SpoVID guides SafA to the spore coat in Bacillus subtilis

Ultrastructure of macromolecular assemblies contributing to bacterial spore resistance revealed by in situ cryo-electron tomography

Bacterial spores owe their incredible resistance capacities to molecular structures that protect the cell content from external aggressions. Among the determinants of resistance are the quaternary structure of the chromosome and an extracellular shell made of proteinaceous layers (the coat), the assembly of which remains poorly understood. Here, in situ cryo-electron tomography on lamellae generated by cryo-focused ion beam micromachining provides insights into the ultrastructural organization of Bacillus subtilis sporangia. The reconstructed tomograms reveal that early during sporulation, the chromosome in the forespore adopts a toroidal structure harboring 5.5-nm thick fibers. At the same stage, coat proteins at the surface of the forespore form a stack of amorphous or structured layers with distinct electron density, dimensions and organization. By analyzing mutant strains using cryo-electron tomography and transmission electron microscopy on resin sections, we distinguish seven nascent coat regions with different molecular properties, and propose a model for the contribution of coat morphogenetic proteins.

New insights into peroxide toxicology: sporulenes help Bacillus subtilis endospores from hydrogen peroxide

AIM

The purpose of the present study was to understand the possible events involved in the toxicity of hydrogen peroxide (H2O2) to wild and sporulene-deficient spores of Bacillus subtilis, as H2O2 was previously shown to have deleterious effects.

METHODS AND RESULTS

The investigation utilized two strains of B. subtilis, namely the wild-type PY79 (WT) and the sporulene-deficient TB10 (ΔsqhC mutant). Following treatment with 0.05% H2O2 (v/v), spore viability was assessed using a plate count assay, which revealed a significant decrease in cultivability of 80% for the ΔsqhC mutant spores. Possible reasons for the loss of spore viability were investigated with microscopic analysis, dipicholinic acid (DPA) quantification and propidium iodide (PI) staining. Microscopic examinations revealed the presence of withered and deflated morphologies in spores of ΔsqhC mutants treated with H2O2, indicating a compromised membrane permeability. This was further substantiated by the absence of DPA and a high frequency (50%-75%) of PI infiltration. The results of fatty acid methyl ester analysis and protein profiling indicated that the potentiation of H2O2-induced cellular responses was manifested in the form of altered spore composition in ΔsqhC B. subtilis. The slowed growth rates of the ΔsqhC mutant and the heightened sporulene biosynthesis pathways in the WT strain, both upon exposure to H2O2, suggested a protective function for sporulenes in vegetative cells.

CONCLUSIONS

Sporulenes serve as a protective layer for the inner membrane of spores, thus assuming a significant role in mitigating the adverse effects of H2O2 in WT B. subtilis. The toxic effects of H2O2 were even more pronounced in the spores of the ΔsqhC mutant, which lacks this protective barrier of sporulenes.

Bacterial developmental checkpoint that directly monitors cell surface morphogenesis

Bacillus subtilis spores are encased in two concentric shells: an outer proteinaceous "coat" and an inner peptidoglycan "cortex," separated by a membrane. Cortex assembly depends on coat assembly initiation, but how cells achieve this coordination across the membrane is unclear. Here, we report that the protein SpoVID monitors the polymerization state of the coat basement layer via an extension to a functional intracellular LysM domain that arrests sporulation when coat assembly is initiated improperly. Whereas extracellular LysM domains bind mature peptidoglycan, SpoVID LysM binds to the membrane-bound lipid II peptidoglycan precursor. We propose that improper coat assembly exposes the SpoVID LysM domain, which then sequesters lipid II and prevents cortex assembly. SpoVID defines a widespread group of firmicute proteins with a characteristic N-terminal domain and C-terminal peptidoglycan-binding domains that might combine coat and cortex assembly roles to mediate a developmental checkpoint linking the morphogenesis of two spatially separated supramolecular structures.

