Improvement of lichenysin production in Bacillus licheniformis by replacement of native promoter of lichenysin biosynthesis operon and medium optimization

Biosurfactant Production by Pseudomonas: a Systematic Review

It is of fundamental interest to research and develop innovative biotechnologies, as well as bioproducts that replace or are alternatives to those of non-renewable origin, such as biosurfactants in relation to traditional surfactants used in various sectors. Consequently, there are a large number of experimental studies addressing different subjects, especially with the use of bacteria of the genus Pseudomonas; however, there is a lack of work that demonstrates the evaluation of this science produced to date. Therefore, this article discusses the production of biosurfactants by Pseudomonas with the aim of surveying and analyzing experimental articles on this topic. To realize this, a systematic search was carried out with well-defined temporal space, databases, and inclusion and exclusion criteria, based on metric studies that guided what information would be collected and the method of evaluation. Therefore, a large number of articles were selected, which demonstrated Pseudomonas aeruginosa as the bioagent mostly used in the tests, which aimed to improve the process in the area. Furthermore, interest in this field has increased over the years, predominantly in emerging market countries, where the most prominent authors on the topic are found. Therefore, it is necessary that there is an expansion of interest in the area to make the production of biosurfactants cheaper in areas that currently have greater development deficiencies, such as means of purifying the bioprocess and reducing foam formation in the bioprocess.

Classification, application, multifarious activities and production improvement of lipopeptides produced by Bacillus

Lipopeptides, a class of compounds consisting of a peptide ring and a fatty acid chain, are secondary metabolites produced by Bacillus spp. As their hydrophilic and oleophilic properties, lipopeptides are widely used in food, medicine, environment and other industrial or agricultural fields. Compared with artificial synthetic surfactants, microbial lipopeptides have the advantages of low toxicity, high efficiency and versatility, resulting in urgent market demand and broad development prospect of lipopeptides. However, due to the complex metabolic network and precursor requirements of synthesis, the specific and strict synthesis pathway, and the coexistence of multiple homologous substances, the production of lipopeptides by microorganisms has the problems of high cost and low production efficiency, limiting the mass production of lipopeptides and large-scale application in industry. This review summarizes the types of Bacillus-produced lipopeptides and their biosynthetic pathways, introduces the versatility of lipopeptides, and describes the methods to improve the production of lipopeptides, including genetic engineering and optimization of fermentation conditions.

Modular metabolic engineering of Bacillus amyloliquefaciens for high-level production of green biosurfactant iturin A

As a kind of biosurfactants, iturin A has attracted people's wide attentions due to their features of biodegradability, environmentally friendly, etc.; however, high production cost limited its extensive application, and the aim of this research wants to improve iturin A production in Bacillus amyloliquefaciens. Firstly, dual promoter was applied to strengthen iturin A synthetase expression, and its yield was increased to 1.25 g/L. Subsequently, original 5'-UTRs of downstream genes (ituA, ituB, and ituC) in iturin A synthetase cluster were optimized, which significantly increased mRNA secondary stability, and iturin A yield produced by resultant strain HZ-T3 reached 2.32 g/L. Secondly, synthetic pathway of α-glucosidase inhibitor 1-deoxynojirimycin was blocked to improve substrate corn starch utilization, and iturin A yield was increased by 34.91% to 3.13 g/L. Thirdly, efficient precursor (fatty acids, Ser, and Pro) supplies were proven as the critical role in iturin A synthesis, and 5.52 g/L iturin A was attained by resultant strain, through overexpressing yngH, serC, and introducing ocD. Meanwhile, genes responsible for poly-γ-glutamic acid, extracellular polysaccharide, and surfactin syntheses were deleted, which led to a 30.98% increase of iturin A yield. Finally, lipopeptide transporters were screened, and iturin A yield was increased by 17.98% in SwrC overexpression strain, reached 8.53 g/L, which is the highest yield of iturin A ever reported. This study laid a foundation for industrial production and application development of iturin A, and provided the guidance of metabolic engineering breeding for efficient production of other metabolites synthesized by non-ribosomal peptide synthetase. KEY POINTS: • Optimizing 5'-UTR is an effective tactics to regulate synthetase cluster expression. • Blocking 1-DNJ synthesis benefited corn starch utilization and iturin A production. • The iturin A yield attained in this work was the highest yield reported so far.

Taxonomic and Predictive Functional Profile of Hydrocarbonoclastic Bacterial Consortia Developed at Three Different Temperatures

Microbial community exhibit shift in composition in response to temperature variation. We report crude oil-degrading activity and high-throughput 16S rRNA gene sequencing (metagenome) profiles of four bacterial consortia enriched at three different temperatures in crude oil-amended Bushnell-Hass Medium from an oily sludge sediment. The consortia were referred to as O (4 ± 2 ℃ in 3% w/v crude oil), A (25 ± 2 ℃ in 1% w/v crude oil), H (25 ± 2 ℃ in 3% w/v crude oil), and X (45 ± 2 ℃ in 3% w/v crude oil). The hydrocarbon-degrading activity was highest for consortium A and H and lowest for consortium O. The metagenome profile revealed the predominance of Proteobacteria (62.12-1.25%) in each consortium, followed by Bacteroidota (18.94-37.77%) in the consortium O, A, and H. Contrarily, consortium X comprised 7.38% Actinomycetota, which was essentially low (< 0.09%) in other consortia, and only 0.41% Bacteroidota. The PICRUSt-based functional analysis predicted major functions associated with the metabolism and 5060 common KEGG Orthology (KOs). A total of 296 KOs were predicted exclusively in consortium X. Additionally, 247 KOs were predicted from xenobiotic biodegradation pathways. This study found that temperature had a stronger influence on the composition and function of the bacterial community than crude oil concentration.

Antimicrobial peptides from Bacillus spp. and strategies to enhance their yield

Antibiotic resistance is a growing concern that is affecting public health globally. The search for alternative antimicrobial agents has become increasingly important. Antimicrobial peptides (AMPs) produced by Bacillus spp. have emerged as a promising alternative to antibiotics, due to their broad-spectrum antimicrobial activity against resistant pathogens. In this review, we provide an overview of Bacillus-derived AMPs, including their classification into ribosomal (bacteriocins) and non-ribosomal peptides (lipopeptides and polyketides). Additionally, we delve into the molecular mechanisms of AMP production and describe the key biosynthetic gene clusters involved. Despite their potential, the low yield of AMPs produced under normal laboratory conditions remains a challenge to large-scale production. This review thus concludes with a comprehensive summary of recent studies aimed at enhancing the productivity of Bacillus-derived AMPs. In addition to medium optimization and genetic manipulation, various molecular strategies have been explored to increase the production of recombinant antimicrobial peptides (AMPs). These include the selection of appropriate expression systems, the engineering of expression promoters, and metabolic engineering. Bacillus-derived AMPs offer great potential as alternative antimicrobial agents, and this review provides valuable insights on the strategies to enhance their production yield, which may have significant implications for combating antibiotic resistance. KEY POINTS: • Bacillus-derived AMP is a potential alternative therapy for resistant pathogens • Bacillus produces two main classes of AMPs: ribosomal and non-ribosomal peptides • AMP yield can be enhanced using culture optimization and molecular approaches.

