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

Abstract

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.

Introduction

Bacillus subtilis is an important industrial microorganism. Its genetics and physiology have been studied intensively. Among bacteria, the understanding of its genetic background and physiology is second only to Escherichia coli (Kunst et al. 1997; Sonenshein et al. 2002). Spore surface display is a method of anchoring exogenous functional proteins on the surface of spores by means of a special structure (Zhang et al. 2019). B. subtilis spore surface display has many advantages. First, spores are resistant to harsh environmental conditions, and this is conducive to the use and stability of exogenous proteins in complex environments (Wang et al. 2011). Second, spores are synthesized in the cytoplasm of bacterial cells, so any heterologous protein to be anchored on the spore surface does not need to cross any membrane (Kim and Schumann 2009). Third, molecular chaperone in the cytoplasm of B. subtilis can appropriately promote the secretion and expression of foreign proteins (Muller et al. 2000). The first spore display system was established by Isticato et al. (2001), using CotB as an anchor protein to display tetanus toxin (TTFC) on the surface of B. subtilis spores. With growing knowledge of the B. subtilis genome and proteome, spore surface display has now been successfully applied in many fields, including oral vaccine development, antibody production, biocatalysis, bioremediation, and creating of diagnostic tools (Fig. 1) (Georgiou et al. 1997; Li et al. 2019).

Fig. 1.

Applications of B. subtilis spore surface display

B. subtilis spore surface display follows two main approaches: a recombinant approach and a nonrecombinant approach (Isticato and Ricca 2014; Ricca et al. 2014). The recombinant approach requires modification of the bacterial genome to express a protein of interest as a fusion with spore coat protein (Hinc et al. 2013; Isticato and Ricca 2014), and the nonrecombinant approach is based on the direct adsorption of heterologous proteins on the spore surface or anchoring exogenous proteins on the spore surface with a cross-linking agent (Isticato et al. 2019; Ricca et al. 2014). The display by recombination approach avoids the isolation and purification steps of foreign proteins, the production process is simple, and it is the mainstream of B. subtilis spore surface display technology (Chen et al. 2017b; Kim and Schumann 2009). In this review, we summarize the application of genetic recombination-based spore surface display technology in many fields, discuss new and developing research, and determine the future prospects of the technology.

Formation and structure of B. subtilis spores

Bacteria have many strategies to cope with the challenges of their environment (Tasaki et al. 2017). These strategies often involve rapid changes in gene expression, which temporarily alter the phenotype of cells and allow them to survive. A more complex and persistent example of stress response is sporulation, in which the bacterial genome is isolated in a protected space (spore) until environmental conditions improve, at which point spores will germinate to form vegetative cells with reproductive capacity (Setlow 2014). Among Gram-positive bacteria, B. subtilis and a few similar species are the most commonly used experimental systems, and many studies have been conducted to assess the process and morphology of sporulation (Higgins and Dworkin 2012).

Formation of B. subtilis spores

It is challenging for B. subtilis to form spores; their formation is controlled by a series of regulatory and structural genes whose expressions themselves are tightly regulated (Bejerano-Sagie et al. 2006). When nutrients are depleted, sporulation is triggered by the activation of histidine sensor kinases, including KinA, KinB, and KinC, which shuttle phosphate through an extended phosphorelay, resulting in phosphorylation of the master regulator of sporulation,transcription factor Spo0A (Molle et al. 2003). KinA is the major kinase responsible for initiation of sporulation and KinA (or KinB) overexpression during exponential growth is sufficient to induce entry into sporulation (Fujita and Losick 2005). In fact, inducing KinA synthesis beyond a certain level leads toentry into sporulation regardless of nutrient availability (Eswaramoorthy et al. 2010). The effect of a kinC mutation on sporulation is weaker than that of kinA or kinB (Lopez et al. 2009). Phosphorylated Spo0A can directly activate or inhibit the transcription of many genes; it indirectly controls genes involved in asymmetric cell division and those involved in the activation of sporulation-specific sigma factors and ultimately promotes spore formation (Hilbert and Piggot 2004).

