Mucosal vaccine delivery by non-recombinant spores of Bacillus subtilis

Development of a spore-based mucosal vaccine against the bovine respiratory pathogen Mannheimia haemolytica

Bovine respiratory disease (BRD) is a significant health issue in the North American feedlot industry, causing substantial financial losses due to morbidity and mortality. A lack of effective vaccines against BRD pathogens has resulted in antibiotics primarily being used for BRD prevention. The aim of this study was to develop a mucosal vaccine against the BRD pathogen, Mannheimia haemolytica, using Bacillus subtilis spores as an adjuvant. A chimeric protein (MhCP) containing a tandem repeat of neutralizing epitopes from M. haemolytica leukotoxin A (NLKT) and outer membrane protein PlpE was expressed to produce antigen for adsorption to B. subtilis spores. Adsorption was optimized by comparing varying amounts of antigen and spores, as well as different buffer pH and reaction temperatures. Using the optimal adsorption parameters, spore-bound antigen (Spore-MhCP) was prepared and administered to mice via two mucosal routes (intranasal and intragastric), while intramuscular administration of free MhCP and unvaccinated mice were used as positive and negative control treatments, respectively. Intramuscular administration of MhCP elicited the strongest serum IgG response. However, intranasal immunization of Spore-MhCP generated the best secretory IgA-specific response against both PlpE and NLKT in all samples evaluated (bronchoalveolar lavage, saliva, and feces). Since proliferation of M. haemolytica in the respiratory tract is a prerequisite to lung infection, this spore-based vaccine may offer protection in cattle by limiting colonization and subsequent infection, and Spore-MhCP warrants further evaluation in cattle as a mucosal vaccine against M. haemolytica.

Bacillus subtilis spores displaying RBD domain of SARS-CoV-2 spike protein

Bacillus subtilis spores are considered to be efficient and useful vehicles for the surface display and delivery of heterologous proteins. In this study, we prepared recombinant spores with the receptor binding domain (RBD) of the SARS-CoV-2 spike glycoprotein displayed on their surface in fusion with the CotZ or CotY spore coat proteins as a possible tool for the development of an oral vaccine against the SARS-CoV-2 virus. The RBD was attached to the N-terminus or C-terminus of the coat proteins. We also directly adsorbed non-recombinantly produced RBD to the spore surface. SDS-PAGE, western blot and fluorescence microscopy were used to analyze RBD surface expression on purified spores. Results obtained from both display systems, recombinant and non-recombinant, demonstrated that RBD was present on the spore surfaces.

Oral Delivery of Bacillus subtilis Expressing Chicken NK-2 Peptide Protects Against Eimeria acervulina Infection in Broiler Chickens

Chicken NK-lysin peptide 2 (cNK-2) is a natural lytic peptide with direct cytotoxicity against many apicomplexan parasites including Eimeria. Developing an effective oral delivery strategy to express cNK-2 in the intestine, where Eimeria parasites interact with the host's gut epithelial cells, may effectively reduce the fecundity of parasites and minimize intestinal damage. Furthermore, cNK-2 modulates gut immune responses to decrease local inflammation elicited by Eimeria infection in the intestine. Therefore, we developed a stable strain of Bacillus subtilis (B. subtilis) that carries cNK-2 to the gut to determine its effectiveness in ameliorating the negative impacts of coccidiosis and to replace the use of antibiotics in controlling coccidiosis in commercial broiler chicken production. Chickens were randomly allocated into eight treatment groups: two control groups (NC: E. acervulina infected non-B. subtilis control; CON: non-infected control); three B. subtilis-empty vector (EV) groups (EV6: 10⁶ cfu/day/bird; EV8: 10⁸ cfu/day/bird; EV10: 10¹⁰ cfu/day/bird), and three B. subtilis-cNK-2 groups (NK6: 10⁶ cfu/day/bird; NK8: 10⁸ cfu/day/bird; NK10: 10¹⁰ cfu/day/bird). All chickens, except those in the CON group, were challenged with 5,000 freshly sporulated E. acervulina oocysts through oral gavage on day 15. Chickens were given an oral dose of B. subtilis on days 14, 15, and 16. Body weight, weight gains, and fecal oocyst shedding were measured. To investigate the efficacy of oral B. subtilis-cNK-2 against coccidiosis, gene expression of gut health-related biomarkers was measured using RT-PCR. Markers included SOD1, CAT, and HMOX1 for oxidative stress in the spleen and intestinal mucosa, OCLN, ZO-1, and JAM2 for tight junction proteins, and MUC2 for mucin gene expression in the gut. The results showed that oral treatment of young chickens with B. subtilis-cNK-2 improved growth performance, enhanced gut integrity, and reduced fecal oocyst shedding. Altogether, these results confirm B. subtilis-cNK-2 treatment as a promising and effective alternative strategy to replace antibiotics against coccidiosis based on its ability to reduce parasite survival, to reduce coccidiosis-induced body weight loss, and to decrease gut damage based on the enhanced expression of proteins associated with gut integrity and intestinal health.