Biosurfactants: Secondary Metabolites Involved in the Process of Bioremediation and Biofilm Removal

The search for environmentally friendly methods to remove persistent substances such as organic pollutants and sessile communities such as biofilms that severely affect the environment and human health resulted in biosurfactant discovery. Owing to their low level of toxicity and high biodegradability, biosurfactants are increasingly preferred to be used for removal of pollutants from nature. These amphipathic molecules can be synthesized inexpensively, employing cheap substrates such as agricultural and industrial wastes. Recent progress has been made in identifying various biosurfactants that can be used to remove organic pollutants and harmful microbial aggregates, as well as novel microbial strains that produce these surface-active molecules to survive in a hydrocarbon-rich environment. This review focuses on the identification and understanding the role of biosurfactants and the microorganisms involved in the removal of biofilms and remediation of xenobiotics and various types of hydrocarbons such as crude oil, aromatic hydrocarbons, n-alkanes, aliphatic hydrocarbons, asphaltenes, naphthenes, and other petroleum products. This property of biosurfactant is very important as biofilms are of great concern due to their impact on the environment, public health, and industries worldwide. This work also includes several advanced molecular methods that can be used to enhance the production of biosurfactants by the microorganisms studied.

Promoting cell growth for bio-chemicals production via boosting the synthesis of L/D-alanine and D-alanyl-D-alanine in Bacillus licheniformis

Metabolic engineering is a substantial approach for escalating the production of biochemical products. Cell biomass is lowered by system constraints and toxication carried on by the aggregation of metabolites that serve as inhibitors of product synthesis. In order to increase the production of biochemical products, it is important to trace the relationship between alanine metabolism and biomass. According to our investigation, the appropriate concentration of additional L/D-alanine (0.1 g/L) raised the cell biomass (OD₆₀₀) in Bacillus licheniformis in contrast to the control strain. Remarkably, it was also determined that high levels of intracellular L/D-alanine and D-alanyl-D-alanine were induced by the overexpression of the ald, dal, and ddl genes to accelerate cell proliferation. Our findings clearly revealed that 0.2 g/L of L-alanine and D-alanine substantially elevated the titer of poly-γ-glutamic acid (γ-PGA) by 14.89% and 6.19%, correspondingly. And the levels of γ-PGA titer were hastened by the overexpression of the ald, dal, and ddl genes by 19.72%, 15.91%, and 16.64%, respectively. Furthermore, overexpression of ald, dal, and ddl genes decreased the by-products (acetoin, 2,3-butanediol, acetic acid and lactic acid) formation by about 14.10%, 8.77%, and 8.84% for augmenting the γ-PGA production. Our results also demonstrated that overexpression of ald gene amplified the production of lichenysin, pulcherrimin and nattokinase by about 18.71%, 19.82% and 21.49%, respectively. This work delineated the importance of the L/D-alanine and D-alanyl-D-alanine synthesis to the cell growth and the high production of bio-products, and provided an effective strategy for producing bio-products.

Bioengineering for the Microbial Degradation of Petroleum Hydrocarbon Contaminants

Petroleum hydrocarbons are relatively recalcitrant compounds, and as contaminants, they are one of the most serious environmental problems. n-Alkanes are important constituents of petroleum hydrocarbons. Advances in synthetic biology and metabolic engineering strategies have made n-alkane biodegradation more designable and maneuverable for solving environmental pollution problems. In the microbial degradation of n-alkanes, more and more degradation pathways, related genes, microbes, and alkane hydroxylases have been discovered, which provide a theoretical basis for the further construction of degrading strains and microbial communities. In this review, the current advances in the microbial degradation of n-alkanes under aerobic condition are summarized in four aspects, including the biodegradation pathways and related genes, alkane hydroxylases, engineered microbial chassis, and microbial community. Especially, the microbial communities of “Alkane-degrader and Alkane-degrader” and “Alkane-degrader and Helper” provide new ideas for the degradation of petroleum hydrocarbons. Surfactant producers and nitrogen providers as a “Helper” are discussed in depth. This review will be helpful to further achieve bioremediation of oil-polluted environments rapidly.

Microbial Lipopeptides: Properties, Mechanics and Engineering for Novel Lipopeptides

Microorganisms produce active surface agents called lipopeptides (LPs) which are amphiphilic in nature. They are cyclic or linear compounds and are predominantly isolated from Bacillus and Pseudomonas species. LPs show antimicrobial activity towards various plant pathogens and act by inhibiting the growth of these organisms. Several mechanisms are exhibited by LPs, such as cell membrane disruption, biofilm production, induced systematic resistance, improving plant growth, inhibition of spores, etc., making them suitable as biocontrol agents and highly advantageous for industrial utilization. The biosynthesis of lipopeptides involves large multimodular enzymes referred to as non-ribosomal peptide synthases. These enzymes unveil a broad range of engineering approaches through which lipopeptides can be overproduced and new LPs can be generated asserting high efficacy. Such approaches involve several synthetic biology systems and metabolic engineering techniques such as promotor engineering, enhanced precursor availability, condensation domain engineering, and adenylation domain engineering. Finally, this review provides an update of the applications of lipopeptides in various fields.

A comprehensive study on microbial-surfactants from bioproduction scale-up toward electrokinetics remediation of environmental pollutants: Challenges and perspectives

Currently, researchers have focused on electrokinetic (EK) bioremediation due to its potential to remove a wide-range of pollutants. Further, to improve their performance, synthetic surfactants are employed as effective additives because of their excellent solubility and mobility. Synthetic surfactants have an excessive position in industries since they are well-established, cheap, and easily available. Nevertheless, these surfactants have adverse environmental effects and could be detrimental to aquatic and terrestrial life. Owing to social and environmental awareness, there is a rising demand for bio-based surfactants in the global market, from environmental sustainability to public health, because of their excellent surface and interfacial activity, higher and stable emulsifying property, biodegradability, non- or low toxicity, better selectivity and specificity at extreme environmental conditions. Unfortunately, challenges to biosurfactants, like expensive raw materials, low yields, and purification processes, hinder their applicability to large-scale. To date, extensive research has already been conducted for production scale-up using multidisciplinary approaches. However, it is still essential to research and develop high-yielding bacteria for bioproduction through traditional and biotechnological advances to reduce production costs. Herein, this review evaluates the recent progress made on microbial-surfactants for bioproduction scale-up and provides detailed information on traditional and advanced genetic engineering approaches for cost-effective bioproduction. Furthermore, this study emphasized the role of electrokinetic (EK) bioremediation and discussed the application of BioS-mediated EK for various pollutants remediation.

