Functional display of active β-galactosidase on Bacillus subtilis spores using crust proteins as carriers

Surface Display of Duck Hepatitis A Virus Type 1 VP1 Protein on Bacillus subtilis Spores Elicits Specific Systemic and Mucosal Immune Responses on Mice

Duck viral hepatitis, primarily caused by duck hepatitis A virus type 1 (DHAV-1), poses a significant threat to the global duck industry. Bacillus subtilis is commonly utilized as a safe probiotic in the development of mucosal vaccines. In this study, a recombinant strain of B. subtilis, designated as B. subtilis RV, was constructed to display the DHAV-1 capsid protein VP1 on its spore surface using the outer coat protein B as an anchoring agent. The immunogenicity of this recombinant strain was evaluated in a mouse model through mixed feeding immunization. The results indicated that B. subtilis RV could elicit specific systemic and mucosal immune responses in mice, as evidenced by the high levels of serum IgG, intestinal secretory IgA, and potent virus-neutralizing antibodies produced. Furthermore, the recombinant strain significantly upregulated the expression levels of IL-2, IL-6, IL-10, TNF-α, and IFN-γ in the intestinal mucosa. Thus, the recombinant strain maintained the balance of the Th1/Th2 immune response and demonstrated an excellent mucosal immune adjuvant function. In summary, this study suggests that B. subtilis RV can be a novel alternative for effectively controlling DHAV-1 infection as a vaccine-based feed additive.

Applications of Bacillus subtilis Protein Display for Medicine, Catalysis, Environmental Remediation, and Protein Engineering

Bacillus subtilis spores offer several advantages that make them attractive for protein display. For example, protein folding issues associated with unfolded polypeptide chains crossing membranes are circumvented. In addition, they can withstand physical and chemical extremes such as heat, desiccation, radiation, ultraviolet light, and oxidizing agents. As a result, the sequence of the displayed protein can be easily obtained even under harsh screening conditions. Next, immobilized proteins have many economic and technological advantages. They can be easily separated from the reaction and the protein stability is increased in harsh environments. In traditional immobilization methods, proteins are expressed and purified and then they are attached to a matrix. In contrast, immobilization occurs naturally during the sporulation process. They can be easily separated from the reaction and the protein stability is increased in harsh environments. Spores are also amenable to high-throughput screening for protein engineering and optimization. Furthermore, they can be used in a wide array of biotechnological and industrial applications such as vaccines, bioabsorbants to remove toxic chemicals, whole-cell catalysts, bioremediation, and biosensors. Lastly, spores are easily produced in large quantities, have a good safety record, and can be used as additives in foods and drugs.

An overview and future prospects of recombinant protein production in Bacillus subtilis

Bacillus subtilis is a well-characterized Gram-positive bacterium and a valuable host for recombinant protein production because of its efficient secretion ability, high yield, and non-toxicity. Here, we comprehensively review the recent studies on recombinant protein production in B. subtilis to update and supplement other previous reviews. We have focused on several aspects, including optimization of B. subtilis strains, enhancement and regulation of expression, improvement of secretion level, surface display of proteins, and fermentation optimization. Among them, optimization of B. subtilis strains mainly involves undirected chemical/physical mutagenesis and selection and genetic manipulation; enhancement and regulation of expression comprises autonomous plasmid and integrated expression, promoter regulation and engineering, and fine-tuning gene expression based on proteases and molecular chaperones; improvement of secretion level predominantly involves secretion pathway and signal peptide screening and optimization; surface display of proteins includes surface display of proteins on spores or vegetative cells; and fermentation optimization incorporates medium optimization, process condition optimization, and feeding strategy optimization. Furthermore, we propose some novel methods and future challenges for recombinant protein production in B. subtilis.Key points• A comprehensive review on recombinant protein production in Bacillus subtilis.• Novel techniques facilitate recombinant protein expression and secretion.• Surface display of proteins has significant potential for different applications.

Constructing an Efficient Bacillus subtilis Spore Display by Using Cohesin-Dockerin Interactions

Bacillus subtilis spore display has become a field of increasing interest in the past two decades. To improve the efficiency of B. subtilis spore display, its directed modification was performed based on the cellulosome architecture by introducing onto them divergent cohesin (Coh) modules that can specifically bind to the target enzyme bearing the matching dockerins (Doc). In this study, five different pairs of cohesins and dockerins, selected from four cellulolytic microbes, were examined for their capabilities in displaying a tetrameric enzyme β-galactosidase from Bacillus stearothermophilus IAM11001 on the surface of B. subtilis WB600 spores. Immunofluorescence microscopy, western blotting, dot blotting, and enzyme assay was applied to confirm its surface expression. All the resultant five Coh-Doc based spore display can hydrolyze o-nitrophenyl-β-D-galactopyranoside. Further, the optimized Coh-Doc based spore display exhibited the highest display efficiency. Overall, the results of current study may open new perspectives on the use of Coh-Doc interaction, which will find application in improving the efficiency of B. subtilis spore display.