Bacillus Toyonensis BCT-7112T Spores as Parenteral Adjuvant of BoHV-5 Vaccine in a Murine Model

Bacterial spores of the genus Bacillus are being evaluated as adjuvant molecules capable of improving the immune response to vaccines. In this study, we investigate whether subcutaneously administered spores of B. toyonensis BCT-7112^T could enhance a vaccine immune response in mice. Three groups of mice were subcutaneously vaccinated on day 0 and received a booster on day 21 of the experiment, with the following vaccine formulations: 40 µg of recombinant glycoprotein D (rgD) from bovine herpesvirus type 5 (BoHV-5) adsorbed in 10% aluminum hydroxide (alum) without B. toyonensis spores (group 1) and B. toyonensis (1 × 10⁶ viable spores) + 40 µg of rgD adsorbed in 10% alum (group 2); and B. toyonensis (1 × 10⁶ viable spores) without rgD (group 3). Group 2 showed significantly higher titers (P < 0.05) of total specific serum IgG, IgG2a, and neutralizing antibodies, when compared with the groups 1 and 3. A significantly higher (P < 0.05) transcription level of cytokines IL-4, IL-12, and IFN-γ was observed in splenocytes from mice that received the B. toyonensis spores in the vaccine formulation. In addition, stimulation of the macrophage-like cell line RAW264.7 with spores of B. toyonensis markedly enhanced the cell proliferation and mRNA transcription levels of IL-4, and IL-12 cytokines in these cells. Our findings indicated that the subcutaneous administration of B. toyonensis BCT-7112^T spores enhanced the humoral and cellular immune response against BoHV-5 in mice.

The Sporobiota of the Human Gut

The human gut microbiome is a diverse and complex ecosystem that plays a critical role in health and disease. The composition of the gut microbiome has been well studied across all stages of life. In recent years, studies have investigated the production of endospores by specific members of the gut microbiome. An endospore is a tough, dormant structure formed by members of the Firmicutes phylum, which allows for greater resistance to otherwise inhospitable conditions. This innate resistance has consequences for human health and disease, as well as in biotechnology. In particular, the formation of endospores is strongly linked to antibiotic resistance and the spread of antibiotic resistance genes, also known as the resistome. The term sporobiota has been used to define the spore-forming cohort of a microbial community. In this review, we present an overview of the current knowledge of the sporobiota in the human gut. We discuss the development of the sporobiota in the infant gut and the perinatal factors that may have an effect on vertical transmission from mother to infant. Finally, we examine the sporobiota of critically important food sources for the developing infant, breast milk and powdered infant formula.

The temperature of growth and sporulation modulates the efficiency of spore-display in Bacillus subtilis

BACKGROUND

Bacterial spores displaying heterologous antigens or enzymes have long been proposed as mucosal vaccines, functionalized probiotics or biocatalysts. Two main strategies have been developed to display heterologous molecules on the surface of Bacillus subtilis spores: (i) a recombinant approach, based on the construction of a gene fusion between a gene coding for a coat protein (carrier) and DNA coding for the protein to be displayed, and (ii) a non-recombinant approach, based on the spontaneous and stable adsorption of heterologous molecules on the spore surface. Both systems have advantages and drawbacks and the selection of one or the other depends on the protein to be displayed and on the final use of the activated spore. It has been recently shown that B. subtilis builds structurally and functionally different spores when grown at different temperatures; based on this finding B. subtilis spores prepared at 25, 37 or 42 °C were compared for their efficiency in displaying various model proteins by either the recombinant or the non-recombinant approach.