The formation of spores can be roughly divided into the following steps (Eichenberger et al. 2003; Higgins and Dworkin 2012): In harsh environments, B. subtilis cells begin to form dormant spores that resist adverse environments, and the activity of σ^H begins to increase. Cells then divide unequally using specific asymmetric septum to form large mother cells and a small forespore. The mother cell is necessary for spore formation, but it is eventually lysis and the prospore eventually produces a mature spore. Mother cell and prospore express different σ factors; σ^E factors are expressed in mother cells, while σ^F factors are expressed in prospores (Losick and Stragier 1992), and phosphorylated Spo0A can induce the activation of σ^E and σ^F factors (Wang et al. 2006). After unequal division is completed, the maternal plasma membrane gradually encapsulates the forespore, so the outer membrane of the forespore encapsulates two layers of membrane structure. After that, the activated or synthesized σ^G and σ^K begin to induce gene expression in the forespore and mother cell. Lastly, specific structures such as spore crust, cortex, and spore coat are gradually synthesized. The cortex is composed of peptidoglycan (PG), which is located between the inner and outer membrane of spore, and spore PG precursors are synthesized in the mother cell (Popham 2002). The spore coat is formed in the mother cell and covers the outer surface of the prospore (Henriques and Moran 2007; Kim et al. 2006). Dipicolinic acid (DPA) synthesized in the mother cell gradually fills the forespores, which could help the forespores dehydrate continuously, the mother cell lyses, and mature spore is generated (Fig. 2) (McKenney et al. 2013).

Fig. 2.

The sporulation and germination cycle in B. subtilis. Adapted from McKenney et al. (2013)

Structure of B. subtilis spores

The B. subtilis spore is a complex structure. The spore core contains the chromosomal DNA that is maintained in a compact state by small acid-soluble proteins (SASPs). The original membrane that surrounded the forespore surrounds the core and the peptidoglycan rich cortex surrounds this membrane. Surrounding the cortex, the spore coat consists of about 80 proteins deposited by the mother cell arranged in inner and outer layers (Fig. 3) (Liu et al. 2016; McKenney et al. 2013).

Fig. 3.

Spore structure

Spore coat

The assembling coat is synthesized in the mother cell and is targeted to the outer forespore membrane by SpoIVA (Wang et al. 2009). SpoIVA binds and hydrolyzes ATP, allowing it to self-assemble into cable-like structures (Setlow 2012; Ramamurthi and Losick 2008) that form a basement layer that serves as a platform for coat assembly. Other proteins involved in assembly are SpoVID that directly interacts with SpoIVA (Mullerova et al. 2009; Wang et al. 2009) and SafA, which is necessary for the encasement of the spore (Mullerova et al. 2009). SafA was found to affect the localization of about 16 inner coat protein fusions (McKenney et al. 2010) substantiating its central role in coat assembly. Three layers of the B. subtilis spore coat are observed in thin-section electron microscopy: an inner coat, an outer coat, and crust (Warth et al. 1963; McKenney et al. 2010). The outer coat is indispensable for spore formation, yet its specific functions remain unclear. Compared with the outer coat of spores, the inner coat is a selective permeability barrier that protects spore DNA from being destroyed by some chemical agents (McKenney et al. 2013). The spore coat makes spores resistant to chemical reagents and external lysozymes, and prevents the nuclei from being degraded or ingested by protozoa (Setlow 2006); however, the resistance to heat, radiation, and some other chemical reagents is poorly understood (Borch-Pedersen et al. 2016).

Spore cortex

The cortex and core of spores are the key structures in the formation and maintenance of dormant spores. The spore cortex is thick, mainly composed of PG, which can reach 10% of the total dry weight of the spore. The structure of the PG is similar to that in vegetative cell wall, but in cortex, the structure is relatively loose, which is extremely important for maintaining spore resistance and dormancy (Aguilar et al. 2007; Higgins and Dworkin 2012). Because some amino acid residues of N-acetylmuramic acid of PG in the spore cortex are replaced by short peptides, the degree of cross-linking of PG in cortex is lower than that of PG in vegetative cell wall (Popham 2002). In addition, B. subtilis spore-cortex PG was found to be O-acetylated, a common PG modification that reduces sensitivity to the innate immune anti-microbial lysozyme (Laaberki et al. 2011). However, since lysozyme is unable to penetrate the outer coat (Driks 1999), this modification would not appear to be useful.

Spore core

The innermost layer of spores is the core, it is surrounded by the inner forespore membrane, germ cell wall (McKenney et al. 2013). The inner forespore membrane is located in the inner of cortex, it It has extremely low permeability, and small molecular substances are difficult to penetrate, which can prevent DNA-damaging substances from penetrating the inner forespore membrane to cause damage to the spore core DNA (Setlow 2006). Spore core contains most of its enzymes, chromosomal DNA, ribosomes, and tRNA, it also contains the special small molecule, DPA, it is chelated with Ca²⁺ in the spore core, exists as a calcium salt, CaDPA, which can promote dehydration of the spore core and increase the thermal resistance of the spore (Higgins and Dworkin 2012; McKenney et al. 2013). SASPs are tightly bound to the core DNA of the spore, which can make the spore tolerate damage from UA radiation, drying, and high temperature, and can be used as a carbon source and energy source during spore germination (Setlow 2007).