Optimization of Medium Components for Fed-Batch Fermentation Using Central Composite Design to Enhance Lichenysin Production by Bacillus licheniformis Ali5

Lichenysin, an amphiphilic biosurfactant with structural and physicochemical properties similar to surfactin, is produced by Bacillus licheniformis. Its low toxicity, good environmental compatibility, solubilization, foaming, emulsification and detergent activities have led to a wide range of applications in agricultural biocontrol, enhanced oil recovery, foaming agents for cosmetics and detergents for household cleaning products. However, despite the extraordinary surface-active properties and potential applications of lichenysin, the number of wild bacteria found so far is relatively low. Low titers and high costs are the main limiting factors for widespread industrial applications. In this study, a factorial design was used to optimize the composition of the medium for the production of lichenysin by Bacillus licheniformis Ali5. Firstly, the solutions of carbon, nitrogen, amino acids, inorganic salts and trace elements in the medium were evaluated in flasks using a single-factor optimization method. Meanwhile, the operating conditions were optimized in the same way. Afterwards, a partial factorial design was used to investigate the effect of six variables (five medium compositions and inoculum size) on lichenysin production. Based on the results obtained, the concentrations of sucrose and ammonium nitrate and the inoculum size were considered to be important for lichenysin production. Subsequently, a full factorial design was used to optimize these three variables. The optimized medium composition were sucrose 19.8 g/L, NH4NO3 3.9 g/L, K2HPO4·3H2O 4.0 g/L, MgSO4·7H2O 0.6 g/L, FeSO4·7H2O 0.1 g/L, CaCl2 0.01 g/L, NaCl 3.0, trace elements 1.2 mL/L. Finally, the titer of lichenysin after fed-batch fermentation reached 1425.85 mg/L, which was approximately 5.5 times higher than the titer of lichenysin from the original medium. Consequently, the method was further demonstrated to be suitable for lichenysin production.

Research Advances of Microbial Enhanced Oil Recovery

Microbial enhanced oil recovery (MEOR), characterized with the virtues of low cost and environmental protection, reflects the prevalent belief in environmental protection, and is attracting the attention of more researchers. Nonetheless, with the prolonged slump in global oil prices, how to further reduce the cost of MEOR has become a key factor in its development. This paper described the recent development of MEOR technology in terms of mechanisms, mathematical models, and field application, meanwhile the novel technologies of MEOR such as genetically engineered microbial enhanced oil recovery (GEMEOR) and enzyme enhanced oil recovery (EEOR) were introduced. The paper proposed three possible methods to decrease the cost of MEOR: using inexpensive nutrients as substrates, applying a mixture of chemical and biological agents, and utilizing crude microbial products. Additionally, in order to reduce the uncertainty in the practical application of MEOR technology, it is essential to refine the reservoir screening criteria and establish a sound mathematical model of MEOR. Eventually, the paper proposes to combine genetic engineering technology and microbial hybrid culture technology to build a microbial consortium with excellent oil displacement efficiency and better environmental adaptability. This may be a vital part of the future research on MEOR technology, which will play a major role in improving its economic efficiency and practicality.

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Mechanisms of microbial enhanced oil recovery.

• The novel technology of microbial enhanced oil recovery.

• Field trails of microbial enhanced oil recovery.

Enhanced production of iturin A by strengthening fatty acid synthesis modules in Bacillus amyloliquefaciens

Iturin A is a biosurfactant with various applications, and its low synthesis capability limits its production and application development. Fatty acids play a critical role in cellular metabolism and target product syntheses, and the relationship between fatty acid supplies and iturin A synthesis is unclear. In this study, we attempted to increase iturin A production via strengthening fatty acid synthesis pathways in Bacillus amyloliquefaciens. First, acetyl-CoA carboxylase AccAD and ACP S-malonyltransferase fabD were overexpressed via promoter replacement, and iturin A yield was increased to 1.36 g/L by 2.78-fold in the resultant strain HZ-ADF1. Then, soluble acyl-ACP thioesterase derived from Escherichia coli showed the best performance for iturin A synthesis, as compared to those derived from B. amyloliquefaciens and Corynebacterium glutamicum, the introduction of which in HZ-ADF1 further led to a 57.35% increase of iturin A yield, reaching 2.14 g/L. Finally, long-chain fatty acid-CoA ligase LcfA was overexpressed in HZ-ADFT to attain the final strain HZ-ADFTL2, and iturin A yield reached 2.96 g/L, increasing by 6.59-fold, and the contents of fatty acids were enhanced significantly in HZ-ADFTL2, as compared to the original strain HZ-12. Taken together, our results implied that strengthening fatty acid supplies was an efficient approach for iturin A production, and this research provided a promising strain for industrial production of iturin A.

A Review on Biotechnological Approaches Applied for Marine Hydrocarbon Spills Remediation

The increasing demand for petroleum products generates needs for innovative and reliable methods for cleaning up crude oil spills. Annually, several oil spills occur around the world, which brings numerous ecological and environmental disasters on the surface of deep seawaters like oceans. Biological and physico-chemical remediation technologies can be efficient in terms of spill cleanup and microorganisms—mainly bacteria—are the main ones responsible for petroleum hydrocarbons (PHCs) degradation such as crude oil. Currently, biodegradation is considered as one of the most sustainable and efficient techniques for the removal of PHCs. However, environmental factors associated with the functioning and performance of microorganisms involved in hydrocarbon-degradation have remained relatively unclear. This has limited our understanding on how to select and inoculate microorganisms within technologies of cleaning and to optimize physico-chemical remediation and degradation methods. This review article presents the latest discoveries in bioremediation techniques such as biostimulation, bioaugmentation, and biosurfactants as well as immobilization strategies for increasing the efficiency. Besides, environmental affecting factors and microbial strains engaged in bioremediation and biodegradation of PHCs in marines are discussed.

Bacillus licheniformis: The unexplored alternative for the anaerobic production of lipopeptide biosurfactants?

Microbial biosurfactants have attracted the attention of researchers and companies for the last decades, as they are considered promising candidates to replace chemical surfactants in numerous applications. Although in the last years, considerable advances were performed regarding strain engineering and the use of low-cost substrates in order to reduce their production costs, one of the main bottlenecks is their production at industrial scale. Conventional aerobic biosurfactant production processes result in excessive foaming, due to the use of high agitation and aeration rates necessary to increase dissolved oxygen concentration to allow microbial growth and biosurfactant production. Different approaches have been studied to overcome this problem, although with limited success. A not widely explored alternative is the development of foam-free processes through the anaerobic growth of biosurfactant-producing microorganisms. Surfactin, produced by Bacillus subtilis, is the most widely studied lipopeptide biosurfactant, and the most powerful biosurfactant known so far. Bacillus licheniformis strains produce lichenysin, a lipopeptide biosurfactant which structure is similar to surfactin. However, despite its extraordinary surface-active properties and potential applications, lichenysin has been scarcely studied. According to previous studies, B. licheniformis is better adapted to anaerobic growth than B. subtilis, and could be a good alternative for the anaerobic production of lipopeptide biosurfactants. In this review, the potential and limitations of surfactin and lichenysin production under anaerobic conditions will be analyzed, and the possibility of implementing foam-free processes for lichenysin production, in order to expand the market and applications of biosurfactants in different fields, will be discussed.