RESULTS

Immune- and fluorescence-based assays were used to analyze the display of several model proteins on spores prepared at 25, 37 or 42 °C. Recombinant spores displayed different amounts of the same fusion protein in response to the temperature of spore production. In spores simultaneously displaying two fusion proteins, each of them was differentially displayed at the various temperatures. The display by the non-recombinant approach was only modestly affected by the temperature of spore production, with spores prepared at 37 or 42 °C slightly more efficient than 25 °C spores in adsorbing at least some of the model proteins tested.

CONCLUSION

Our results indicate that the temperature of spore production allows control of the display of heterologous proteins on spores and, therefore, that the spore-display strategy can be optimized for the specific final use of the activated spores by selecting the display approach, the carrier protein and the temperature of spore production.

Bacillus subtilis spore vaccines displaying protective antigen induce functional antibodies and protective potency

BACKGROUND

Bacillus anthracis is the causative agent of anthrax, a disease of both humans and various animal species, and can be used as a bioterror agent. Effective vaccines are available, but those could benefit from improvements, including increasing the immunity duration, reducing the shot frequency and adverse reactions. In addition, more sophisticated antigen delivery and potentiation systems are urgently required. The protective antigen (PA), one of three major virulence factors associated with anthrax was displayed on the surface of Bacillus subtilis spores, which is a vaccine production host and delivery vector with several advantages such as a low production cost, straightforward administration as it is safe for human consumption and the particulate adjuvanticity. Mice were immunized orally (PO), intranasally (IN), sublingually (SL) or intraperitoneally (IP) with the PA displaying probiotic spore vaccine. Clinical observation, serological analysis and challenge experiment were conducted to investigate the safety and efficacy of the vaccine.

RESULTS

A/J mice immunized with the PA spore vaccine via PO, IN, SL, and IP were observed to have increased levels of active antibody titer, isotype profiles and toxin neutralizing antibody in sera, and IgA in saliva. The immunized mice were demonstrated to raise protective immunity against the challenge with lethal B. anthracis spores.

CONCLUSIONS

In this study, we developed a B. subtilis spore vaccine that displays the PA on its surface and showed that the PA-displaying spore vaccine was able to confer active immunity to a murine model based on the results of antibody isotype titration, mucosal antibody identification, and a lethal challenge experiment.

A probiotic treatment increases the immune response induced by the nasal delivery of spore-adsorbed TTFC

Background

Spore-forming bacteria of the Bacillus genus are widely used probiotics known to exert their beneficial effects also through the stimulation of the host immune response. The oral delivery of B. toyonensis spores has been shown to improve the immune response to a parenterally administered viral antigen in mice, suggesting that probiotics may increase the efficiency of systemic vaccines. We used the C fragment of the tetanus toxin (TTFC) as a model antigen to evaluate whether a treatment with B. toyonensis spores affected the immune response to a mucosal antigen.

Results

Purified TTFC was given to mice by the nasal route either as a free protein or adsorbed to B. subtilis spores, a mucosal vaccine delivery system proved effective with several antigens, including TTFC. Spore adsorption was extremely efficient and TTFC was shown to be exposed on the spore surface. Spore-adsorbed TTFC was more efficient than the free antigen in inducing an immune response and the probiotic treatment improved the response, increasing the production of TTFC-specific secretory immunoglobin A (sIgA) and causing a faster production of serum IgG. The analysis of the induced cytokines indicated that also the cellular immune response was increased by the probiotic treatment. A 16S RNA-based analysis of the gut microbial composition did not show dramatic differences due to the probiotic treatment. However, the abundance of members of the Ruminiclostridium 6 genus was found to correlate with the increased immune response of animals immunized with the spore-adsorbed antigen and treated with the probiotic.

