Bifunctional Malic/Malolactic Enzyme Provides a Novel Mechanism for NADPH-Balancing in Bacillus subtilis

Unveiling malic acid biorefinery: Comprehensive insights into feedstocks, microbial strains, and metabolic pathways

The over-reliance on fossil fuels and resultant environmental issues necessitate sustainable alternatives. Microbial fermentation of biomass for malic acid production offers a viable, eco-friendly solution, enhancing resource efficiency and minimizing ecological damage. This review covers three core aspects of malic acid biorefining: feedstocks, microbial strains, and metabolic pathways. It emphasizes the significance of utilizing biomass sugars, including the co-fermentation of different sugar types to improve feedstock efficiency. The review discusses microbial strains for malic acid fermentation, addressing challenges related to by-products from biomass breakdown and strategies for overcoming them. It delves into the crucial pathways and enzymes for malic acid production, outlining methods to optimize its metabolism, focusing on enzyme regulation, energy balance, and yield enhancement. These insights contribute to advancing the field of consolidated bioprocessing in malic acid biorefining.

Optimization of hydrogen production in Enterobacter aerogenes by Complex I peripheral fragments destruction and maeA overexpression

As a concentrated energy source with high added value, hydrogen has great development prospects, with special emphasis on sustainable microbial production as a replacement for traditional fossil fuels. In this study, λ-Red recombination was used to alter the activity of Complex I by single and combined knockout of nuoE, nuoF and nuoG. In addition, the conversion of malic to pyruvic acid was promoted by overexpressing the maeA gene, which could increase the content of NADH and formic acid in the bacterial cells. Compared to the original strain, hydrogen production was 65% higher in the optimized strain IAM1183-EFG/M, in which the flux of the formic acid pathway was increased by 257%, the flux of the NADH pathway was increased by 13%, and the content of metabolites also changed significantly. In further bioreactor, the total hydrogen production of the scale-up IAM1183-EFG/M after 44 h of fermentation was 4.76 L, which increased by 18% compared with the starting strain. This study provides a new direction for future exploration of microbial hydrogen production by combinatorial modification of multiple genes.

mleS in Staphylococcus aureus Contributes to Microaerobic Metabolic Activity, Abscess Formation, and Survival in Macrophages

Staphylococcus aureus is subdivided into lineages termed sequence types (STs), infections of which necessitate the expression of virulence factors and metabolic adaptation to the host niche. Given that mechanisms underlying the dynamic replacement of sequence types in S. aureus populations have yet to be sufficiently determined, we investigated the role of metabolic determinants in epidemic clones. mleS, encoding the NAD+-dependent malolactic enzyme, was found to be carried by the epidemic clones ST59 and ST398, although not by ST239 and ST5. The genomic location of mleS in the metabolism-associated region flanked by the thiol-specific redox system and glycolysis operon implies that it plays significant roles in metabolism and pathogenesis. Mouse skin abscess caused by the BS19-mleS mutant strain (isogenic mleS mutant in an ST59 isolate) was significantly attenuated and associated with reductions in interleukin-6 (IL-6) and lactic acid production. mleS deletion also impaired S. aureus biofilm formation and survival in RAW264.7 cells. The BS19-mleS-mutant was also characterized by reduced ATP and lactic acid production under microaerobic conditions; however, NAD+/NADH levels remained unaffected. mleS is thus identified as an epidemiological marker that plays an important role in the microaerobic metabolism and pathogenesis of epidemic S. aureus clones. IMPORTANCE Given the importance of metabolic adaptation during infection, new insights are required regarding the pathogenesis of S. aureus, particularly for epidemic clones. We accordingly investigated the role of metabolic determinants that are unique to the epidemic clones ST59 and ST398. Our results provide evidence that the NAD+-dependent malolactic enzyme-coding gene mleS is an epidemiological marker that plays an important role in the microaerobic metabolism and pathogenesis of epidemic S. aureus clones.

An NADPH-auxotrophic Corynebacterium glutamicum recombinant strain and used it to construct L-leucine high-yielding strain

The NADPH-regeneration enzymes in Corynebacterium glutamicum were inactivated to construct an NADPH-auxotrophic C. glutamicum strain by gene knockout and gene replacement. The resultant NADPH-auxotrophic C. glutamicum XL-1 ΔZMI_Cg::I_Sm (i.e., strain Leu-1) grew well in the basic medium only with gluconate as carbon source. Replacement of the native glyceraldehyde 3-phosphate dehydrogenase (NAD-GapDH_Cg) by NADP-GapDH_Ca from Clostridium acetobutylicum is an effective strategy for producing L-leucine in NADPH-prototrophic strain XL-1 and NADPH-auxotrophic strain Leu-1, whereas the L-leucine yield did not differ significantly between these strains (14.1 ± 1.8 g/L vs 16.2 ± 1.1 g/L). Enhancing the carbon flux in biosynthetic pathway by recombinant expression plasmid pEC-ABNCE promoted L-leucine production, but the shortage NADPH supply limited the L-leucine yield. The mutated promoters of zwf and icd_Cg were introduced into C. glutamicum with NADP-GapDH_Ca and pEC-ABNCE increased L-leucine yield (54.3 ± 2.9 g/L) and improved cell growth (OD₅₆₂ = 83.4 ± 7.5) in fed-batch fermentation because the resultant strain C. glutamicum XL-1 ΔMI_Cg::I_Sm G_Cg::G_Ca P_zwf-D1 P_icd-D2/pEC-ABNCE (i.e., strain Leu-9) exhibited the proper intracellular NADPH and NADH level. This is the first report of constructing an L-leucine high-yielding strain that reasonably supplies NADPH by optimizing the biosynthetic pathway of NADPH from an NADPH-auxotrophic strain.

Occurrence and potential mechanism of holin-mediated non-lytic protein translocation in bacteria

Holins are generally believed to generate large membrane lesions that permit the passage of endolysins across the cytoplasmic membrane of prokaryotes, ultimately resulting in cell wall degradation and cell lysis. However, there are more and more examples known for non-lytic holin-dependent secretion of proteins by bacteria, indicating that holins somehow can transport proteins without causing large membrane lesions. Phage-derived holins can be used for a non-lytic endolysin translocation to permeabilize the cell wall for the passage of secreted proteins. In addition, clostridia, which do not possess the Tat pathway for transport of folded proteins, most likely employ non-lytic holin-mediated transport also for secretion of toxins and bacteriocins that are incompatible with the general Sec pathway. The mechanism for non-lytic holin-mediated transport is unknown, but the recent finding that the small holin TpeE mediates a non-lytic toxin secretion in Clostridium perfringens opened new perspectives. TpeE contains only one short transmembrane helix that is followed by an amphipathic helix, which is reminiscent of TatA, the membrane-permeabilizing component of the Tat translocon for folded proteins. Here we review the known cases of non-lytic holin-mediated transport and then focus on the structural and functional comparison of TatA and TpeE, resulting in a mechanistic model for holin-mediated transport. This model is strongly supported by a so far not recognized naturally occurring holin-endolysin fusion protein.