γ-Polyglutamic Acid Production, Biocontrol, and Stress Tolerance: Multifunction of Bacillus subtilis A-5 and the Complete Genome Analysis

Bacillus subtilis A-5 has the capabilities of high-molecular-weight γ-PGA production, antagonism to plant pathogenic fungi, and salt/alkaline tolerance. This multifunctional bacterium has great potential for enhancing soil fertility and plant security in agricultural ecosystem. The genome size of B. subtilis A-5 was 4,190,775 bp, containing 1 Chr and 2 plasmids (pA and pB) with 43.37% guanine-cytosine content and 4605 coding sequences. The γ-PGA synthase gene cluster was predicted to consist of pgsBCA and factor (pgsE). The γ-PGA-degrading enzymes were mainly pgdS, GGT, and cwlO. Nine gene clusters producing secondary metabolite substances, namely, four unknown function gene clusters and five antibiotic synthesis gene clusters (surfactin, fengycin, bacillibactin, subtilosin_A, and bacilysin), were predicted in the genome of B. subtilis A-5 using antiSMASH. In addition, B. subtilis A-5 contained genes related to carbohydrate and protein decomposition, proline synthesis, pyruvate kinase, and stress-resistant proteins. This affords significant insights into the survival and application of B. subtilis A-5 in adverse agricultural environmental conditions.

Lichenysin Production by Bacillus licheniformis Food Isolates and Toxicity to Human Cells

Bacillus licheniformis can cause foodborne intoxication due to the production of the surfactant lichenysin. The aim of this study was to measure the production of lichenysin by food isolates of B. licheniformis in LB medium and skimmed milk and its cytotoxicity for intestinal cells. Out of 11 B. licheniformis isolates tested, most showed robust growth in high salt (1M NaCl), 4% ethanol, at 37 or 55°C, and aerobic and anaerobic conditions. All strains produced lichenysin (in varying amounts), but not all strains were hemolytic. Production of this stable compound by selected strains (high producers B4094 and B4123, and type strain DSM13^T) was subsequently determined using LB medium and milk, at 37 and 55°C. Lichenysin production in LB broth and milk was not detected at cell densities < 5 log₁₀ CFU/ml. The highest concentrations were found in the stationary phase of growth. Total production of lichenysin was 4–20 times lower in milk than in LB broth (maximum 36 μg/ml), and ∼10 times lower in the biomass obtained from milk agar than LB agar. Under all conditions tested, strain B4094 consistently yielded the highest amounts. Besides strain variation and medium composition, temperature also had an effect on lichenysin production, with twofold lower amounts of lichenysin produced at 55°C than at 37°C. All three strains produced lichenysin A with varying acyl chain lengths (C11–C18). The relative abundance of the C14 variant was highest in milk and the C15 variant highest in LB. The concentration of lichenysin needed to reduce cell viability by 50% (IC₅₀) was 16.6 μg/ml for Caco-2 human intestinal epithelial cells and 16.8 μg/ml for pig ileum organoids. Taken together, the presence of low levels (<5 log₁₀ CFU/ml) of B. licheniformis in foods is unlikely to pose a foodborne hazard related to lichenysin production. However, depending on the strain present, the composition, and storage condition of the food, a risk of foodborne intoxication may arise if growth to high levels is supported and such product is ingested.

Microbial production of small peptide: pathway engineering and synthetic biology

Small peptides are a group of natural products with low molecular weights and complex structures. The diverse structures of small peptides endow them with broad bioactivities and suggest their potential therapeutic use in the medical field. The remaining challenge is methods to address the main limitations, namely (i) the low amount of available small peptides from natural sources, and (ii) complex processes required for traditional chemical synthesis. Therefore, harnessing microbial cells as workhorse appears to be a promising approach to synthesize these bioactive peptides. As an emerging engineering technology, synthetic biology aims to create standard, well-characterized and controllable synthetic systems for the biosynthesis of natural products. In this review, we describe the recent developments in the microbial production of small peptides. More importantly, synthetic biology approaches are considered for the production of small peptides, with an emphasis on chassis cells, the evolution of biosynthetic pathways, strain improvements and fermentation.

Metabolic Engineering of Aspartic Acid Supply Modules for Enhanced Production of Bacitracin in Bacillus licheniformis

Bacitracin, a type of cyclic dodecapeptide antibiotic mainly produced by Bacillus, is widely used in fields of veterinary drug and feed additive. Modularization of metabolic pathways based on the concept of synthetic biology has been widely used in the efficient synthesis of target products. Here, we want to improve bacitracin production through strengthening aspartic acid (Asp) supply in B. licheniformis DW2. First, exogenous Asp addition assays implied that strengthening Asp supply benefited bacitracin production. Second, Asp synthetic pathways were strengthened via overexpressing aspartate dehydrogenase AspD and asparaginase AnsB, attaining recombinant strain DW2-ASP2, and bacitracin yield produced by DW2-ASP2 was 862.81 U/mL, increased by 14.05% compared with that of DW2 (756.49 U/mL). Then, to improve precursor oxaloacetate (OAA) accumulation for Asp synthesis, pyruvate carboxylase PycA and carbonic anhydrase EcaA were co-overexpressed in DW2-ASP2, and malic enzyme gene malS was deleted to weak overflow metabolism of tricarboxylic acid, and the attained strain DW2-ASP7 showed further increased bacitracin production from 862.81 to 989.23 U/mL. Subsequently, transporter YveA was identified as an Asp exporter, and bacitracin yield was increased to 1025.26 U/mL via deleting yveA, attaining strain DW2-ASP9. Finally, Asp ammonia-lyase gene aspA was disrupted to weaken Asp degradation, and bacitracin yield of attained strain DW2-ASP10 reached 1059.86 U/mL, increased by 40.10% compared to DW2. Taken together, this research demonstrated that metabolic engineering of Asp metabolic modules is an efficient strategy for enhancing bacitracin production, and these strategies could also be applied in the production of other peptide-related metabolites.

Construction and Characterization of a Gradient Strength Promoter Library for Fine-Tuned Gene Expression in Bacillus licheniformis

Bacillus licheniformis DW2 is an important industrial strain for bacitracin production, and it is also used for biochemical production, however, the lack of effective toolkit for precise regulation of gene expression hindered its application seriously. Here, a gradient strength promoter library was constructed based on bacitracin synthetase gene cluster promoter PbacA. First, different PbacA promoter variants were constructed via coupling PbacA with various 5'-UTRs, and expression ranges of 32.6-741.8% were attained among these promoters. Then, three promoters, PUbay (strong), PbacA (middle), and PUndh (weakest), were applied for red fluorescent protein (RFP) and keratinase expression assays, and these promoters were proven to have good universality for different proteins. Second, the promoter of bacitracin synthetase gene cluster was replaced by these three promoters, and bacitraicn titer was enhanced by 14.62% when PUbay was applied, which was decreased by 98.05% under the mediation of PUndh compared with that of the original strain DW2. Third, promoters PUbay, PUyvgO, and PUndh were selected to regulate the expression levels of critical genes that are responsible for pucheriminic acid synthesis, and pucheriminic acid yield was increased by 194.1% via manipulating synthetic and competitive pathways. Finally, promoters PUbay, PbacA, and PUndh were applied for green fluorescent protein (GFP) and RFP expression in Escherichia coli, and consistent effects were attained based on our results. Taken together, a gradient strength promoter library was constructed in this research, which provided an effective toolkit for fine-tuning gene expression and reprogramming metabolite metabolic flux in B. licheniformis.