Conclusion

Our results indicate that B. toyonensis spores significantly contribute to the humoral and cellular responses elicited by a mucosal immunization with spore-adsorbed TTFC, pointing to the probiotic treatment as an alternative to the use of adjuvants for mucosal vaccinations.

Induction of a Specific Humoral Immune Response by Nasal Delivery of Bcla2ctd of Clostridioides difficile

Clostridioides difficile, formerly known as Clostridium difficile, is a spore-forming bacterium considered as the most common cause of nosocomial infections in developed countries. The spore of C. difficile is involved in the transmission of the pathogen and in its first interaction with the host; therefore, a therapeutic approach able to control C. difficile spores would improve the clearance of the infection. The C-terminal (CTD) end of BclA2, a spore surface protein of C. difficile responsible of the interaction with the host intestinal cells, was selected as a putative mucosal antigen. The BclA2 fragment, BclA2_CTD, was purified and used to nasally immunize mice both as a free protein and after adsorption to the spore of Bacillus subtilis, a well-established mucosal delivery vehicle. While the adsorption to spores increased the in vitro stability of BclA2_CTD, in vivo both free and spore-adsorbed BclA2_CTD were able to induce a similar, specific humoral immune response in a murine model. Although in the experimental conditions utilized the immune response was not protective, the induction of specific IgG indicates that free or spore-bound BclA2_CTD could act as a putative mucosal antigen targeting C. difficile spores.

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.

Oral delivery of Bacillus subtilis spores expressing Clonorchis sinensis paramyosin protects grass carp from cercaria infection

Clonorchis sinensis (C. sinensis), an important fishborne zoonotic parasite threatening public health, is of major socioeconomic importance in epidemic areas. Effective strategies are still urgently expected to prevent against C. sinensis infection. In the present study, paramyosin of C. sinensis (CsPmy) was stably and abundantly expressed on the surface of Bacillus subtilis spores. The recombinant spores (B.s-CotC-CsPmy) were incorporated in the basal pellets diet in three different dosages (1 × 10⁵, 1 × 10⁸, 1 × 10¹¹ CFU/g pellets) and orally administrated to grass carp (Ctenopharyngodon idella). The immune responses and intestinal microbiota in the treated grass carp were investigated. Results showed that specific anti-CsPmy IgM levels in sera, skin mucus, bile, and intestinal mucus, as well as mRNA levels of IgM and IgZ in the spleen and head kidney, were significantly increased in B.s-CotC-CsPmy-10¹¹ group. Besides, transcripts levels of IL-8 and TNF-αin the spleen and head kidney were also significantly elevated than the control groups. Moreover, mRNA levels of tight junction proteins in the intestines of B.s-CotC-CsPmy-10¹¹ group increased. Potential pathogenetic bacteria with lower abundance and higher abundances of candidate probiotics and bacteria associated with digestion in 1 × 10¹¹ CFU/g B.s-CotC-CsPmy spores administrated fishes could be detected compared with control group. The amount of metacercaria in per gram fish flesh was statistically decreased in 1 × 10¹¹ CFU/g B.s-CotC-CsPmy spores orally immunized group. Our work demonstrated that B. subtilis spores presenting CsPmy on the surface could be a promising effective, safe, and needle-free candidate vaccine against C. sinensis infection for grass carp.

Nanoparticles in influenza subunit vaccine development: Immunogenicity enhancement

The threat of novel influenza infections has sparked research efforts to develop subunit vaccines that can induce a more broadly protective immunity by targeting selected regions of the virus. In general, subunit vaccines are safer but may be less immunogenic than whole cell inactivated or live attenuated vaccines. Hence, novel adjuvants that boost immunogenicity are increasingly needed as we move toward the era of modern vaccines. In addition, targeting, delivery, and display of the selected antigens on the surface of professional antigen-presenting cells are also important in vaccine design and development. The use of nanosized particles can be one of the strategies to enhance immunogenicity as they can be efficiently recognized by antigen-presenting cells. They can act as both immunopotentiators and delivery system for the selected antigens. This review will discuss on the applications, advantages, limitations, and types of nanoparticles (NPs) used in the preparation of influenza subunit vaccine candidates to enhance humoral and cellular immune responses.