A dynamic, ring-forming MucB / RseB-like protein influences spore shape in Bacillus subtilis

How organisms develop into specific shapes is a central question in biology. The maintenance of bacterial shape is connected to the assembly and remodelling of the cell envelope. In endospore-forming bacteria, the pre-spore compartment (the forespore) undergoes morphological changes that result in a spore of defined shape, with a complex, multi-layered cell envelope. However, the mechanisms that govern spore shape remain poorly understood. Here, using a combination of fluorescence microscopy, quantitative image analysis, molecular genetics and transmission electron microscopy, we show that SsdC (formerly YdcC), a poorly-characterized new member of the MucB / RseB family of proteins that bind lipopolysaccharide in diderm bacteria, influences spore shape in the monoderm Bacillus subtilis. Sporulating cells lacking SsdC fail to adopt the typical oblong shape of wild-type forespores and are instead rounder. 2D and 3D-fluorescence microscopy suggest that SsdC forms a discontinuous, dynamic ring-like structure in the peripheral membrane of the mother cell, near the mother cell proximal pole of the forespore. A synthetic sporulation screen identified genetic relationships between ssdC and genes involved in the assembly of the spore coat. Phenotypic characterization of these mutants revealed that spore shape, and SsdC localization, depend on the coat basement layer proteins SpoVM and SpoIVA, the encasement protein SpoVID and the inner coat protein SafA. Importantly, we found that the ΔssdC mutant produces spores with an abnormal-looking cortex, and abolishing cortex synthesis in the mutant largely suppresses its shape defects. Thus, SsdC appears to play a role in the proper assembly of the spore cortex, through connections to the spore coat. Collectively, our data suggest functional diversification of the MucB / RseB protein domain between diderm and monoderm bacteria and identify SsdC as an important factor in spore shape development.

Cell shape is an important cellular attribute linked to cellular function and environmental adaptation. Bacterial endospores are one of the toughest cell types on Earth, with a defined shape and complex, highly-resistant, multi-layered cell envelope. Although decades of research have focused on defining the composition and assembly of the multi-layered spore envelope, little is known about how these layers contribute to spore shape. Here, we identify SsdC, a poorly-characterized new member of the MucB / RseB family of proteins that bind lipopolysaccharide in diderm bacteria. We show that SsdC is an important factor in spore shape development in the monoderm, model organism Bacillus subtilis. Our data suggest that SsdC influences the assembly of the spore cortex, through connections to the spore coat, by forming an intriguing, dynamic ring-like structure adjacent to the developing spore. Furthermore, our identification of SsdC suggests evolutionary diversification of the MucB /RseB protein domain between diderm and monoderm bacteria.

From Root to Tips: Sporulation Evolution and Specialization in Bacillus subtilis and the Intestinal Pathogen Clostridioides difficile

Bacteria of the Firmicutes phylum are able to enter a developmental pathway that culminates with the formation of highly resistant, dormant endospores. Endospores allow environmental persistence, dissemination and for pathogens, are also infection vehicles. In both the model Bacillus subtilis, an aerobic organism, and in the intestinal pathogen Clostridioides difficile, an obligate anaerobe, sporulation mobilizes hundreds of genes. Their expression is coordinated between the forespore and the mother cell, the two cells that participate in the process, and is kept in close register with the course of morphogenesis. The evolutionary mechanisms by which sporulation emerged and evolved in these two species, and more broadly across Firmicutes, remain largely unknown. Here, we trace the origin and evolution of sporulation using the genes known to be involved in the process in B. subtilis and C. difficile, and estimating their gain-loss dynamics in a comprehensive bacterial macroevolutionary framework. We show that sporulation evolution was driven by two major gene gain events, the first at the base of the Firmicutes and the second at the base of the B. subtilis group and within the Peptostreptococcaceae family, which includes C. difficile. We also show that early and late sporulation regulons have been coevolving and that sporulation genes entail greater innovation in B. subtilis with many Bacilli lineage-restricted genes. In contrast, C. difficile more often recruits new sporulation genes by horizontal gene transfer, which reflects both its highly mobile genome, the complexity of the gut microbiota, and an adjustment of sporulation to the gut ecosystem.