A review on new aspects of lipopeptide biosurfactant: Types, production, properties and its application in the bioremediation process

Nowadays, the worldwide search regarding renewable products from natural resources is increasing due to the toxicity of chemical counterparts. Biosurfactants are surface-active compounds that contain several physiological functions that are used in industries like food, pharmaceutical, petroleum and agriculture. Microbial lipopeptides have gained more attention among the researchers for their low toxicity, efficient action and good biodegradability when compared with other surfactants. Because of their versatile properties, lipopeptide compounds are utilized in the remediation of organic and inorganic pollutants. This review presented a depth evaluation of lipopeptide surfactants in the bioremediation process and their properties to maintain a sustainable environment. Lipopeptide can acts as a replacement to chemical surfactants only if they meet industrial-scale production and low-cost substrates. This review also demonstrated the production of a lipopeptide biosurfactant from a low-cost substrate and depicted plausible techniques to manage the substrate residues to determine its ability in the different applications particularly in the bioremediation process.

Development and Genetic Engineering of Hyper-Producing Microbial Strains for Improved Synthesis of Biosurfactants

Current research energies are fixated on the synthesis of environmentally friendly and non-hazardous products, which include finding and recognizing biosurfactants that can substitute synthetic surfactants. Microbial biosurfactants are surface-active compounds synthesized intracellularly or extracellularly. To use biosurfactants in various industries, it is essential to understand scientific engagements that demonstrate its potentials as real advancement in the 21st century. Other than applying a substantial effect on the world economic market, engineered hyper-producing microbial strains in combination with optimized cultivation parameters have made it probable for many industrial companies to receive the profits of 'green' biosurfactant innovation. There needs to be an emphasis on the worldwide state of biosurfactant synthesis, expression of biosurfactant genes in expressive host systems, the recent developments, and prospects in this line of research. Thus, molecular dynamics with respect to genetic engineering of biosynthetic genes are proposed as new biotechnological tools for development, improved synthesis, and applications of biosurfactants. For example, mutant and hyper-producing recombinants have been designed efficaciously to advance the nature, quantity, and quality of biosurfactants. The fastidious and deliberate investigation will prompt a comprehension of the molecular dynamics and phenomena in new microorganisms. Throughout the decade, valuable data on the molecular genetics of biosurfactant have been produced, and this solid foundation would encourage application-oriented yields of the biosurfactant production industry and expand its utilization in diverse fields. Therefore, the conversations among different interdisciplinary experts from various scientific interests such as microbiology, biochemistry, molecular biology, and genetics are indispensable and significant to accomplish these objectives.

Bioremediation Potential of Streptomyces sp. MOE6 for Toxic Metals and Oil

Toxic metal contamination has serious effects on human health. Crude oil that may contain toxic metals and oil spills can further contaminate the environment and lead to increased exposure. This being the case, we chose to study the bio-production of inexpensive, environmentally safe materials for remediation. Streptomyces sp. MOE6 is a Gram-positive, filamentous bacterium from soil that produces an extracellular polysaccharide (MOE6-EPS). A one-factor-at-a-time experiments showed that the maximum production of MOE6-EPS was achieved at 35 °C, pH 6, after nine days of incubation with soluble starch and yeast extract as carbon sources and the latter as the nitrogen source. We demonstrated that MOE6-EPS has the capacity to remove toxic metals such as Co(II), Cr(VI), Cu(II) and U(VI) and from solution either by chelation and/or reduction. Additionally, the bacterium was found to produce siderophores, which contribute to the removal of metals, specifically Fe(III). Additionally, purified MOE6-EPS showed emulsifying activities against various hydrophobic substances, including olive oil, corn oil, benzene, toluene and engine oil. These results indicate that EPS from Streptomyces sp. MOE6 may be useful to sequester toxic metals and oil in contaminated environments.

Engineering Expression Cassette of pgdS for Efficient Production of Poly-γ-Glutamic Acids With Specific Molecular Weights in Bacillus licheniformis

Poly-γ-glutamic acid (γ-PGA) is an emerging biopolymer with various applications and γ-PGAs with different molecular weights exhibit distinctive properties. However, studies on the controllable molecular weights of biopolymers are limited. The purpose of this study is to achieve production of γ-PGAs with a wide range of molecular weights through manipulating the expression of γ-PGA depolymerase (PgdS) in Bacillus licheniformis WX-02. Firstly, the expression and secretion of PgdS were regulated through engineering its expression elements (four promoters and eight signal peptides), which generated γ-PGAs with molecular weights ranging from 6.82 × 10⁴ to 1.78 × 10⁶ Da. Subsequently, through combination of promoters with signal peptides, the production of γ-PGAs with a specific molecular weight could be efficiently obtained. Interestingly, the γ-PGA yield increased with the reduced molecular weight in flask cultures (Pearson correlation coefficient of −0.968, P < 0.01). Finally, in batch fermentation, the highest yield of γ-PGA with a weight-average molecular weight of 7.80 × 10⁴ Da reached 39.13 g/L under glutamate-free medium. Collectively, we developed an efficient strategy for one-step production of γ-PGAs with specific molecular weights, which have potential application for industrial production of desirable γ-PGAs.

Systematic engineering of branch chain amino acid supply modules for the enhanced production of bacitracin from Bacillus licheniformis

Bacitracin is a broad-spectrum cyclic peptide antibiotic mainly produced by Bacillus, precursor amino acid supply served as the critical role during its synthesis. In this study, we systematically engineered branch-chain amino acid (BCAA) supply modules for bacitracin production. Firstly, we demonstrated that Ile and Leu acted as limiting precursors for bacitracin synthesis, and that BCAA synthetic pathways were strengthened via simultaneous overexpression of, feedback-resistance acetolactate synthase IlvBN^fbr, 2-isopropylmalate synthetase LeuA^fbr and BCAA aminotransferase YbgE. Using this approach, bacitracin yield from strain DW-BCAA2 was 892.54 U/mL, an increase of 18.32% compared with that DW2 (754.32 U/mL). Secondly, the BCAA permeases, YvbW and BraB, which have higher affinities for Leu and Ile transportation, respectively, were both identified as BCAA importers, with their overexpression improving intracellular BCAA accumulations and bacitracin yields. Finally, the leucine-responsive family regulator, lrpC was deleted to generate the final strain DW-BCAA6, with intracellular concentrations of Ile, Leu and Val increased by 2.26-, 1.90- and 0.72-fold, respectively. The bacitracin yield from DW-BCAA6 was 1029.83 U/mL, an increase of 36.52%, and is the highest bacitracin yield reported. Equally, concentrations of other byproducts including acetic acid, acetoin and 2,3-butanediol were all reduced. Taken together, we devised an efficient strategy for the enhanced production of bacitracin, and a promising B. licheniformis DW-BCAA6 strain was constructed for industrial production of bacitracin.