Mouse intestinal microbiota reduction favors local intestinal immunity triggered by antigens displayed in Bacillus subtilis biofilm

BACKGROUND

We previously engineered Bacillus subtilis to express an antigen of interest fused to TasA in a biofilm. B. subtilis has several properties such as sporulation, biofilm formation and probiotic ability that were used for the oral application of recombinant spores harboring Echinococcus granulosus paramyosin and tropomyosin immunogenic peptides that resulted in the elicitation of a specific humoral immune response in a dog model.

RESULTS

In order to advance our understanding of the research in oral immunization practices using recombinant B. subtilis spores, we describe here an affordable animal model. In this study, we show clear evidence indicating that a niche is required for B. subtilis recombinant spores to colonize the densely populated mice intestinal microbiota. The reduction of intestinal microbiota with an antibiotic treatment resulted in a positive elicitation of local humoral immune response in BALB/c mice after oral application of recombinant B. subtilis spores harboring TasA fused to E. granulosus (102-207) EgTrp immunogenic peptide. Our results were supported by a lasting prevalence of spores in mice feces up to 50 days after immunization and by the presence of specific secretory IgA, isolated from feces, against E. granulosus tropomyosin.

CONCLUSIONS

The reduction of mouse intestinal microbiota allowed the elicitation of a local humoral immune response in mice after oral application with spores of B. subtilis harboring immunogenic peptides against E. granulosus.

IL-1 Fragment Modulates Immune Response Elicited by Recombinant Bacillus subtilis Spores Presenting an Antigen/Adjuvant Chimeric Protein

Mucosal immunizations are convenient ways of vaccination, which do not require any trained personnel for administration. One of the major challenges for developing an effective mucosal vaccine is finding appropriate adjuvant. Bacillus subtilis endospores have been shown to help solving these obstacles while serving as a platform for presentation of both, antigens and adjuvants. In this study, we have successfully designed and constructed recombinant spores displaying an antigen/adjuvant chimeric protein. We have used a fragment of Clostridium difficile flagellar cap FliD protein as antigen and VQGEESNDK peptide, a fragment of human IL-1β, as adjuvant. Recombinant spores presenting FliD were able to elicit immune response in orally immunized mice which could be evaluated by detection of FliD-specific IgA antibodies in feces of immunized animals. Moreover, the presence of IL-1β fragment significantly changed characteristics of elicited immune response. Obtained results show that recombinant spores presenting an antigen/adjuvant chimeric protein exhibit both properties in mucosal immunization of mice. Moreover, IL-1β fragment could serve as valuable adjuvant in B. subtilis spore-based mucosal vaccines.

Bacillus subtilis spore with surface display of paramyosin from Clonorchis sinensis potentializes a promising oral vaccine candidate

Background

Clonorchiasis caused by Clonorchis sinensis has become increasingly prevalent in recent years. Effective prevention strategies are urgently needed to control this food-borne infectious disease. Previous studies indicated that paramyosin of C. sinensis (CsPmy) is a potential vaccine candidate.

Methods

We constructed a recombinant plasmid of PEB03-CotC-CsPmy, transformed it into Bacillus subtilis WB600 strain (B.s-CotC-CsPmy), and confirmed CsPmy expression on the spore surface by SDS-PAGE, Western blotting and immunofluorescence assay. The immune response and protective efficacy of the recombinant spore were investigated in BALB/c mice after intragastrical or intraperitoneal immunization. Additionally, biochemical enzyme activities in sera, the intestinal histopathology and gut microflora of spore-treated mice were investigated.

Results

CsPmy was successfully expressed on the spore surface and the fusion protein on the spore surface with thermostability. Specific IgG in sera and intestinal mucus were increased after intraperitoneal and intragastrical immunization. The sIgA level in intestinal mucus, feces and bile of B.s-CotC-CsPmy orally treated mice were also significantly raised. Furthermore, numerous IgA-secreting cells were detected in intestinal mucosa of intragastrically immunized mice. No inflammatory injury was observed in the intestinal tissues and there was no significant difference in levels of enzyme-indicated liver function among the groups. Additionally, the diversity and abundance of gut microbiota were not changed after oral immunization. Intragastric and intraperitoneal immunization of B.s-CotC-CsPmy spores in mice resulted in egg reduction rates of 48.3 and 51.2% after challenge infection, respectively. Liver fibrosis degree in B.s-CotC-CsPmy spores treated groups was also significantly reduced.