SpoIVA-SipL Complex Formation Is Essential for Clostridioides difficile Spore Assembly

Spores are the major infectious particle of the Gram-positive nosocomial pathogen Clostridioides difficile (formerly Clostridium difficile), but the molecular details of how this organism forms these metabolically dormant cells remain poorly characterized. The composition of the spore coat in C. difficile differs markedly from that defined in the well-studied organism Bacillus subtilis, with only 25% of the ∼70 spore coat proteins being conserved between the two organisms and with only 2 of 9 coat assembly (morphogenetic) proteins defined in B. subtilis having homologs in C. difficile We previously identified SipL as a clostridium-specific coat protein essential for functional spore formation. Heterologous expression analyses in Escherichia coli revealed that SipL directly interacts with C. difficile SpoIVA, a coat-morphogenetic protein conserved in all spore-forming organisms, through SipL's C-terminal LysM domain. In this study, we show that SpoIVA-SipL binding is essential for C. difficile spore formation and identify specific residues within the LysM domain that stabilize this interaction. Fluorescence microscopy analyses indicate that binding of SipL's LysM domain to SpoIVA is required for SipL to localize to the forespore while SpoIVA requires SipL to promote encasement of SpoIVA around the forespore. Since we also show that clostridial LysM domains are functionally interchangeable at least in C. difficile, the basic mechanism for SipL-dependent assembly of clostridial spore coats may be conserved.IMPORTANCE The metabolically dormant spore form of the major nosocomial pathogen Clostridioides difficile is its major infectious particle. However, the mechanisms controlling the formation of this resistant cell type are not well understood, particularly with respect to its outermost layer, the spore coat. We previously identified two spore-morphogenetic proteins in C. difficile: SpoIVA, which is conserved in all spore-forming organisms, and SipL, which is conserved only in the clostridia. Both SpoIVA and SipL are essential for heat-resistant spore formation and directly interact through SipL's C-terminal LysM domain. In this study, we demonstrate that the LysM domain is critical for SipL and SpoIVA function, likely by helping recruit SipL to the forespore during spore morphogenesis. We further identified residues within the LysM domain that are important for binding SpoIVA and, thus, functional spore formation. These findings provide important insight into the molecular mechanisms controlling the assembly of infectious C. difficile spores.

Temporal and spatial regulation of protein cross-linking by the pre-assembled substrates of a Bacillus subtilis spore coat transglutaminase

In many cases protein assemblies are stabilized by covalent bonds, one example of which is the formation of intra- or intermolecular ε-(γ-glutamyl)lysil cross-links catalyzed by transglutaminases (TGases). Because of the potential for unwanted cross-linking reactions, the activities of many TGases have been shown to be tightly controlled. Bacterial endospores are highly resilient cells in part because they are surrounded by a complex protein coat. Proteins in the coat that surrounds Bacillus subtilis endospores are crosslinked by a TGase (Tgl). Unlike other TGases, however, Tgl is produced in an active form, and efficiently catalyzes amine incorporation and protein cross-linking in vitro with no known additional requirements. The absence of regulatory factors raises questions as to how the activity of Tgl is controlled during spore coat assembly. Here, we show that substrates assembled onto the spore coat prior to Tgl production govern the localization of Tgl to the surface of the developing spore. We also show that Tgl residues important for substrate recognition are crucial for its localization. We identified the glutamyl (Q) and lysil (K) substrate docking sites and we show that residues on the Q side of Tgl are more important for the assembly of Tgl than those on the K side. Thus, the first step in the reaction cycle, the interaction with Q-substrates and formation of an acyl-enzyme intermediate, is also the determinant step in the localization of Tgl. Consistent with the idea that Tg exerts a “spotwelding” activity, cross-linking pre-formed assemblies, we show that C30 is an oblong hexamer in solution that is cross-linked in vitro into high molecular weight forms. Moreover, during the reaction, Tgl becomes part of the cross-linked products. We suggest that the dependency of Tgl on its substrates is used to accurately control the time, location and extent of the enzyme´s activity, directed at the covalent fortification of pre-assembled complexes at the surface of the developing spore.

The orderly recruitment of proteins during the assembly of complex macromolecular structures poses challenges throughout cell biology. During endospore development in the bacterium Bacillus subtilis at least 80 proteins synthesized in the mother cell are assembled around the developing spore to form a protective coat. Regulation of coat gene expression has been described in detail but it is unknown how the information encoded by the structures of the proteins guide their assembly. We have examined the assembly of a transglutaminase, Tgl, which introduces ε-(γ-glutamyl)lysil cross-links in coat protein substrates. We describe with molecular detail a substrate-driven assembly model that directs the enzyme to the locations of its substrates where, as we suggest, it exerts a “spotwelding” activity to fortify pre-assembled complexes. The catalytic cysteine, located in a tunnel that spans the Tgl structure, first forms an acyl enzyme intermediate with a glutamine (Q) donor substrate. Then, it engages a lysine (K) donor substrate to form the cross-linked product. We have identified the Q and K acceptor ends of the Tgl tunnel, and we show that substitutions in substrate recognition residues at the Q side impair assembly more strongly than at the K side. Thus, assembly of Tgl parallels its catalytic cycle, directing the enzyme to the pre-formed complexes that are to be cross-linked.

