Improvement of acetoin reductase activity enhances bacitracin production by Bacillus licheniformis

Microbial synthesis of bacitracin: Recent progress, challenges, and prospects

Microorganisms are important sources of various natural products that have been commercialized for human medicine and animal healthcare. Bacitracin is an important antibacterial natural product predominantly produced by Bacillus licheniformis and Bacillus subtilis, and it is characterized by a broad antimicrobial spectrum, strong activity and low resistance, thus bacitracin is extensively applied in animal feed and veterinary medicine industries. In recent years, various strategies have been proposed to improve bacitracin production. Herein, we systematically describe the regulation of bacitracin biosynthesis in genus Bacillus and its associated mechanism, to provide a theoretical basis for bacitracin overproduction. The metabolic engineering strategies applied for bacitracin production are explored, including improving substrate utilization, using an enlarged precursor amino acid pool, increasing ATP supply and NADPH generation, and engineering transcription regulators. We also present several approaches of fermentation process optimization to facilitate the industrial large-scale production of bacitracin. Finally, the challenges and prospects associated with microbial bacitracin synthesis are discussed to facilitate the establishment of high-yield and low-cost biological factories.

Enhancement of precursor amino acid supplies for improving bacitracin production by activation of branched chain amino acid transporter BrnQ and deletion of its regulator gene lrp in Bacillus licheniformis

Bacitracin, a new type of cyclic peptide antibiotic, is widely used as the feed additive in feed industry. Branched chain amino acids (BCAAs) are the key precursors for bacitracin synthesis. In this research, soybean meal was served as the raw material to supply precursor amino acids for bacitracin synthesis, and enhanced production of bacitracin was attempted by engineering BCAA transporter BrnQ and its regulator Lrp in the bacitracin industrial production strain Bacillus licheniformis DW2. Firstly, our results confirmed that Lrp negatively affected bacitracin synthesis in DW2, and deletion of lrp improved intracellular BCAA accumulations, as well as the expression level of BCAA transporter BrnQ, which further led to a 14.71% increase of bacitracin yield, compared with that of DW2. On the contrary, overexpression of Lrp decreased bacitracin yield by 12.28%. Secondly, it was suggested that BrnQ acted as a BCAA importer in DW2, and overexpression of BrnQ enhanced the intracellular BCAA accumulations and 10.43% of bacitracin yield. While, the bacitracin yield decreased by 18.27% in the brnQ deletion strain DW2△brnQ. Finally, BrnQ was further overexpressed in lrp deletion strain DW2△lrp, and bacitracin yield produced by the final strain DW2△lrp::BrnQ was 965.34 U/mL, increased by 22.42% compared with that of DW2 (788.48 U/mL). Collectively, this research confirmed that Lrp affected bacitracin synthesis via regulating the expression of BCAA transporter BrnQ and BCAA distributions, and provided a promising strain for industrial production of bacitracin.

Enhancement of Bacitracin Production by NADPH Generation via Overexpressing Glucose-6-Phosphate Dehydrogenase Zwf in Bacillus licheniformis

Bacitracin, a kind of cyclic peptide antibiotic mainly produced by Bacillus, has wide ranges of applications. NADPH generation plays an important role in amino acid synthesis, which might influence precursor amino acid supply for bacitracin production. In this study, we want to improve bacitracin yield by enhancing intracellular precursor amino acids via strengthening NAPDH generation pathways in the bacitracin industrial production strain Bacillus licheniformis DW2. Based on our results, strengthening of NADPH pathway genes (zwf, gnd, ppnk, pntAB, and udhA) could all improve bacitracin yields in DW2, and the glucose-6-phosphate dehydrogenase Zwf overexpression strain DW2::Zwf displayed the best performance, the yield of which (886.43 U/mL) was increased by 12.43% compared to DW2 (788.40 U/mL). Then, the zwf transcriptional level and Zwf activity of DW2::Zwf were increased by 12.24-fold and 1.57-fold; NADPH and NADPH/NADH were enhanced by 61.24% and 90.63%, compared with those of DW2, respectively. Moreover, the concentrations of intracellular precursor amino acids (isoleucine, leucine, cysteine, ornithine, lysine, glutamic acid) were all enhanced obviously for bacitracin production in DW2::Zwf. Collectively, this research constructed a promising B. licheniformis strain for industrial production of bacitracin, more importantly, which revealed that strengthening of NADPH generation is an efficient strategy to improve precursor amino acid supplies for bacitracin production.

Application of enzymatic apple pomace hydrolysate to production of 2,3-butanediol by alkaliphilic Bacillus licheniformis NCIMB 8059

2,3-Butanediol (2,3-BD) synthesis by a nonpathogenic bacterium Bacillus licheniformis NCIMB 8059 from enzymatic hydrolysate of depectinized apple pomace and its blend with glucose was studied. In shake flasks, the maximum diol concentration in fed-batch fermentations was 113 g/L (in 163 h, from the hydrolysate, feedings with glucose) while in batch processes it was around 27 g/L (in 32 h, from the hydrolysate and glucose blend). Fed-batch fermentations in the 0.75 and 30 L fermenters yielded 87.71 g/L 2,3-BD in 160 h, and 72.39 g/L 2,3-BD in 94 h, respectively (from the hydrolysate and glucose blend, feedings with glucose). The hydrolysate of apple pomace, which was for the first time used for microbial 2,3-BD production is not only a source of sugars but also essential minerals.