Highly sensitive protein detection using recombinant spores and lateral flow immunoassay

Lateral flow immunoassays (LFIs) can be used to detect intact bacteria or spores; when gold nanoparticles (AuNPs) are used as the signal reporters, the detection limits are very low. Spore-based surface display has been widely studied for enzyme immobilization and live-nontoxic oral vaccines. In this study, recombinant spores were used to improve the sensitivity of a LFI. We developed a test kit that combines streptavidin-displayed spores with a LFI assay for rapid protein detection. The recombinant spores served as a signal amplifier and AuNPs were used as the signal reporters. For detection of β-galactosidase, which was used as the model protein, the detection limit was about 10^-15 mol, while that of the conventional LFI is about 10^-12 mol. In both methods, nanogold was used as the colorimetric signal and could be observed with the naked eye. This method improved LFI sensitivity without sacrificing its advantages. Furthermore, enhanced green fluorescent protein (eGFP) was also displayed on the surface of the streptavidin-displayed spores. Without AuNPs, the fluorescent recombinant spores acted as the signal, which could be detected by a fluorescence detector, such as a fluorescence microscope. The detection limit was 10^-16 mol under fluorescence microscopy whose magnification was 25-fold. Therefore, in conclusion, in this proof of concept study, the detection limits of both proposed methods were far superior to those of traditional LFI assay.

Applications of Bacillus subtilis Spores in Biotechnology and Advanced Materials

The bacterium Bacillus subtilis has long been an important subject for basic studies. However, this organism has also had industrial applications due to its easy genetic manipulation, favorable culturing characteristics for large-scale fermentation, superior capacity for protein secretion, and generally recognized as safe (GRAS) status. In addition, as the metabolically dormant form of B. subtilis, its spores have attracted great interest due to their extreme resistance to many environmental stresses, which makes spores a novel platform for a variety of applications. In this review, we summarize both conventional and emerging applications of B. subtilis spores, with a focus on how their unique characteristics have led to innovative applications in many areas of technology, including generation of stable and recyclable enzymes, synthetic biology, drug delivery, and material sciences. Ultimately, this review hopes to inspire the scientific community to leverage interdisciplinary approaches using spores to address global concerns about food shortages, environmental protection, and health care.

Detection of biotin with zeptomole sensitivity using recombinant spores and a competition assay

Detection of protein-binding analytes is important for many applications. Currently, various instrument-based techniques are used for detecting protein-binding analytes. However, such techniques have several limitations including high cost and time-consuming sample processing. In order to overcome these limitations, we developed a sensitive competition assay for the detection of protein-binding analytes using recombinant endospores as a sensing element. The method is based on the competition between the biotin, the model analyte, and a biotin-magnetic bead complex to bind the recombinant spores containing the biotin binding region of streptavidin. After magnetic attraction, the residual spores in the suspension are spread on plates to form colonies which are used to count the amount of the residual spores; the higher the residual ratio of spores, the more biotin in the samples. The linear range was from 150 zmol to 1.5 fmol and the limit of detection of the assay was 150 zmol. The assay proposed herein is sensitive and does not require any expensive equipment. It is suitable for qualitative or semi-quantitative analysis such as screening tests for the detection of toxic chemicals.

Progress in research and application development of surface display technology using Bacillus subtilis spores

Bacillus subtilis is a widely distributed aerobic Gram-positive species of bacteria. As a tool in the lab, it has the advantages of nonpathogenicity and limited likelihood of becoming drug resistant. It is a probiotic strain that can be directly used in humans and animals. It can be induced to produce spores under nutrient deficiency or other adverse conditions. B. subtilis spores have unique physical, chemical, and biochemical characteristics. Expression of heterologous antigens or proteins on the surface of B. subtilis spores has been successfully performed for over a decade. As an update and supplement to previously published research, this paper reviews the latest research on spore surface display technology using B. subtilis. We have mainly focused on the regulation of spore coat protein expression, display and application of exogenous proteins, and identification of developing research areas of spore surface display technology.