A LysM Domain Intervenes in Sequential Protein-Protein and Protein-Peptidoglycan Interactions Important for Spore Coat Assembly in Bacillus subtilis

At a late stage in spore development in Bacillus subtilis, the mother cell directs synthesis of a layer of peptidoglycan known as the cortex between the two forespore membranes, as well as the assembly of a protective protein coat at the surface of the forespore outer membrane. SafA, the key determinant of inner coat assembly, is first recruited to the surface of the developing spore and then encases the spore under the control of the morphogenetic protein SpoVID. SafA has a LysM peptidoglycan-binding domain, SafA_LysM, and localizes to the cortex-coat interface in mature spores. SafA_LysM is followed by a region, A, required for an interaction with SpoVID and encasement. We now show that residues D10 and N30 in SafA_LysM, while involved in the interaction with peptidoglycan, are also required for the interaction with SpoVID and encasement. We further show that single alanine substitutions on residues S11, L12, and I39 of SafA_LysM that strongly impair binding to purified cortex peptidoglycan affect a later stage in the localization of SafA that is also dependent on the activity of SpoVE, a transglycosylase required for cortex formation. The assembly of SafA thus involves sequential protein-protein and protein-peptidoglycan interactions, mediated by the LysM domain, which are required first for encasement then for the final localization of the protein in mature spores.IMPORTANCE Bacillus subtilis spores are encased in a multiprotein coat that surrounds an underlying peptidoglycan layer, the cortex. How the connection between the two layers is enforced is not well established. Here, we elucidate the role of the peptidoglycan-binding LysM domain, present in two proteins, SafA and SpoVID, that govern the localization of additional proteins to the coat. We found that SafA_LysM is a protein-protein interaction module during the early stages of coat assembly and a cortex-binding module at late stages in morphogenesis, with the cortex-binding function promoting a tight connection between the cortex and the coat. In contrast, SpoVID_LysM functions only as a protein-protein interaction domain that targets SpoVID to the spore surface at the onset of coat assembly.

Coordinated Assembly of the Bacillus anthracis Coat and Exosporium during Bacterial Spore Outer Layer Formation

This work dramatically improves our understanding of the assembly of the outermost layer of the B. anthracis spore, the exosporium, a layer that encases spores from many bacterial species and likely plays important roles in the spore’s interactions with the environment, including host tissues. Nonetheless, the mechanisms directing exosporium assembly into a shell surrounding the spore are still very poorly understood. In this study, we clarify these mechanisms by the identification of a novel protein interaction network that directs assembly to initiate at a specific subcellular location in the developing cell. Our results further suggest that the presence or absence of an exosporium has a major impact on the assembly of other more interior spore layers, thereby potentially explaining long-noted differences in spore assembly between B. anthracis and the model organism B. subtilis.

Bacterial spores produced by the Bacillales are composed of concentric shells, each of which contributes to spore function. Spores from all species possess a cortex and coat, but spores from many species possess additional outer layers. The outermost layer of Bacillus anthracis spores, the exosporium, is separated from the coat by a gap known as the interspace. Exosporium and interspace assembly remains largely mysterious. As a result, we have a poor understanding of the overarching mechanisms driving the assembly of one of the most ubiquitous cell types in nature. To elucidate the mechanisms directing exosporium assembly, we generated strains bearing mutations in candidate exosporium-controlling genes and analyzed the effect on exosporium formation. Biochemical and cell biological analyses argue that CotE directs the assembly of CotO into the spore and that CotO might be located at or close to the interior side of the cap. Taken together with data showing that CotE and CotO interact directly in vitro, we propose a model in which CotE and CotO are important components of a protein interaction network that connects the exosporium to the forespore during cap formation and exosporium elongation. Our data also suggest that the cap interferes with coat assembly at one pole of the spore, altering the pattern of coat deposition compared to the model organism Bacillus subtilis. We propose that the difference in coat assembly patterns between these two species is due to an inherent flexibility in coat assembly, which may facilitate the evolution of spore outer layer complexity.