Regulation of B. subtilis spore coat protein expression

The anchoring proteins used in B. subtilis spore surface display can be linked to exogenous proteins through their C- or N-termini. The correct selection of anchoring proteins is key to successfully displaying exogenous proteins on the spore surface. A suitable anchoring protein needs to meet the following requirements: (1) it must have a strong anchoring domain to ensure that foreign proteins can be immobilized on the spore surface (Potocki et al. 2017); (2) they must be compatible with foreign proteins, be able to form fusion proteins, and should not be able to interact with each other (Lee et al. 2003); and (3) anchored proteins must be resistant to protease hydrolysis (Lee et al. 2003; Potocki et al. 2017). To date, various spore coat proteins, such as CotB, CotC, CotE, CotG, CotX, CotY, CotZ, CgeA, and OxdD, have been successfully used as the anchoring proteins to display exogenous proteins on the spore surface of B. subtilis.

Spore surface display of B. subtilis using CotB as an anchoring protein

CotB was the first spore coat protein to be used in B. subtilis spore surface display. Its expression and assembly require the assistance of a variety of regulatory factors and proteins (Kodama et al. 2011; Zilhao et al. 2004). The expression of cotB is regulated by the maternal cell-specific sigma factors and transcription regulators GerE and GerR (Cangiano et al. 2010). CotB has a strongly hydrophilic C-terminus, which is composed of three serine-rich repeats; the serine residues accounts for more than 50% of the CotB C-terminus. Some studies have shown that CotB modification requires the involvement of CotG and CotH (Zilhao et al. 2004), and CotG is known to interact directly with CotB. Mutation of cotG results in the accumulation of a 46-kDa CotB protein in cells, but the specific mechanisms for this remain unclear. CotH, or proteins regulated by CotH, can prevent CotG from being hydrolyzed by proteases in the cell before assembling into spores, and it has an indirect regulatory effect on CotB (Nguyen et al. 2016). Isticato et al. deleted the amino acid residue in position 105 of the CotB C-terminus (CotBΔ105), used CotBΔ105 as an anchoring protein, and integrated the tetanus toxin gene into the amylase gene locus of the cotB-deleted B. subtilis genome. It was found that exogenous protein could not be expressed in spores, which proved that the fusion protein could not be assembled on the surface of spores in the absence of the original cotB gene (Henriques and Moran 2007). Therefore, the cotB, cotG, and cotH genes of B. subtilis should be retained when CotB is being used as an anchor to display exogenous proteins.

Spore surface display of B. subtilis using CotC as an anchoring protein

CotC is an abundant, 66-amino-acid protein known to assemble in the outer coat in various forms: a monomer of 12 kDa, a homodimer of 21 kDa, and two less abundant forms of 12.5 and 30 kDa, probably due to posttranslational modifications of CotC (Isticato et al. 2010; Isticato et al. 2008). Assembly of CotC requires expression of both cotH and cotE, but CotC does not accumulate in the mother cell compartment when its assembly is prevented by mutation of CotH (Isticato et al. 2004). In contrast, overexpression of cotH allows the accumulation of CotC in the mother cell compartment, suggesting that CotH, or a CotH-dependent factor, acts to prevent degradation of CotC in the mother cell and then allows its assembly within the coat (Baccigalupi et al. 2004). The mechanism of assembly of CotC is of interest, as the abundant CotC protein has been used as a vehicle for the display of heterologous proteins at the spore surface (Isticato et al. 2007). At present, heat-labile enterotoxin B subunit, urea, ethanol dehydrogenase, β-galactosidase, proinsulin, enolase, and trehalose synthase have all been successfully displayed on the spore surface using CotC as molecular carrier, which improves their tolerance to harsh environments (Hinc et al. 2010b; Romero et al. 2007).

Spore surface display of B. subtilis using CotG as an anchoring protein

CotG is a 24-kDa protein regulated by mother cell RNA polymerase σ^K and transcription regulator GerE. Like CotC, the expression of CotG is also indirectly regulated by GerR because GerR can activate SpoVIF, which plays an active role in GerE and GerE-dependent genes (Cangiano et al. 2010). The assembly of CotG on spore surfaces is mainly as 32- and 36-kDa proteins. Thirty-two kilodalton CotG may be formed by abnormal migration of unmodified initial proteins. Thirty-six kilodalton CotG may be produced by extensive cross-linking of proteins when proteins are assembled into the spore coat (Eichenberger et al. 2004). Like CotB and CotC, CotG assembly also requires cotH expression. CotH protects CotG from protease hydrolysis before sporulation, which is essential for the formation and assembly of CotG (Naclerio et al. 1996; Zilhao et al. 2004). Therefore, cotH should be retained when CotG is used as anchoring protein to display exogenous proteins (Saggese et al. 2014).

