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.

With the increasing use of animal and plant proteins, pollution due to nitrogen sources is attracting increasing attention. In particular, the amount of nitrogen-containing sewage discharged into the environment has increased significantly, causing eutrophication of water bodies and environmental degradation of water quality. Traditionally, nitrifying bacteria perform ammonia nitrification under aerobic conditions, while denitrifying bacteria perform nitrate/nitrite denitrification under anaerobic conditions. However, heterotrophic nitrifying and aerobic denitrifying microorganisms (HNADs) perform ammonia nitrification and nitrate/nitrite denitrification under the same aerobic conditions using an organic carbon source, which is a much simpler and more efficient process. In this review, the distribution and evolutionary relationships of novel HNADs strains are presented, and the influencing factors, metabolic pathways, key enzymes, and practical applications of HNADs are reviewed.

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.

Aquaculture relies heavily on formulated feed, with feed costs accounting for approximately 50% of the total production expenses. High-protein feed is expensive, and some raw materials are dependent on imports. Organic waste, primarily from food scraps and agricultural residues, can be considered feed ingredients to reduce costs. Through fermentation, the nutritional value of these feeds can be enhanced, promoting the growth of aquatic animals, such as fish, and reducing overall costs. Enzymatic hydrolysis and microbial fermentation are commonly used bioprocessing methods, particularly suitable for unconventional feed sources. These methods not only enhance the nutritional value of the feed but also reduce the content of anti-nutritional factors, improving feed digestibility. Enzymatic hydrolysis is often combined with microbial fermentation to produce more efficient novel biofeeds. This synergistic approach enhances microbial fermentation efficiency and the degradation of macromolecules, further improving the nutritional value of the feed, promoting intestinal digestion and absorption, and enhancing animal production performance. Compared to treatments with single strains or enzymes, the synergistic fermentation of feed with microorganisms and enzymes shows more significant effects. This review summarizes the fermentation mechanisms and classifications of enzymatic hydrolysis and microbial fermentation in producing novel biofeeds. It also outlines the advantages of this synergistic process and its progress in aquatic animal production, providing a scientific basis for its broader application in aquaculture.

L-asparaginase, which catalyses the hydrolysis of L-asparagine to L-aspartate, has attracted the attention of researchers due to its expanded applications in medicine and the food industry. In this study, a novel thermostable L-asparaginase from Pyrococcus yayanosii CH1 was cloned and over-expressed in Bacillus subtilis 168. To obtain thermostable L-asparaginase mutants with higher activity, a robust high-throughput screening process was developed specifically for thermophilic enzymes. In this process, cell disruption and enzyme activity assays are simultaneously performed in 96-deep well plates. By combining error-prone PCR and screening, six brilliant positive variants and four key amino acid residue mutations were identified. Combined mutation of the four residues showed relatively high specific activity (3108 U/mg) that was 2.1 times greater than that of the wild-type enzyme. Fermentation with the mutant strain in a 5-L fermenter yielded L-asparaginase activity of 2168 U/mL.

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.