Conclusions

CsPmy expressed on the spore surface maintained its immunogenicity. Both intragastrical and intraperitoneal immunization with B.s-CotC-CsPmy spores induced systemic and local mucosal immune response in mice. Although both intragastric and intraperitoneal immunization elicited a similar protective effect, intragastric immunization induced stronger mucosal immune response without side effects to the liver, intestine and gut microbiota, compared with intraperitoneal immunization. Oral immunization with B. subtilis spore expressing CsPmy on the surface was a promising, safe and needle-free vaccination strategy against clonorchiasis.

Conversion of xylan by recyclable spores of Bacillus subtilis displaying thermophilic enzymes

BACKGROUND

The Bacillus subtilis spore has long been used to display antigens and enzymes. Spore display can be accomplished by a recombinant and a non-recombinant approach, with the latter proved more efficient than the recombinant one. We used the non-recombinant approach to independently adsorb two thermophilic enzymes, GH10-XA, an endo-1,4-β-xylanase (EC 3.2.1.8) from Alicyclobacillus acidocaldarius, and GH3-XT, a β-xylosidase (EC 3.2.1.37) from Thermotoga thermarum. These enzymes catalyze, respectively, the endohydrolysis of (1-4)-β-D-xylosidic linkages of xylans and the hydrolysis of (1-4)-β-D-xylans to remove successive D-xylose residues from the non-reducing termini.

RESULTS

We report that both purified enzymes were independently adsorbed on purified spores of B. subtilis. The adsorption was tight and both enzymes retained part of their specific activity. When spores displaying either GH10-XA or GH3-XT were mixed together, xylan was hydrolysed more efficiently than by a mixture of the two free, not spore-adsorbed, enzymes. The high total activity of the spore-bound enzymes is most likely due to a stabilization of the enzymes that, upon adsorption on the spore, remained active at the reaction conditions for longer than the free enzymes. Spore-adsorbed enzymes, collected after the two-step reaction and incubated with fresh substrate, were still active and able to continue xylan degradation. The recycling of the mixed spore-bound enzymes allowed a strong increase of xylan degradation.

CONCLUSION

Our results indicate that the two-step degradation of xylans can be accomplished by mixing spores displaying either one of two required enzymes. The two-step process occurs more efficiently than with the two un-adsorbed, free enzymes and adsorbed spores can be reused for at least one other reaction round. The efficiency of the process, the reusability of the adsorbed enzymes, and the well documented robustness of spores of B. subtilis indicate the spore as a suitable platform to display enzymes for single as well as multi-step reactions.

The combination of recombinant and non-recombinant Bacillus subtilis spore display technology for presentation of antigen and adjuvant on single spore

BACKGROUND

Bacillus subtilis spores can be used for presentation of heterologous proteins. Two main approaches have been developed, the recombinant one, requiring modification of bacterial genome to express a protein of interest as a fusion with spore-coat protein, and non-recombinant, based on the adsorption of a heterologous protein onto the spore. So far only single proteins have been displayed on the spore surface.

RESULTS

We have used a combined approach to adsorb and display FliD protein of Clostridium difficile on the surface of recombinant IL-2-presenting spores. Such spores presented FliD protein with efficiency comparable to FliD-adsorbed spores produced by wild-type 168 strain and elicited FliD-specific immune response in intranasally immunized mice.

CONCLUSIONS

Our results indicate that such dual display technology may be useful in creation of spores simultaneously presenting adjuvant and antigen molecules. Regarding the characteristics of elicited immune response it seems plausible that such recombinant IL-2-presenting spores with adsorbed FliD protein might be an interesting candidate for vaccine against infections with Clostridium difficile.