Spore surface display of B. subtilis using other anchoring proteins

OxdD is a secondary component of the spore shell and has oxalate decarboxylase activity. It can catalyze the conversion of oxalate into formate and CO₂. Its molecular weight is approximately 43 kDa (Garcia-Ramon et al. 2017). oxdD gene is transcribed by a σ^K-recognized promoter during sporulation and is negatively regulated by GerE. Therefore, OxdD is produced in the mother cell chamber of sporangia and depends on SafA assembly in the coat. Genetic and cytobiological analyses have shown that OxdD is located in the outer layer of the spore. As an anchoring protein, OxdD could encapsulate the exogenous proteins under the spore surface, providing more effective protection for the exogenous proteins and reducing effects on spore formation (Romero et al. 2007).

CotH is an intermediate morphogenetic protein that plays a role in the assembly of the spore shell, but differs from CotG. CotH, as an inner layer protein of 42.8 kDa, has a strong correlation with CotB and CotG. The expression of cotH is regulated by σ^K. As mentioned earlier, the assembly of spore coat proteins CotB, CotC, and CotG in CotH-mutant strains also has multiple validity defect, indicating that the inner and outer layers of the spore coat require CotH function (Isticato et al. 2015).

CotZ, a key component of the crust belongs to the last encasement class and is more abundant at the mother cell proximal pole of the forespore (Imamura et al. 2010). It is dependent on σ^E, σ^K and the transcription factor GerE for expression (McKenney et al. 2013). CotZ is a 16-kDa protein, and it has been found to act as a new anchoring motif for the efficient display of UreA of Helicobacter acinonychis on the spores (Imamura et al. 2011; Hinc et al. 2013). In the case of the CotZ-UreA fusion protein, the calculated number of recombinant protein molecules is 2.5 × 10² from a single spore. This fusion protein is more effective in stimulating immunological response than other antigens in mice.

Similar to CotZ, CgeA is another 14 kDa crust protein. CgeA is dependent on σ^K and the transcription factor GerE for expression (Imamura et al. 2011). Iwanicki et al. (2014) described an example application of presented vector system to display CagA protein of Helicobacter pylori in fusion with CgeA spore coat protein.

Applications of B. subtilis spore surface display

B. subtilis has a well-established fermentation and production technology, and spores are resistant to harsh environmental conditions (Wang et al. 2011), so the application of spore surface display technology is very extensive. To date, this technology has been used in the production of multimeric proteins, oral vaccine preparations, and industrial enzyme production (Guoyan et al. 2019). Table 1 summarizes the related applications of spore surface display of foreign proteins based on recombinant approach in previous studies.

Table 1.

List of fusion and target proteins, used vectors, and application of Bacillus subtilis spore surface-displayed proteins based on recombinant approach