SpoVID functions as a non-competitive hub that connects the modules for assembly of the inner and outer spore coat layers in Bacillus subtilis

During sporulation in Bacillus subtilis, a group of mother cell‐specific proteins guides the assembly of the coat, a multiprotein structure that protects the spore and influences many of its environmental interactions. SafA and CotE behave as party hubs, governing assembly of the inner and outer coat layers. Targeting of coat proteins to the developing spore is followed by encasement. Encasement by SafA and CotE requires E, a region of 11 amino acids in the encasement protein SpoVID, with which CotE interacts directly. Here, we identified two single alanine substitutions in E that prevent binding of SafA, but not of CotE, to SpoVID, and block encasement. The substitutions result in the accumulation of SafA, CotE and their dependent proteins at the mother cell proximal spore pole, phenocopying a spoVID null mutant and suggesting that mislocalized SafA acts as an attractor for the rest of the coat. The requirement for E in SafA binding is bypassed by a peptide with the sequence of E provided in trans. We suggest that E allows binding of SafA to a second region in SpoVID, enabling CotE to interact with E and SpoVID to function as a non‐competitive hub during spore encasement.

New perspectives on Mega plasmid sequence (poh1) in Bacillus thuringiensis ATCC 10792 harbouring antimicrobial, insecticidal and antibiotic resistance genes

Bacillus thuringiensis promotes the growth of numerous economically important crops. The present study presents the complete genome sequence for a mega plasmid present in the type strain of B. thuringiensis ATCC 10792, a typical spore-forming Gram-positive bacterium with insecticidal activity, and investigates its genetic characteristics. The genome was sequenced and assembled de novo using Pac-Bio sequencers and the Hierarchical Genome Assembly Process, respectively. Further genome annotation was performed, and a total of 489 proteins and a novel mega-plasmid (poh1) with 584,623 bps were identified. The organization of poh1 revealed the genes involved in the insecticidal toxin pathway. The genes responsible for antimicrobial, insecticidal and antibiotic activities were well conserved in poh1, indicating an intimate association with plant hosts. The poh1 plasmid contains the gene encoding a novel crystal protein kinase responsible for production of zeta toxin, which poisons insects and other Gram-negative bacteria through the global inhibition of peptidoglycan synthesis. Lantibiotics are a group of bacteriocins that include the biologically active antimicrobial peptide Paenibacillin. Further, poh1 also contains the genes that encode the gramicidin S prototypical antibiotic peptide and tetracycline resistance protein. In conclusion, the strain-specific genes of B. thuringiensis strain ATCC 10792 were identified through complete genome sequencing and bioinformatics data based on major pathogenic factors that contribute to further studies of the pathogenic mechanism and phenotype analyses.

Autoregulation of SafA Assembly through Recruitment of a Protein Cross-Linking Enzyme

The coat of Bacillus subtilis spores is a multiprotein protective structure that also arbitrates many of the environmental interactions of the spore. The coat assembles around the cortex peptidoglycan layer and is differentiated into an inner and an outer layer and a crust. SafA governs assembly of the inner coat, whereas CotE drives outer coat assembly. SafA localizes to the cortex-coat interface. Both SafA and its short form C30 are substrates for Tgl, a coat-associated transglutaminase that cross-links proteins through ε-(γ-glutamyl)lysyl isopeptide bonds. We show that SafA and C30 are distributed between the coat and cortex layers. The deletion of tgl increases the extractability of SafA, mainly from the cortex. Tgl itself is mostly located in the inner coat and cortex. The localization of Tgl-cyan fluorescent protein (Tgl-CFP) is strongly, but not exclusively, dependent on safA However, the association of Tgl with the cortex requires safA Together, our results suggest an assembly pathway in which Tgl is first recruited to the forming spore in a manner that is only partially dependent on SafA and then is drafted to the cortex by SafA. Tgl, in turn, promotes the conversion of coat- and cortex-associated SafA into forms that resist extraction, possibly by catalyzing the cross-linking of SafA to other coat proteins, to the cortex, and/or to cortex-associated proteins. Therefore, the final assembly state of SafA relies on an autoregulatory pathway that requires the subcellular localization of a protein cross-linking enzyme. Tgl most likely exerts a "spotwelding" activity, cross-linking preformed complexes in the cortex and inner coat layers of spores.IMPORTANCE In this work, we show how two proteins work together to determine their subcellular location within the coat of bacterial endospores. Bacillus subtilis endospores are surrounded by a multilayer protein coat composed of over 80 proteins, which surrounds an underlying peptidoglycan layer (the spore cortex) protecting it from lytic enzymes. How specific coat proteins are targeted to specific layers of the coat is not well understood. We found that the protein SafA recruits a protein-cross-linking enzyme (a transglutaminase) to the cortex and inner layers of the coat, where both are cemented, by cross-linking, into macromolecular complexes.