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Enhancing intracellular BCAA accumulations benefited bacitracin synthesis.

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YvbW and BraB were both identified as BCAA importers in B. licheniformis.

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Deleting lrp increased brnQ transcription and intracellular BCAA concentrations.

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Bacitracin yield produced by DW-BCAA6 was the highest currently reported.

Heterologous Expression of Sfp-Type Phosphopantetheinyl Transferase is Indispensable in the Biosynthesis of Lipopeptide Biosurfactant

Phosphopantetheinyl transferases are of tremendous enthusiasm inferable from their fundamental parts in activating polyketide, fatty acid, and non-ribosomal peptide synthetase enzymes and additionally an increasing number of biotechnological applications. The present study reports the identification of sfp gene from the Paenibacillus sp. D9, which encompasses 693 bp encoding a 230-amino acid protein with a molecular weight of 25.3 kDa. The amino acid sequence Paenibacillus sp. D9 Sfp revealed more than 90% sequence identity to other Sfp proteins from other Paenibacillus. The sfp gene was cloned and recovered efficiently using affinity chromatography with maximal specific phosphopantetheinyl transferase activity at an optimal pH of 8.0 and temperature of 30 °C. The enzyme also exhibited stability under a wide-ranging pH and temperature. The presence of Zn²⁺, Cu²⁺, and Fe²⁺ ions improved the enzymatic activity, while other metals such as Ni²⁺, Co²⁺, and Mg²⁺ had inhibitory effects. The introduction of EDTA also displayed no inhibition. Kinetic parameters were obtained having values of 4.52 mg/mL, 35.33 U/mg, 3.64 s^-1, and 0.104 mM^-1 s^-1 for K_m, V_max, k_cat, and k_cat/K_m, respectively. The biosurfactant synthesized by the recombinant BioSp was found to be surface active, reducing the surface tension to 33.7 mN/m on the glucose substrate after 5 days of incubation at 37 °C. The recombinant Escherichia coli strain also exhibited an improvement in biosurfactant yield (1.11 g/L) when contrasted with 0.52 g/L from Paenibacillus sp. D9. High esterase activity of 2.55 IU/mL using p-nitrophenyl acetate was observed on the recombinant strain, as the protein connected with the release of the biosurfactant was observed to be an esterase. The characteristics of improved biosurfactant and esterase synthesis by hyper-producing recombinant strain possess numerous values from biotechnology standpoint.

Biosurfactant: A new frontier for greener technology and environmental sustainability

Petroleum hydrocarbons, oil, heavy metals pollution is becoming additional severe problem due to the growing call for crude oil and crude oil products related products in several fields of application. Such pollution have fascinated much considerations and attractions as it leads to ecological damages in both marines, aquatic and terrestrial ecosystems. Thus, different techniques including chemical surfactants and complex technologies have been proposed for their clean up from the environment, which in turn has detrimental effects on the environment. As of late, biosurfactant compounds have added much deliberation since they are considered as a reasonable option and eco-accommodating materials for remediation technology. The present society is confronting a few difficulties of usage, authorizing ecological protection and environmental change for the next generations. Biosurfactants hold the special property of minimizing and reducing the interfacial tension of liquids. Such features endure biosurfactants to afford a major part in emulsification, de-emulsification, biodegradability, foam formation, washing performance, surface activity, and detergent formulation, which have potential applications in the diverse industrial set-up. Conversations on cost-effective technologies, renewable materials, novel synthesis, downstream, upstream, emerging characterization techniques, molecular, and genetical engineering are substantial to produce biosurfactant of quality and quantity. Therefore, greater attention is being paid to biosurfactant production by identifying their environmental, and biotechnological applications. Be that as it may, the extravagant cost drew in with biosurfactants biotechnological synthesis and recovery can hamper their application in those areas. Notwithstanding these costs, biosurfactants can be used as these parts shows outstandingly high benefits that can at present beat the expenses incurred in the initial purification and downstream processes. Biosurfactant production by microorganisms is relatively considered one of the crucial know-how for improvement, growth, advancement, and environmental sustainability of the 21st century. There is a developing conversation around environmental safety and the significant role that biosurfactants will progressively play soon, for instance, the use of renewable by-products as substrates, potential reduction, re-use and recycling of waste and waste products. The review confers the usefulness of biosurfactants in the removal of environmental contaminants and, consequently, expanding environmental safety and drive towards greener technology.

Establishment and application of multiplexed CRISPR interference system in Bacillus licheniformis

Bacillus licheniformis has been regarded as an outstanding microbial cell factory for the production of biochemicals and enzymes. Due to lack of genetic tools to repress gene expression, metabolic engineering and gene function elucidation are limited in this microbe. In this study, an integrated CRISPR interference (CRISPRi) system was constructed in B. licheniformis. Several endogenous genes, including yvmC, cypX, alsD, pta, ldh, and essential gene rpsC, were severed as the targets to test this CRISPRi system, and the repression efficiencies were ranged from 45.02 to 94.00%. Moreover, the multiple genes were simultaneously repressed with high efficiency using this CRISPRi system. As a case study, the genes involved in by-product synthetic and L-valine degradation pathways were selected as the silence targets to redivert metabolic flux toward L-valine synthesis. Repression of acetolactate decarboxylase (alsD) and leucine dehydrogenase (bcd) led to 90.48% and 80.09 % increases in L-valine titer, respectively. Compared with the control strain DW9i△leuA (1.47 g/L and 1.79 g/L), the L-valine titers of combinatorial strain DW9i△leuA/pHYi-alsD-bcd were increased by 1.27-fold and 2.89-fold, respectively, in flask and bioreactor. Collectively, this work provides a feasible approach for multiplex metabolic engineering and functional genome studies of B. licheniformis.

An alternative genome-integrated method for undomesticated Bacillus subtilis and related species

Given their applicability in genetic engineering, undomesticated Bacillus strains are extensively used as non-natural hosts for chemical production due to their high tolerance of toxic substrates or products. However, they are difficult to genomically modify due to their low transformation efficiencies. In this study, the Bacillus-E. coli shuttle vector pHY300PLK, which is widely used in gram-positive bacteria, was adopted for genome integration in organic solvent-tolerant Bacillus isolates. The Bacillus-replicative vector was used to deliver homologous recombinant DNA and propagate itself inside the host cell, increasing the likelihood of genome integration of the recombinant DNA. Then, the unintegrated vectors were cured by cell cultivation in antibiotic-free medium with facilitation of nickel ions. The developed protocol was successfully demonstrated and validated by the disruption of amyE gene in B. subtilis 168. With an improved clonal selection protocol, the probability of clonal selection of the amyE::cat genome-integrated mutants was increased up to 42.0 ± 10.2%. Genome integration in undomesticated, organic solvent tolerant Bacillus strains was also successfully demonstrated with amyE as well as proB gene creating the gene-disrupted mutants with the corresponding phenotype and genotype. Not only was this technique effectively applied to several strains of undomesticated B. subtilis, but it was also successfully applied to B. cereus. This study validates the possibility of the application of Bacillus-replicative vector as well as the developed protocol in a variety of genome modification of undomesticated Bacillus species.