Fusion protein	Bacterial strain	Target protein	Used vector	Substrate/antibody	Product	Application	Reference
CotB	B. subtilis PY79	TTFC	pGEM	Anti-TTFC	―*	Oral vaccination	(Isticato et al. 2001)
	B. subtilis PY79 and RH103	TTFC	pET28b	Anti-TTFC	―	Oral vaccination	(Duc et al. 2003)
	B. subtilis PY79 and PP108	TcdA	―	Anti-TcdA	―	Oral vaccination	(Hong et al. 2017)
	B. subtilis DB403	Tm1350	pHS	p-Nitrophenyl butyrate	p-Nitrophenyl	Industrial biocatalysis	(Chen et al. 2015a)
	B. subtilis DB403	DSM	pHS	p-Nitrophenyl butyrate	p-Nitrophenyl	Industrial biocatalysis	(Chen et al. 2015b)
	B. subtilis PY79	VP28	pDG364	White Spot Syndrome virus	―	Vaccine for shrimps	(Nguyen et al. 2014; Pham et al. 2016)
	B. subtilis PY79	UreA	pGEM	Anti-UreA	―	Anti-Helicobacter vaccine	(Hinc et al. 2010b)
	B. subtilis HU58	MPT64	pcotVac	Anti-MPT64	―	Vaccine against tuberculosis	(Sibley et al. 2014)
	B. subtilis PY79	RSM2e3	pDG1664	Anti-RSM2e3	―	Influenza vaccine	(Zhao et al. 2014)
	B. subtilis 168	FliD	pDL	Anti-FliD	―	C. difficile oral vaccines	(Negri et al. 2013)
	B. subtilis PY79	GST-Cpa247-370	pDG1664	Anti-Cpa247–370	―	Vaccine against necrotic enteritis	(Hoang et al. 2008)
	B. subtilis PY79	PA	pDG364	Anti-PA	―	Anthrax vaccine	(Le et al. 2007)
	B. subtilis PY79 and RH201		pDHAFB	Anti-His	―	Bioremediation	(Hinc et al. 2010a)
CotC	B. subtilis PY79 (Spo+)	TTFC and LTB	pRH22 and pIM51	Anti-TTFC and anti-LTB	―	Clostridium tetani and E. coli vaccine	(Mauriello et al. 2004)
	B subtilis 168 (trp−)	BmADH	pJS700	Ethanol and NAD+	Acetaldehyde and NADH	Industrial biocatalysis	(Wang et al. 2011)
	B. subtilis PY79 and PP108	TcdA	―	Anti-TcdA	―	Oral vaccination	(Hong et al. 2017)
	B subtilis 168 (trp−)	OmpC	pDG364	―	―	Vaccine against Salmonella	(Dai et al. 2018)
	B. subtilis DB431 and BB80	VP28 and VP26	pDG1662	Anti-Vp28 and anti-Vp26	―	Oral vaccination	(Valdez et al. 2014)
	B. subtilis WB600	Urease B and CTB	pUS186	Rat anti UreB serum	―	Oral vaccine for H. pylori	(Zhou et al. 2017)
	B. subtilis WB600	CsCP	pEB03	Rat anti-rCsCP serum	―	Vaccine against Clonorchis sinensis	(Tang et al. 2016; Tang et al. 2017)
	B. subtilis WB600	TP20.8	pGEX	TP20.8-specific antibody	―	Vaccine against Clonorchis sinensis	(Zhou et al. 2008b)
	B. subtilis WB600	CsPmy	PEB03	Rat anti-rCsPmy serum	―	Vaccine against Clonorchis sinensis	(Sun et al. 2018)
	B. subtilis WB600	CsTP22.3	pGEX	Rat anti-TP22.3 sera	―	Vaccine against Clonorchis sinensis	(Zhou et al. 2008a)
	B. subtilis WB600	CsLAP2	PEB03	Rat anti-CsLAP2 serum	―	Vaccine against Clonorchis sinensis	(Qu et al. 2014)
	B. subtilis WB800N	TreS	pDG1730	D-maltosee	D-trehalose	Industrial biocatalysis	(Liu et al. 2019)
	B. subtilis WB600	VP4	pEB03	Rabbit anti-rVP4 serum	―	Vaccine against grass carp reovirus	(Jiang et al. 2018)
	B subtilis 168 (trp−)	hGH	pJS700	Anti-hGH	―	Oral vaccination	(Lian et al. 2014)
	B. subtilis PY79	UreA	pGEM	Anti-UreA	―	Anti-Helicobacter vaccine	(Hinc et al. 2010b)
	B subtilis 168	β-galactosidase	pKH40	ONPG	ONP	Industrial biocatalysis	(Tavassoli et al. 2013)
	B. subtilis 168 (trp−)	GP64	pJS700	GP64-specific antibody	―	Vaccine against Bombyx mori Nucleopolyhedrovirus	(Li et al. 2011)
	B. subtilis 168 (trp−)	HSA	pJS700	HSA-specific antibody	―	Oral vaccination	(Mao et al. 2012)
CotE	B. subtilis DB104	Tyrosinase	pCSK1	L-tyrosine	―	Industrial, medical, and environmental applications	(Hosseini-Abari et al. 2016)
	B. subtilis DB104	β-galactosidase	pDG1728	Anti β-galactosidase, antibody mouse IgM	―	Industrial biocatalysis	(Hwang et al. 2013)
	B. subtilis DB104	Lipase A and Lipase B	pHPS9	pNPP	―	Industrial biocatalysis	(Kim 2017)
CotG	B.subtilis DB403	Nitrilase	pHS	Tomalononitrile, Succinonitrile, Glutaronitrile	2-cyanoacetic acid, 3-cyanopropionic acid, 4-cyanobutyric acid	Industrial biocatalysis	(Chen et al. 2015c)
	B. subtilis DB104	DhaA	pHY300PLK	2-CEES	Chloride	Bioremediation	(Wang et al. 2019)
	B. subtilis DB104	β-galactosidase	pDG1728	Anti β-galactosidase, antibody mouse IgM	―	Industrial biocatalysis	(Hwang et al. 2013)
	B. subtilis DB104	ω-transaminase	pHPS9	(S)-α-methylbenzylamine and pyruvate	Acetophenone	Industrial biocatalysis	(Bum-Yeol et al. 2011)
	B. subtilis DB104	GFPUV	pCSK1	―	―	Diagnosis	(Kim et al. 2007)
	B. subtilis MI111	Phytase	pHT304	Sodium phytate	Inorganic phosphate	Industrial biocatalysis and animal probiosis	(Mingmongkolchai and Panbangred 2018)
	B. subtilis WB800N	TreS	pDG1730	D-Maltosee	D-Trehalose	Industrial biocatalysis	(Liu et al. 2019)
	B. subtilis 168 c-trp	ChiS	pDHAFB	Chitin	3,5-dinitrosalicylic acids and N-acetyl glucosamine	Biopesticide	(Rostami et al. 2017)
	B. subtilis DB403	Nitrilase	pHS.	3-Cyanopyridine	3-Carboxypyridine	Industrial biocatalysis	(Chen et al. 2017a)
	B. subtilis DB403	L-arabinose isomerase	pHS	D-galactose	D-tagatose	Industrial biocatalysis	(Qi et al. 2018)
	B. subtilis WB600	NanA	pEASY	Pyruvate	Neu5Ac	Industrial biocatalysis	(Xu et al. 2011)
	B. subtilis DB104	Streptavidin	pHPS9	Anti-streptavidin, Antibody	―	Biological diagnosis	(Kim et al. 2005)
CotX	B. subtilis 168 (trpC2)	β-galactosidase	pJS700a	ONPG	ONP	Industrial biocatalysis	(He et al. 2015; Wang et al. 2016)
CotY	B. subtilis 168(trp−)	β-galactosidase	pJS700a	ONPG	ONP	Industrial biocatalysis	(He et al. 2015)
CotZ	B. subtilis 168(trp−)	β-galactosidase	pJS700a	ONPG	ONP	Industrial biocatalysis	(He et al. 2015)
	B. subtilis WB800(trp-)	DPEase	pET22b(+)	D-fructose	D-allulose	Industrial biocatalysis	(He et al. 2016)
CgeA	B. subtilis 168	CagA	pMUTIN4	Mouse anti-CagA antibody	―	Vaccine formulation	(Iwanicki et al. 2014)
OxdD	B. subtilis PY79	Phytase	pDG364	Sodium phytate	Inorganic phosphate	Industrial biocatalysis and animal probiosis	(Potot et al. 2010)