The Conserved Spore Coat Protein SpoVM Is Largely Dispensable in Clostridium difficile Spore Formation

The spore-forming obligate anaerobe Clostridium difficile is the leading cause of antibiotic-associated diarrheal disease in the United States. When C. difficile spores are ingested by susceptible individuals, they germinate within the gut and transform into vegetative, toxin-secreting cells. During infection, C. difficile must also induce spore formation to survive exit from the host. Since spore formation is essential for transmission, understanding the basic mechanisms underlying sporulation in C. difficile could inform the development of therapeutic strategies targeting spores. In this study, we determine the requirement of the C. difficile homolog of SpoVM, a protein that is essential for spore formation in Bacillus subtilis due to its regulation of coat and cortex formation. We observed that SpoVM plays a minor role in C. difficile spore formation, in contrast with B. subtilis, indicating that this protein would not be a good target for inhibiting spore formation.

The spore-forming bacterial pathogen Clostridium difficile is a leading cause of health care-associated infections in the United States. In order for this obligate anaerobe to transmit infection, it must form metabolically dormant spores prior to exiting the host. A key step during this process is the assembly of a protective, multilayered proteinaceous coat around the spore. Coat assembly depends on coat morphogenetic proteins recruiting distinct subsets of coat proteins to the developing spore. While 10 coat morphogenetic proteins have been identified in Bacillus subtilis, only two of these morphogenetic proteins have homologs in the Clostridia: SpoIVA and SpoVM. C. difficile SpoIVA is critical for proper coat assembly and functional spore formation, but the requirement for SpoVM during this process was unknown. Here, we show that SpoVM is largely dispensable for C. difficile spore formation, in contrast with B. subtilis. Loss of C. difficile SpoVM resulted in modest decreases (~3-fold) in heat- and chloroform-resistant spore formation, while morphological defects such as coat detachment from the forespore and abnormal cortex thickness were observed in ~30% of spoVM mutant cells. Biochemical analyses revealed that C. difficile SpoIVA and SpoVM directly interact, similarly to their B. subtilis counterparts. However, in contrast with B. subtilis, C. difficile SpoVM was not essential for SpoIVA to encase the forespore. Since C. difficile coat morphogenesis requires SpoIVA-interacting protein L (SipL), which is conserved exclusively in the Clostridia, but not the more broadly conserved SpoVM, our results reveal another key difference between C. difficile and B. subtilis spore assembly pathways.

IMPORTANCE The spore-forming obligate anaerobe Clostridium difficile is the leading cause of antibiotic-associated diarrheal disease in the United States. When C. difficile spores are ingested by susceptible individuals, they germinate within the gut and transform into vegetative, toxin-secreting cells. During infection, C. difficile must also induce spore formation to survive exit from the host. Since spore formation is essential for transmission, understanding the basic mechanisms underlying sporulation in C. difficile could inform the development of therapeutic strategies targeting spores. In this study, we determine the requirement of the C. difficile homolog of SpoVM, a protein that is essential for spore formation in Bacillus subtilis due to its regulation of coat and cortex formation. We observed that SpoVM plays a minor role in C. difficile spore formation, in contrast with B. subtilis, indicating that this protein would not be a good target for inhibiting spore formation.

