Antimicrobial peptides (AMPs) are bioactive macromolecules that exhibit antibacterial, antiviral, and immunomodulatory functions. They come from a wide range of sources and are found in all forms of life, from bacteria to plants, vertebrates, and invertebrates, and play an important role in controlling the spread of pathogens, promoting wound healing and treating tumors. Consequently, AMPs have emerged as promising alternatives to next-generation antibiotics. With advancements in systems biology and synthetic biology technologies, it has become possible to synthesize AMPs artificially. We can better understand their functional activities for further modification and development by investigating the mechanism of action underlying their antimicrobial properties. This review focuses on the structural aspects of AMPs while highlighting their significance for biological activity. Furthermore, it elucidates the membrane targeting mechanism and intracellular targets of these peptides while summarizing molecular modification approaches aimed at enhancing their antibacterial efficacy. Finally, this article outlines future challenges in the functional development of AMPs along with proposed strategies to overcome them.

Bacillus amyloliquefaciens B10-127 exhibited an excellent ability for industrial-scale microbial fermentation of 2,3-butanediol (2,3-BD) from biodiesel-derived glycerol. However, the accumulation of by-products (acetoin, acetoin, lactate and succinate) and the 2,3-BD yield remains prohibitively low for commercial production. Several strategies were developed to manipulate the carbon flux to 2,3-BD branch in a designed B. amyloliquefaciens. Firstly, extra copies of NADH/NAD+ regeneration system were introduced into B. amyloliquefaciens by co-overproduction of glycerol dehydrogenase and acetoin reductase, which resulting in improvement of 2,3-BD production and suppression of by-products accumulation. Subsequently, the transcriptional regulator ALsR under the control of a moderate promoter PbdhA was introduced into B. amyloliquefaciens, which increased carbon flux to 2,3-BD branch. Finally, a three-stage dissolved oxygen control strategy were proposed based on analysis of the characteristic of 2,3-BD fermentation, and a two-stage pH control strategy were proposed based on different pH preferences of ACR for reduction and oxidation. Following these strategies, a high titer (102.3 g/L), yield (0.44 g/g), and productivity (1.16 g/L/h) of 2,3-BD were achieved. To our knowledge, this is the highest reported 2,3-BD production using biodiesel-derived glycerol as substrate, and this designed B. amyloliquefaciens should be an excellent candidate for producing 2,3-BD on an industrial scale.

L-valine is an essential amino acid that has wide and expanding applications with a suspected growing market demand. Its applicability ranges from animal feed additive, ingredient in cosmetic and special nutrients in pharmaceutical and agriculture fields. Currently, fermentation with the aid of model organisms, is a major method for the production of L-valine. However, achieving the optimal production has often been limited because of the metabolic imbalance in recombinant strains. In this review, the constrains in L-valine biosynthesis are discussed first. Then, we summarize the current advances in engineering of microbial cell factories that have been developed to address and overcome major challenges in the L-valine production process. Future prospects for enhancing the current L-valine production strategies are also discussed.

Acetoin reductase (Acr) catalyzes the conversion of acetoin to 2,3-butanediol (2,3-BD) with concomitant oxidation of NADH to NAD+. Therefore, intracellular 2,3-BD production is likely governed by the quantities of rate-limiting factor(s) Acr and/or NADH. Previously, we showed that a high level of Acr was beneficial for 2,3-BD accumulation. Metabolic engineering strategies were proposed to redistribute carbon flux to 2,3-BD by manipulating NADH levels. The disruption of NADH oxidase (YodC, encoded by yodC) by insertion of a formate dehydrogenase gene in Bacillus subtilis was more efficient for enhancing 2,3-BD production and decreasing acetoin formation than the disruption of YodC by the insertion of a Cat expression cassette. This was because the former resulted in the recombinant strain AFY in which an extra NADH regeneration system was introduced and NADH oxidase was disrupted simultaneously. On fermentation by strain AFY, the highest 2,3-BD concentration increased by 19.9 % while the acetoin titer decreased by 71.9 %, relative to the parental strain. However, the concentration of lactate, the main byproduct, increased by 47.2 %. To further improve carbon flux and NADH to 2,3-BD, the pathway to lactate was blocked using the insertional mutation technique to disrupt the lactate dehydrogenase gene ldhA. The resultant engineered strain B. subtilis AFYL could efficiently convert glucose into 2,3-BD with little acetoin and lactate accumulation. Through increasing the availability of NADH and decreasing the concentration of unwanted byproducts, this work demonstrates an important strategy in the metabolic engineering of 2,3-BD production by integrative recombinant hosts.