^*Not available

Application in polyprotein production

B. subtilis can spontaneously form spores in harsh or nutrient poor environments. Spores have strong resistance to adverse environments, such as high temperature, chemical reagents, ultraviolet rays, and lysozymes. The spore coat is a complex structure comprising at least 70 different proteins. Spore surface display requires the expression of exogenous proteins fused with coat proteins, so that the exogenous proteins are assembled on the spore surface directly without transmembrane localization after synthesis in the mother cell. The fusion proteins can be immobilized by spore surface display, which improves the stability of the protein and makes isolation and purification easier. Liu H et al. fused trehalose synthase with spore-anchoring proteins CotC and CotG for display on the surface of B. subtilis spores, and immunofluorescence, Western blot analysis, and enzyme activity assays showed that trehalose synthase was indeed present on the spore surface. The trehalose synthase on the surface of the recombinant spore can react with maltose as a substrate to form trehalose, after reused four times, the recombinant spore retained most of the enzymatic activity. (Liu et al. 2019). β-Galactosidase is a high molecular weight protein (116 kDa). It is active in a tetramer state and can affect the structure of host cells in general surface display systems. To date, this protein has been displayed on the spore surface using the B. subtilis spore coat proteins CotC, CotE, CotG, CotX, CotY, and CotZ, the enzyme expressed on the surface of the spore still retains its activity (He et al. 2015). The active polyproteins were anchored on the spore surface by spore surface display technology, which demonstrates that this technology represents a new method for the production of polyproteins.

Application in preparation of oral vaccine

CotB and CotC were selected as anchoring proteins to display antigens on the surface of B. subtilis spores. Since the first successful display of surface antigens, the list of displayed antigens has grown steadily (Amuguni and Tzipori 2012; Rosales-Mendoza and Angulo 2015). Spores have good resistance to stress; therefore, vaccines developed with this method can tolerate the acidic environment of the gastrointestinal tract and have a long shelf life (Zhou et al. 2008a). They can pass through the gastrointestinal mucosa smoothly and quickly induce the body to produce a protective immune response. In addition, the use of spores as vaccine carriers can improve the efficiency of the immune response (Batista et al. 2014; Vogt et al. 2016).