The Spore Coat

Spores of Clostridiales and Bacillales are encased in a complex series of concentric shells that provide protection, facilitate germination, and mediate interactions with the environment. Analysis of diverse spore-forming species by thin-section transmission electron microscopy reveals that the number and morphology of these encasing shells vary greatly. In some species, they appear to be composed of a small number of discrete layers. In other species, they can comprise multiple, morphologically complex layers. In addition, spore surfaces can possess elaborate appendages. For all their variability, there is a consistent architecture to the layers encasing the spore. A hallmark of all Clostridiales and Bacillales spores is the cortex, a layer made of peptidoglycan. In close association with the cortex, all species examined possess, at a minimum, a series of proteinaceous layers, called the coat. In some species, including Bacillus subtilis, only the coat is present. In other species, including Bacillus anthracis, an additional layer, called the exosporium, surrounds the coat. Our goals here are to review the present understanding of the structure, composition, assembly, and functions of the coat, primarily in the model organism B. subtilis, but also in the small but growing number of other spore-forming species where new data are showing that there is much to be learned beyond the relatively well-developed basis of knowledge in B. subtilis. To help summarize this large field and define future directions for research, we will focus on key findings in recent years.

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.

Unraveling the predator-prey relationship of Cupriavidus necator and Bacillus subtilis

Cupriavidus necator is a non-obligate bacterial predator of Gram-negative and Gram-positive bacteria. In this study, we set out to determine the conditions, which are necessary to observe predatory behavior of C. necator. Using Bacillus subtilis as a prey organism, we confirmed that the predatory performance of C. necator is correlated with the available copper level, and that the killing is mediated, at least in part, by secreted extracellular factors. The predatory activity depends on the nutrition status of C. necator, but does not require a quorum of predator cells. This suggests that C. necator is no group predator. Further analyses revealed that sporulation enables B. subtilis to avoid predation by C. necator. In contrast to the interaction with predatory myxobacteria, however, an intact spore coat is not required for resistance. Instead resistance is possibly mediated by quiescence.

Diverse supramolecular structures formed by self-assembling proteins of the Bacillus subtilis spore coat

Bacterial spores (endospores), such as those of the pathogens Clostridium difficile and Bacillus anthracis, are uniquely stable cell forms, highly resistant to harsh environmental insults. Bacillus subtilis is the best studied spore‐former and we have used it to address the question of how the spore coat is assembled from multiple components to form a robust, protective superstructure. B. subtilis coat proteins (CotY, CotE, CotV and CotW) expressed in Escherichia coli can arrange intracellularly into highly stable macro‐structures through processes of self‐assembly. Using electron microscopy, we demonstrate the capacity of these proteins to generate ordered one‐dimensional fibres, two‐dimensional sheets and three‐dimensional stacks. In one case (CotY), the high degree of order favours strong, cooperative intracellular disulfide cross‐linking. Assemblies of this kind could form exquisitely adapted building blocks for higher‐order assembly across all spore‐formers. These physically robust arrayed units could also have novel applications in nano‐biotechnology processes.

Spore Surface Display

A variety of bioactive peptides and proteins have been successfully displayed on the surface of recombinant spores of Bacillus subtilis and other sporeformers. In most cases, spore display has been achieved by stably anchoring the foreign molecules to endogenous surface proteins or parts of them. Recombinant spores have been proposed for a large number of potential applications ranging from oral vaccine vehicles to bioremediation tools, and including biocatalysts, probiotics for animal or human use, as well as the generation and screening of mutagenesis libraries. In addition, a nonrecombinant approach has been recently developed to adsorb antigens and enzymes on the spore surface. This nonrecombinant approach appears particularly well suited for applications involving the delivery of active molecules to human or animal mucosal surfaces. Both the recombinant and nonrecombinant spore display systems have a number of advantages over cell- or phage-based systems. The stability, safety of spores of several bacterial species, and amenability to laboratory manipulations, together with the lack of some constraints limiting the use of other systems, make the spore a highly efficient platform to display heterologous proteins.

Clostridium difficile spore biology: sporulation, germination, and spore structural proteins

Clostridium difficile is a Gram-positive, spore-forming obligate anaerobe and a major nosocomial pathogen of worldwide concern. Owing to its strict anaerobic requirements, the infectious and transmissible morphotype is the dormant spore. In susceptible patients, C. difficile spores germinate in the colon to form the vegetative cells that initiate Clostridium difficile infections (CDI). During CDI, C. difficile induces a sporulation pathway that produces more spores; these spores are responsible for the persistence of C. difficile in patients and horizontal transmission between hospitalized patients. Although important to the C. difficile lifecycle, the C. difficile spore proteome is poorly conserved when compared to members of the Bacillus genus. Further, recent studies have revealed significant differences between C. difficile and Bacillus subtilis at the level of sporulation, germination, and spore coat and exosporium morphogenesis. In this review, the regulation of the sporulation and germination pathways and the morphogenesis of the spore coat and exosporium will be discussed.

