QR3, QR4). QR1 and QR2 fermentation can enhance the quality of Huangjiu. This work reveals the correlation between microorganisms and volatile flavor compounds and is beneficial for regulating the micro-ecosystem and flavor of the broomcorn millet Huangjiu.

Toward bioproduction of oxo chemicals from C1 feedstocks using isobutyraldehyde as an example

Oxo chemicals are valuable chemicals for synthesizing a wide array of industrial and consumer products. However, producing of oxo chemicals is predominately through the chemical process called hydroformylation, which requires petroleum-sourced materials and generates abundant greenhouse gas. Current concerns on global climate change have renewed the interest in reducing greenhouse gas emissions and recycling the plentiful greenhouse gas. A carbon–neutral manner in this regard is producing oxo chemicals biotechnologically using greenhouse gas as C1 feedstocks. Exemplifying isobutyraldehyde, this review demonstrates the significance of using greenhouse gas for oxo chemicals production. We highlight the current state and the potential of isobutyraldehyde synthesis with a special focus on the in vivo and in vitro scheme of C1-based biomanufacturing. Specifically, perspectives and scenarios toward carbon– and nitrogen–neutral isobutyraldehyde production are proposed. In addition, key challenges and promising approaches for enhancing isobutyraldehyde bioproduction are thoroughly discussed. This study will serve as a reference case in exploring the biotechnological potential and advancing oxo chemicals production derived from C1 feedstocks.

Microbial succession and exploration of higher alcohols-producing core bacteria in northern Huangjiu fermentation

Higher alcohols (HAs) are abundant compounds that provide important flavors in Huangjiu, but they also cause hangover. Previous studies have shown the production of HAs to be related to yeast, but the correlations between HAs and other microorganisms are rarely reported. In this study, we detected changes in levels of HAs and microbial dynamics during the Huangjiu fermentation process. Relationships were characterized using Pearson’s correlation coefficient. The functional core HA-producing bacteria were selected by bidirectional orthogonal partial least squares (O2PLS). The result showed that 2-methyl-1-propanol, phenethyl alcohol and 3-methyl-1-butanol were the principle HAs present at high levels. Lactococcus and Saccharomyces were predominant at the genus level of bacteria and fungi, respectively. A total of 684 correlations between HAs and microorganisms were established. Five genera were screened as functional core HA-producing bacteria. Our findings might provide some new inspiration for controlling the content of HAs, enhancing international prestige and market expansion of Huangjiu.

A synthetic anhydrotetracycline-controllable gene expression system in Ralstonia eutropha H16

Controllable gene expression systems that are orthogonal to the host's native gene regulation network are invaluable tools for synthetic biology. In Ralstonia eutropha H16, such systems are extremely limited despite the importance of this organism in microbiological research and biotechnological application. Here we developed an anhydrotetracycline (aTc)-inducible gene expression system, which is composed of a synthetic promoter containing the operator tetO, the repressor TetR, and the inducer aTc. Using a reporter-activity based promoter library screen, we first identified the active hybrids between the tetO operators and the R. eutropha native rrsC promoter (PrrsC). Next, we showed that the hybrid promoters are repressable by TetR. To optimize the dynamic range of the system, a high-throughput screening of 300 mutants of R. eutropha phaC1 promoter was conducted to identify suitable promoters to tune the tetR expression level. The final controllable expression system contains the modified PrrsC with two copies of the tetO1 operator integrated and the tetR driven by the mutated PphaC1. The system has decreased basal expression level and can be tuned by different aTc concentrations with greater than 10-fold dynamic range. The system was used to alleviate cellular toxicity caused by AlsS overexpression, which impeded our metabolic engineering work on isobutanol and 3-methyl-1-butanol production in R. eutropha H16.

Metabolic engineering of microorganisms for the production of higher alcohols

Due to the increasing concerns about limited fossil resources and environmental problems, there has been much interest in developing biofuels from renewable biomass. Ethanol is currently used as a major biofuel, as it can be easily produced by existing fermentation technology, but it is not the best biofuel due to its low energy density, high vapor pressure, hygroscopy, and incompatibility with current infrastructure. Higher alcohols, including 1-propanol, 1-butanol, isobutanol, 2-methyl-1-butanol, and 3-methyl-1-butanol, which possess fuel properties more similar to those of petroleum-based fuel, have attracted particular interest as alternatives to ethanol. Since microorganisms isolated from nature do not allow production of these alcohols at high enough efficiencies, metabolic engineering has been employed to enhance their production. Here, we review recent advances in metabolic engineering of microorganisms for the production of higher alcohols.

Model-driven redox pathway manipulation for improved isobutanol production in Bacillus subtilis complemented with experimental validation and metabolic profiling analysis

To rationally guide the improvement of isobutanol production, metabolic network and metabolic profiling analysis were performed to provide global and profound insights into cell metabolism of isobutanol-producing Bacillus subtilis. The metabolic flux distribution of strains with different isobutanol production capacity (BSUL03, BSUL04 and BSUL05) drops a hint of the importance of NADPH on isobutanol biosynthesis. Therefore, the redox pathways were redesigned in this study. To increase NADPH concentration, glucose-6-phosphate isomerase was inactivated (BSUL06) and glucose-6-phosphate dehydrogenase was overexpressed (BSUL07) successively. As expected, NADPH pool size in BSUL07 was 4.4-fold higher than that in parental strain BSUL05. However, cell growth, isobutanol yield and production were decreased by 46%, 22%, and 80%, respectively. Metabolic profiling analysis suggested that the severely imbalanced redox status might be the primary reason. To solve this problem, gene udhA of Escherichia coli encoding transhydrogenase was further overexpressed (BSUL08), which not only well balanced the cellular ratio of NAD(P)H/NAD(P)⁺, but also increased NADH and ATP concentration. In addition, a straightforward engineering approach for improving NADPH concentrations was employed in BSUL05 by overexpressing exogenous gene pntAB and obtained BSUL09. The performance for isobutanol production by BSUL09 was poorer than BSUL08 but better than other engineered strains. Furthermore, in fed-batch fermentation the isobutanol production and yield of BSUL08 increased by 11% and 19%, up to the value of 6.12 g/L and 0.37 C-mol isobutanol/C-mol glucose (63% of the theoretical value), respectively, compared with parental strain BSUL05. These results demonstrated that model-driven complemented with metabolic profiling analysis could serve as a useful approach in the strain improvement for higher bio-productivity in further application.