Enhanced production of antifungal lipopeptide iturin A by Bacillus amyloliquefaciens LL3 through metabolic engineering and culture conditions optimization

Background

Iturins, which belong to antibiotic cyclic lipopeptides mainly produced by Bacillus sp., have the potential for application in biomedicine and biocontrol because of their hemolytic and antifungal properties. Bacillus amyloliquefaciens LL3, isolated previously by our lab, possesses a complete iturin A biosynthetic pathway as shown by genomic analysis. Nevertheless, iturin A could not be synthesized by strain LL3, possibly resulting from low transcription level of the itu operon.

Results

In this work, enhanced transcription of the iturin A biosynthetic genes was implemented by inserting a strong constitutive promoter C2up into upstream of the itu operon, leading to the production of iturin A with a titer of 37.35 mg l⁻¹. Liquid chromatography-mass spectrometry analyses demonstrated that the strain produced four iturin A homologs with molecular ion peaks at m/z 1044, 1058, 1072 and 1086 corresponding to [C₁₄ + 2H]²⁺, [C₁₅ + 2H]²⁺, [C₁₆ + 2H]²⁺ and [C₁₇ + 2H]²⁺. The iturin A extract exhibited strong inhibitory activity against several common plant pathogens. The yield of iturin A was improved to 99.73 mg l⁻¹ by the optimization of the fermentation conditions using a response surface methodology. Furthermore, the yield of iturin A was increased to 113.1 mg l⁻¹ by overexpression of a pleiotropic regulator DegQ.

Conclusions

To our knowledge, this is the first report on simultaneous production of four iturin A homologs (C₁₄–C₁₇) by a Bacillus strain. In addition, this study suggests that metabolic engineering in combination with culture conditions optimization may be a feasible method for enhanced production of bacterial secondary metabolites.

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Deciphering metabolic responses of biosurfactant lichenysin on biosynthesis of poly-γ-glutamic acid

Poly-γ-glutamic acid (γ-PGA) is an extracellularly produced biodegradable polymer, which has been widely used as agricultural fertilizer, mineral fortifier, cosmetic moisturizer, and drug carrier. This study firstly discovered that lichenysin, as a biosurfactant, showed the capability to enhance γ-PGA production in Bacillus licheniformis. The exogenous addition of lichenysin improved the γ-PGA yield up to 17.9% and 21.9%, respectively, in the native strain B. licheniformis WX-02 and the lichenysin-deficient strain B. licheniformis WX02-ΔlchAC. The capability of intracellular biosynthesis of lichenysin was positively correlated with γ-PGA production. The yield of γ-PGA increased by 25.1% in the lichenysin-enhanced strain B. licheniformis WX02-Psrflch and decreased by 12.2% in the lichenysin-deficient strain WX02-ΔlchAC. Analysis of key enzyme activities and gene expression in the TCA cycle, precursor glutamate synthesis, and γ-PGA synthesis pathway revealed that the existence of lichenysin led to increased γ-PGA via shifting the carbon flux in the TCA cycle towards glutamate and γ-PGA biosynthetic pathways, minimizing by-product formation, and facilitating the uptake of extracellular substrates and the polymerization of glutamate to γ-PGA. Insight into the mechanisms of enhanced production of γ-PGA by lichenysin would define the essential parameters involved in γ-PGA biosynthesis and provide the basis for large-scale production of γ-PGA.

Surface Activity and Ca2+-Dependent Aggregation Property of Lichenysin Produced by Bacillus licheniformis NBRC 104464

Bacillus licheniformis NBRC 104464 produces a cyclic lipopeptide different from surfactin. After we performed liquid chromatography fractionation and purification, we used structural analyses to identify the cyclic lipopeptide as lichenysin. Surface tension measurements of lichenysin sodium salt in water yielded a critical micelle concentration (CMC) of 1.0×10^-5 M. The surface tension at the CMC was 28.9 mN/m. Comparative analysis of Ca²⁺-influenced micellar aggregation of lichenysin and surfactin revealed that the formation rate of the lichenysin-Ca²⁺ complex aggregate remained low up to a [Ca²⁺]/[lichenysin] molar ratio of 80, whereas the surfactin-Ca²⁺ complex formed micellar aggregates at the same molar ratio. Further excessive addition of Ca²⁺ to the micellar solution of lichenysin induced higher turbidity than surfactin.

Enhanced synthesis of poly gamma glutamic acid by increasing the intracellular reactive oxygen species in the Bacillus licheniformis Δ1-pyrroline-5-carboxylate dehydrogenase gene ycgN-deficient strain

Poly gamma glutamic acid (γ-PGA) is an anionic polyamide with numerous applications. Previous studies revealed that L-proline metabolism is implicated in a wide range of cellular processes by increasing intercellular reactive oxygen species (ROS) generation. However, the relationship between L-proline metabolism and γ-PGA synthesis has not yet been analyzed. In this study, our results confirmed that deletion of Δ1-pyrroline-5-carboxylate dehydrogenase gene ycgN in Bacillus licheniformis WX-02 increased γ-PGA yield to 13.91 g L^-1, 85.22% higher than that of the wild type (7.51 g L^-1). However, deletion of proline dehydrogenase gene ycgM had no effect on γ-PGA synthesis. Furthermore, a 2.92-fold higher P5C content (19.24 μmol gDCW^-1) was detected in the ycgN deficient strain WXΔycgN, while the P5C levels of WXΔycgM and the double mutant strain WXΔycgMN showed no difference, compared to WX-02. Moreover, the ROS level of WXΔycgN was increased by 1.18-fold, and addition of n-acetylcysteine (antioxidant) decreased its ROS level, which further reduced γ-PGA synthesis capability of WXΔycgN. Collectively, our results demonstrated that proline catabolism played an important role in maintaining ROS homeostasis, and deletion of ycgN-enhanced P5C accumulation, which induced a transient ROS signal to promote γ-PGA synthesis in B. licheniformis.