In recent years, Clonorchiasis sinensis, caused by Clonorchis sinensis, has become increasingly prevalent. Effective prevention strategies are urgently needed to control this food-borne infectious disease. Previous studies have shown that C. sinensis paramyosin (CsPmy) functions as a preferred vaccine. Sun et al. (2018) displayed CsPmy on the spore surface using CotC as anchoring protein. The expression of CsPmy on the spore surface was analyzed by SDS-PAGE, Western blot analysis, and immunofluorescence assay, and the results showed that CsPmy was successfully expressed on spore surfaces and the fusion protein had good thermostability. Specific IgGs in sera and intestinal mucosa were increased after intraperitoneal and intragastrical immunization. Oral immunization with B. subtilis spore expressing CsPmy on the surface was a promising, safe, and needle-free vaccination strategy against clonorchiasis (Mingmongkolchai and Panbangred 2018). In addition, CsPmy, CsCP, TP20.8, CsTP22.3, and CsLAP2 have also been successfully displayed on spore surfaces for immunization against clonorchiasis sinensis (Tang et al. 2017; Zhou et al. 2008a). Salmonellosis is a major public health problem throughout the world. Dai et al. have assessed the potential use of B. subtilis spores for the expression of a major protective antigen of Salmonella serovar pullorum, OmpC. Mice immunized with recombinant spores expressing the OmpC antigen presented significant higher levels of OmpC-specific serum IgG and mucosal SIgA antibodies than mice immunized with nonrecombinant spores (p < 0.01) (Dai et al. 2018). These results indicate that B. subtilis spores have broad applicability in vaccine development.

Application in the production of industrial enzymes

Industrial enzymes are at the core of the biocatalysis and biotransformation industries. They are characterized by high catalytic efficiency, high specificity, and low pollution in the production process. It can be difficult to separate enzymes from substrates, and the reaction conditions are usually strictly controlled. This leads the enzymes to be easily inactivated and makes their reuse difficult. However, enzymes can be easily separated from their substrates by displaying them on the surface of spores. The excellent stress resistance of spores can enhance the stability of enzymes in complex environments and promote the reuseabilty of enzymes. He et al. produced D-allulose by using D-psicose 3-epimerase (DPEase) expressed and displayed on the surface of B. subtilis spores. DPEase was fused at the C-terminus of the anchoring protein, CotZ, via a peptide linker, and trophic genes were used as selection markers during chromosomal integration. The optimal temperature and pH of the fusion protein CotZ-DPEase were 55 °C and pH 7.5–8.0, respectively, and the anchored DPEase exhibited high thermostability. Under optimal conditions, 60% of the yield was maintained after five cycles of utilization. Therefore, this biocatalyst system, capable of expressing and immobilizing DPEase on the spore surface of B. subtilis, was an appropriate alternative for D-allulose production (He et al. 2016).

Lipases expressed in microbial hosts have great commercial value, but their applications are restricted by the high costs of production and harsh conditions used in industrial processes. Chen et al. successfully displayed the thermophilic lipase Tm1350 on the B. subtilis spore surface. The results showed that spore surface-displayed Tm1350 had more stable enzyme activity than free enzyme. Meanwhile, recycling experiments showed that the recombinant spores could be used for up to three reaction cycles without a significant decrease in catalytic rate (84%) (Chen et al. 2015a). These studies have played a positive role in development of the application of spore surface-displayed enzymes in the industrial field.

Application in the field of biological control of environmental pollution

Enzymatic technology has been applied to the treatment of environmental pollution due to its advantages of stability against environmental stress and high catalytic efficiency. Tyrosinases, which are copper-containing monooxygenases, could be used for bioremediation of phenol-polluted environments and production of L-DOPA and melanin from L-tyrosine, are widely used for environmental applications (Sok and Fragoso 2018). Hosseini-Abari et al. displayed tyrosinase on spore surfaces using CotE as a molecular carrier. Tyrosinase activity on spores was monitored in the presence of L-tyrosine and CuSO₄. Recombinant spores could be used repeatedly, with 62% of enzymatic activity remaining after washing six times with Tris-HCl buffer (Hosseini-Abari et al. 2016).

Chitinase is a hydrolytic enzyme that has the specific function of hydrolyzing chitin into chitosan or N-acetylglucosamine. Chitinase is mainly used to control pests in agriculture. It can be used alone as an insecticide or used in conjunction with other microorganisms to control pests (Rishad et al. 2016). Rostami et al. fused chitinase with CotG and successfully displayed it on the surface of B. subtilis spores. Enzyme activity assays showed that the surface-displayed chitinase was active and was also able to inhibit the growth of Rhizoctonia solani and Trichoderma harzianum fungi (Rostami et al. 2017) This suggests a new bioremediation method to treat the problem of residual organophosphorus pesticides in the environment.