The Exosporium of Bacillus megaterium QM B1551 Is Permeable to the Red Fluorescence Protein of the Coral Discosoma sp

Bacterial spores spontaneously interact and tightly bind heterologous proteins. A variety of antigens and enzymes have been efficiently displayed on spores of Bacillus subtilis, the model system for spore formers. Adsorption on B. subtilis spores has then been proposed as a non-recombinant approach for the development of mucosal vaccine/drug delivery vehicles, biocatalysts, bioremediation, and diagnostic tools. We used spores of B. megaterium QM B1551 to evaluate their efficiency as an adsorption platform. Spores of B. megaterium are significantly larger than those of B. subtilis and of other Bacillus species and are surrounded by the exosporium, an outermost surface layer present only in some Bacillus species and lacking in B. subtilis. Strain QM B1551 of B. megaterium and a derivative strain totally lacking the exosporium were used to localize the adsorbed monomeric Red Fluorescent Protein (mRFP) of the coral Discosoma sp., used as a model heterologous protein. Our results indicate that spores of B. megaterium adsorb mRFP more efficiently than B. subtilis spores, that the exosporium is essential for mRFP adsorption, and that most of the adsorbed mRFP molecules are not exposed on the spore surface but rather localized in the space between the outer coat and the exosporium.

Localization of a red fluorescence protein adsorbed on wild type and mutant spores of Bacillus subtilis

BACKGROUND

Bacterial spores have been proposed as vehicles to display heterologous proteins for the development of mucosal vaccines, biocatalysts, bioremediation and diagnostic tools. Two approaches have been developed to display proteins on the spore surface: a recombinant approach, based on the construction of gene fusions between DNA molecules coding for a spore surface protein (carrier) and for the heterologous protein to be displayed (passenger); and a non-recombinant approach based on spore adsorption, a spontaneous interaction between negatively charged, hydrophobic spores and purified proteins. The molecular details of spore adsorption have not been fully clarified yet.

RESULTS

We used the monomeric Red Fluorescent Protein (mRFP) of the coral Discosoma sp. and Bacillus subtilis spores of a wild type and an isogenic mutant strain lacking the CotH protein to clarify the adsorption process. Mutant spores, characterized by a strongly altered coat, were more efficient than wild type spores in adsorbing mRFP but the interaction was less stable and mRFP could be in part released by raising the pH of the spore suspension. A collection of isogenic strains carrying GFP fused to proteins restricted in different compartments of the B. subtilis spore was used to localize adsorbed mRFP molecules. In wild type spores mRFP infiltrated through crust and outer coat, localized in the inner coat and was not surface exposed. In mutant spores mRFP was present in all surface layers, inner, outer coat and crust and was exposed on the spore surface.

CONCLUSIONS

Our results indicate that different spores can be selected for different applications. Wild type spores are preferable when a very tight protein-spore interaction is needed, for example to develop reusable biocatalysts or bioremediation systems for field applications. cotH mutant spores are instead preferable when the heterologous protein has to be displayed on the spore surface or has to be released, as could be the case in mucosal delivery systems for antigens and drugs, respectively.

A versatile nano display platform from bacterial spore coat proteins

Dormant bacterial spores are encased in a thick protein shell, the ‘coat', which contains ∼70 different proteins. The coat protects the spore from environmental insults, and is among the most durable static structures in biology. Owing to extensive cross-linking among coat proteins, this structure has been recalcitrant to detailed biochemical analysis, so molecular details of how it assembles are largely unknown. Here, we reconstitute the basement layer of the coat atop spherical membranes supported by silica beads to create artificial spore-like particles. We report that these synthetic spore husk-encased lipid bilayers (SSHELs) assemble and polymerize into a static structure, mimicking in vivo basement layer assembly during sporulation in Bacillus subtilis. In addition, we demonstrate that SSHELs may be easily covalently modified with small molecules and proteins. We propose that SSHELs may be versatile display platforms for drugs and vaccines in clinical settings, or for enzymes that neutralize pollutants for environmental remediation.

The densely crosslinked protein coats of bacterial spores are among the most durable static structures in biology. Wu et al. reconstitute the basement layer of a bacterial spore coat on membrane-coated beads, and generate covalently-modified spore-like particles with therapeutic potential.