Display of Escherichia coli Phytase on the Surface of Bacillus subtilis Spore Using CotG as an Anchor Protein

Escherichia coli phytase (AppA) has been widely used as an exogenous feed enzyme for monogastric animals; however, the production of this enzyme has been examined primarily in E. coli and yeast expression systems. As an alternative to production of soluble phytase, an enzyme immobilization method using the Bacillus subtilis spore outer-coat protein CotG as an anchoring motif for the display of the AppA was attempted. Using this motif, AppA was successfully produced on the spore surface of B. subtilis as verified by Western blot analysis and phytase activity measurements. Analysis of the pH stability indicated that more than 50% activity was retained after incubation at four different pH values (2.0, 4.0, 7.0, and 8.0) for up to 12 h, with maximum activity observed at pH 4.5. The highest enzyme activity seen at 55 °C and thermal stability measurements demonstrated that more than 30% activity remained after 30 min incubation at 60 °C. The spore surface-displayed AppA was resistant to pepsin, and more stable than phytase produced previously using a yeast expression system. Furthermore, we present data indicating that the use of peptide linkers may help improve the bioactivity of displayed enzymes on the spore surface of B. subtilis.

Sporobeads: The Utilization of the Bacillus subtilis Endospore Crust as a Protein Display Platform

Upon starvation, the soil bacterium Bacillus subtilis forms an intracellular, metabolically inactive endospore. Its core contains the DNA, encased by three protein layers protecting it against a multitude of environmental threats. The outermost layer, the crust, harbors great potential as a protein-displaying platform: a gene of interest can be translationally fused to a crust protein gene, resulting in endospores displaying the desired protein on their surface. To unlock this potential in a standardized fashion, we designed a suite of 12 vectors (Sporovectors), based on the BioBrick cloning standard. With these vectors, proteins can easily be fused N- or C-terminally to the six crust proteins CotV, CotW, CotX, CotY, CotZ, and CgeA under the control of the strongest crust gene promoter P_cotYZ. All Sporovectors were evaluated with GFP and two different laccases. On the basis of our data, CotY and CotZ represent the best anchor proteins. But there are significant differences in activity and functional stability between the two tested laccases. Our vector suite is a powerful tool to generate and evaluate a vast variety of functionalized endospores. It allows quickly identifying the best anchor and fusion site for the protein of interest. Our findings demonstrate that the crust of B. subtilis endospores is an inexpensive and easy platform for displaying different proteins of interest.

Impact of orientation and flexibility of peptide linkers on T. maritima lipase Tm1350 displayed on Bacillus subtilis spores surface using CotB as fusion partner

Fusion protein construction often requires peptide linkers for prolonged conformation, extended stability and enzyme activity. In this study a series of fusion between Thermotoga maritima lipase Tm1350 and Bacillus subtillis coat protein CotB, comprising of several peptide linkers, with different length, flexibility and orientations were constructed. Effects of temperature, pH and chemicals were examined, on the activity of displayed enzyme. The fusion protein with longer flexible linkers L9 [(GGGGS)₄] and L7 (GGGGS-GGGGS-EAAAK-EAAAK-GGGGS-GGGGS) possess 1.29 and 1.16-fold higher activity than the original, under optimum temperature and pH respectively. Moreover, spore surface displaying Tm1350 with L3 (EAAAK-GGGGS) and L9 ((GGGGS)₄) showed extended thermostably, maintaining 1.40 and 1.35-fold higher activity than the original respectively, at 80 °C after 5 h of incubation. The enzyme activity of linkers with different orientation, including L5, L6 and L7 was determined, where L7 maintained 1.05 and 1.27-fold higher activity than L5 and L6. Effect of 0.1% proteinase K, bromelain, 20% ethanol and 30% methanol was investigated. Linkers with appropriate Glycine residues (flexible) showed higher activity than Alanine residues (rigid). The activity of the displayed enzyme can be improved by maintaining orientation and flexibility of peptide linkers, to evaluate high activity and stability in industrial processes.

Recent progress in Bacillus subtilis spore-surface display: concept, progress, and future

With the increased knowledge on spore structure and advances in biotechnology engineering, the newly developed spore-surface display system confers several inherent advantages over other microbial cell-surface display systems including enhanced stability and high safety. Bacillus subtilis is the most commonly used Bacillus species for spore-surface display. The expression of heterologous antigen or protein on the surface of B. subtilis spores has now been practiced for over a decade with noteworthy success. As an update and supplement to other previous reviews, we comprehensively summarize recent studies in the B. subtilis spore-surface display technique. We focus on its benefits as well as the critical factors affecting its display efficiency and offer suggestions for the future success of this field.