Decreased Rhamnose Metabolic Flux Improved Production of Target Proteins and Cell Flocculation in Pichia pastoris

Previously, several genes, including LRA1–LRA4 and LRAR, involved in rhamnose utilization pathway, were discovered in Pichia pastoris GS115; among them, LRA3 and LRA4 were considered as key rate-determining step enzymes. A P. pastoris expression platform based on the strong rhamnose-inducible promoter P_LRA3 did not meet the demands of industrial application due to poor production of recombinant proteins. To enhance recombinant protein production of this expression platform, a genetically engineered strain, P. pastoris GS115m, with decreased rhamnose metabolic flux was developed from P. pastoris GS115 by replacement of the rhamnose-inducible promoter P_LRA4 with another much weaker rhamnose-inducible promoter, P_LRA2. Grown in MRH and YPR media using rhamnose as the main carbon source, the engineered strain showed decreased growth rate and maximal biomass compared with the parental strain. More importantly, grown in rhamnose-containing MRH and YPR media, the recombinant engineered strain harboring a β-galactosidase gene lacB, whose expression was regulated by rhamnose-inducible P_LRA3, yielded substantial increases, of 2.5- and 1.5-fold, respectively, in target protein production over the parental strain. Additionally, grown in MRH and YPR media, the engineered strain had remarkable cell flocculation and rapid sedimentation with the increasing of cell density, providing an effective and convenient separation of the fermentation supernatant from strain cells. The engineered strain is a promising expression host for industrial production of target proteins due to its advantages over the parental strain as follows: (i) improved production of recombinant proteins, and (ii) remarkable cell flocculation and rapid sedimentation.

Effects of transcriptional mode on promoter substitution and tandem engineering for the production of epothilones in Myxococcus xanthus

Promoter optimization is an economical and effective approach to overexpress heterologous genes and improve the biosynthesis of valuable products. In this study, we swapped the original promoter of the epothilone biosynthetic gene cluster in Myxococcus xanthus with two endogenous strong promoters P pilA and P groEL1 , respectively, which, however, decreased the epothilone production ability. The transcriptional abilities by the two promoters were found to be bloomed in the growth stage but markedly decreased after the growth, whereas the original promoter P epo functioned majorly after the exponential growth stage. Tandem repeat engineering on the original promoter P epo remarkably increased epothilone production. The tandem promoter exerted similar expressional pattern as P epo did in M. xanthus. We demonstrated that differential transcriptional modes markedly affected the efficiency of promoters in controlling the gene expressions for the production of the secondary metabolite epothilones. Our study provides an insight into exploiting powerful promoters to produce valuable secondary metabolites, especially in host with limited known promoters.

Genetic Tools and Techniques for Recombinant Expression in Thermophilic Bacillaceae

Although Escherichia coli and Bacillus subtilis are the most prominent bacterial hosts for recombinant protein production by far, additional species are being explored as alternatives for production of difficult-to-express proteins. In particular, for thermostable proteins, there is a need for hosts able to properly synthesize, fold, and excrete these in high yields, and thermophilic Bacillaceae represent one potentially interesting group of microorganisms for such purposes. A number of thermophilic Bacillaceae including B.methanolicus, B.coagulans, B.smithii, B.licheniformis, Geobacillus thermoglucosidasius, G. kaustophilus, and G. stearothermophilus are investigated concerning physiology, genomics, genetic tools, and technologies, altogether paving the way for their utilization as hosts for recombinant production of thermostable and other difficult-to-express proteins. Moreover, recent successful deployments of CRISPR/Cas9 in several of these species have accelerated the progress in their metabolic engineering, which should increase their attractiveness for future industrial-scale production of proteins. This review describes the biology of thermophilic Bacillaceae and in particular focuses on genetic tools and methods enabling use of these organisms as hosts for recombinant protein production.

Improvement of glycerol catabolism in Bacillus licheniformis for production of poly-γ-glutamic acid

Bacillus licheniformis WX-02 is a well-studied strain to produce poly-γ-glutamic acid (γ-PGA) with numerous applications. This study is to improve WX-02 strain's capability of assimilating glycerol, a major byproduct of biofuels industries, through metabolic manipulation. Through gene knockout, the GlpK pathway was identified as the sole functional glycerol catabolism pathway, while the DhaK pathway was inactive for this strain under either aerobic or anaerobic conditions. The enhancement of glycerol utilization was attempted by substituting the native glpFK promoter with the constitutive promoter (P43), ytzE promoter (PytzE), and bacABC operon promoter (PbacA), respectively. The glycerol consumptions of the corresponding mutant strains WX02-P43glpFK, WX02-PytzEglpFK, and WX02-PbacAglpFK were 30.9, 26.42, and 18.8% higher than that of the WX-02 strain, respectively. The γ-PGA concentrations produced by the three mutant strains were 33.71, 23.39, and 30.05% higher than that of WX-02 strain, respectively. When biodiesel-derived crude glycerol was used as the carbon source, the mutant WX02-P43glpFK produced 16.63 g L^-1 of γ-PGA, with a productivity of 0.35 g L^-1 h^-1. Collectively, this study demonstrated that glycerol can be used as an effective substrate for producing γ-PGA by metabolic engineering B. licheniformis strains.

Identification and High-level Production of Pulcherrimin in Bacillus licheniformis DW2

Pulcherrimin, a potential biocontrol agent produced by microorganisms, has the promising applications in the agricultural, medical, and food areas, and the low yield of pulcherrimin has hindered its applications. In this study, the red pigment produced by Bacillus licheniformis DW2 was identified as pulcherrimin through the spectrometry analysis and genetic manipulation, and the component of the medium used for pulcherrimin production was optimized. Based on our results, the addition of 1.0 g L^-1 Tween 80 could improve the yield of pulcherrimin, and glucose and (NH₄)₂SO₄ were served as the optimal carbon and nitrogen sources for pulcherrimin synthesis, respectively. Furthermore, an orthogonal array design was applied for optimization of the medium. Under optimized condition, the maximum yield of pulcherrimin was 331.17 mg L^-1, 5.30-fold higher than that of the initial condition, which was the maximum yield reported for pulcherrimin production. Collectively, this study provided a promising strain and a feasible approach to achieve the high-level production of antimicrobial pulcherrimin.

A novel strategy to improve protein secretion via overexpression of the SppA signal peptide peptidase in Bacillus licheniformis

Background

Signal peptide peptidases play an important role in the removal of remnant signal peptides in the cell membrane, a critical step for extracellular protein production. Although these proteins are likely a central component for extracellular protein production, there has been a lack of research on whether protein secretion could be enhanced via overexpression of signal peptide peptidases.

Results

In this study, both nattokinase and α-amylase were employed as prototypical secreted target proteins to evaluate the function of putative signal peptide peptidases (SppA and TepA) in Bacillus licheniformis. We observed dramatic decreases in the concentrations of both target proteins (45 and 49%, respectively) in a sppA deficient strain, while the extracellular protein yields of nattokinase and α-amylase were increased by 30 and 67% respectively in a strain overexpressing SppA. In addition, biomass, specific enzyme activities and the relative gene transcriptional levels were also enhanced due to the overexpression of sppA, while altering the expression levels of tepA had no effect on the concentrations of the secreted target proteins.

Conclusions

Our results confirm that SppA, but not TepA, plays an important functional role for protein secretion in B. licheniformis. Our results indicate that the sppA overexpression strain, B. licheniformis BL10GS, could be used as a promising host strain for the industrial production of heterologous secreted proteins.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0688-7) contains supplementary material, which is available to authorized users.