Application in animal feed preparation

Feed enzymes must remain active under the harsh conditions of feed preparation and the gastrointestinal tract. The strong stress resistance of spores enables them to be used as new tools for improving bioactive molecular preparations. E. coli phytase (AppA) has been widely used as an exogenous feed enzyme for monogastric animals. Sirima et al. displayed AppA on the spore surface of B. subtilis using spore coat protein CotG as an anchoring protein. AppA was successfully produced on the spore surface as verified by Western blot analysis and phytase activity assays. The highest enzyme activity was observed at 55 °C and thermal stability measurements demonstrated that more than 30% activity remained after 30 min incubated at 60 °C (Mingmongkolchai and Panbangred 2018).

Research hotspots on surface display of B. subtilis spores

As mentioned above, many B. subtilis spore surface display systems have been developed. However, up to now, most of these studies have been confined to the laboratory. Therefore, research on how to scale up the production of target proteins has become an active area of research. Strategies include introducing linker peptide chains (Huang et al. 2015), using multiple anchoring proteins to display exogenous proteins at the same time (Iwanicki et al. 2014), and increasing the number of copies of exogenous genes (Xu et al. 2011). Research on improving the sporulation efficiency of B. subtilis is another recent approach to optimizing spore surface display (Devi et al. 2015; Tojo et al. 2013).

An appropriate intermediate ligand can improve the folding efficiency of foreign target proteins and anchoring proteins. It can also change the interactions between foreign target proteins and anchoring proteins, as well as between target foreign proteins and the cell surface. Strauss et al. found that the activity of lipase on the spore surface was positively correlated with the length of the intermediate. Lipase activity increased from 0.8 to 83 U/mg when the length of the intermediate increased from 10 to 92 amino acids (Strauss and Götz 1996). Hinc et al. found that the binding mode of anchoring proteins and foreign target proteins was the key factor for the success of spore display. The conformation of linker peptides could affect the results of spore surface display, and alpha helices have shown to be most effective under some conditions (Hinc et al. 2013).

Using multiple anchoring proteins to display exogenous proteins at the same time can also improve spore display efficiency. The structure of B. subtilis spores is complex and contains dozens of different proteins. The number of potential anchoring proteins in spores is an important factor that restricts display efficiency. Therefore, the simultaneous display of various exogenous proteins by multiple anchoring proteins has become a hot area of research (Liu et al. 2019). At present, the chromosome insertion sites selected by the researchers are all the growth non-essential amyE gene. Iwanicki et al. constructed spore surface display integrative vectors using the non-essential genes lacA and pyrD as insertion sites, and using CotC and CotG as anchoring proteins, thus creating a multi-anchoring protein display system (Iwanicki et al. 2014).

Conclusions and future perspectives

B. subtilis spore surface display technology has developed rapidly over the past decade, and many coat proteins, including CotB, CotC, CotE, CotG, CotZ, CgeA, and OxdD, have been successfully used to display exogenous proteins or polypeptides on the spore surface. The nonpathogenicity of B. subtilis make this technology applicable to food and biological industries. The resistance of spores to stress makes industrial enzymes displayed on their surface more stable, and also provides the advantages of easy purification and recycling of immobilized enzyme, which can greatly reduce the cost of industrialization. B. subtilis spore surface display provides feasible avenues to improve industrial production efficiency, while providing for food and biological safety. At the same time, there are still some problems in spore surface display, such as the limited number of anchoring proteins on the spore surface, which is not conducive to a large number of exogenous proteins. Further, the success of surface display on spores depends on the fusion of anchored and target proteins, so it is critical to choose the correct fusion and anchor partner (Hinc et al. 2010b).

B. subtilis spore surface display technology has shown great promise for use in vaccine and drug preparation, enzymatic catalysis, biological detection, and other areas because of its unique advantages. It is believed that with further research on surface display using B. subtilis, this technology will play an important role in even more fields in the future.

Funding information

This work was supported by the National Natural Science Foundation of China (31501413); Shandong key project of Research & Development plan (2017GSF221019); Young doctorate Cooperation Fund Project, QiLu University of Technology (Shandong Academy of Sciences) (2017BSHZ021); State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences (ZZ20190314); Natural Science Foundation of Shandong Province (ZR2017BC072, ZR2019PC060); The Dragon City Excellent Researcher Award Program from Zhucheng and Taishan Industry Leading Talents Program; and A Project of Shandong Province Higher Educational Science and Technology Program (A18KA116).

Compliance with ethical standards

Conflict of interest

The authors declare that they have no competing interests.

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.