SpoIVA and SipL are Clostridium difficile spore morphogenetic proteins

Clostridium difficile is a major nosocomial pathogen whose infections are difficult to treat because of their frequent recurrence. The spores of C. difficile are responsible for these clinical features, as they resist common disinfectants and antibiotic treatment. Although spores are the major transmissive form of C. difficile, little is known about their composition or morphogenesis. Spore morphogenesis has been well characterized for Bacillus sp., but Bacillus sp. spore coat proteins are poorly conserved in Clostridium sp. Of the known spore morphogenetic proteins in Bacillus subtilis, SpoIVA is one of the mostly highly conserved in the Bacilli and the Clostridia. Using genetic analyses, we demonstrate that SpoIVA is required for proper spore morphogenesis in C. difficile. In particular, a spoIVA mutant exhibits defects in spore coat localization but not cortex formation. Our study also identifies SipL, a previously uncharacterized protein found in proteomic studies of C. difficile spores, as another critical spore morphogenetic protein, since a sipL mutant phenocopies a spoIVA mutant. Biochemical analyses and mutational analyses indicate that SpoIVA and SipL directly interact. This interaction depends on the Walker A ATP binding motif of SpoIVA and the LysM domain of SipL. Collectively, these results provide the first insights into spore morphogenesis in C. difficile.

The Bacillus subtilis endospore: assembly and functions of the multilayered coat

Sporulation in Bacillus subtilis involves an asymmetric cell division followed by differentiation into two cell types, the endospore and the mother cell. The endospore coat is a multilayered shell that protects the bacterial genome during stress conditions and is composed of dozens of proteins. Recently, fluorescence microscopy coupled with high-resolution image analysis has been applied to the dynamic process of coat assembly and has shown that the coat is organized into at least four distinct layers. In this Review, we provide a brief summary of B. subtilis sporulation, describe the function of the spore surface layers and discuss the recent progress that has improved our understanding of the structure of the endospore coat and the mechanisms of coat assembly.

Physical interaction between coat morphogenetic proteins SpoVID and CotE is necessary for spore encasement in Bacillus subtilis

Endospore formation by Bacillus subtilis is a complex and dynamic process. One of the major challenges of sporulation is the assembly of a protective, multilayered, proteinaceous spore coat, composed of at least 70 different proteins. Spore coat formation can be divided into two distinct stages. The first is the recruitment of proteins to the spore surface, dependent on the morphogenetic protein SpoIVA. The second step, known as encasement, involves the migration of the coat proteins around the circumference of the spore in successive waves, a process dependent on the morphogenetic protein SpoVID and the transcriptional regulation of individual coat genes. We provide genetic and biochemical evidence supporting the hypothesis that SpoVID promotes encasement of the spore by establishing direct protein-protein interactions with other coat morphogenetic proteins. It was previously demonstrated that SpoVID directly interacts with SpoIVA and the inner coat morphogenetic protein, SafA. Here, we show by yeast two-hybrid and pulldown assays that SpoVID also interacts directly with the outer coat morphogenetic protein, CotE. Furthermore, by mutational analysis, we identified a specific residue in the N-terminal domain of SpoVID that is essential for the interaction with CotE but dispensable for the interaction with SafA. We propose an updated model of coat assembly and spore encasement that incorporates several physical interactions between the principal coat morphogenetic proteins.

Display of recombinant proteins on Bacillus subtilis spores, using a coat-associated enzyme as the carrier

The display of proteins such as feed enzymes at the surface of bacterial spore systems has a great potential use for animal feed. Feed enzymes increase the digestibility of nutrients, leading to greater efficiency in the manufacturing of animal products and minimizing the environmental impact of increased animal production. To deliver their full potential in the gut, feed enzymes must survive the harsh conditions of the feed preparation and the gastrointestinal tract. The well-documented resistance of spores to harsh environments, together with the ability to use proteins that compose the spore as carriers for the display of passenger proteins, suggests that spores could be used as innovative tools to improve the formulation of bioactive molecules. Although some successful examples have been reported, in which abundant structural proteins of the Bacillus subtilis spore outer-coat layer were used as carriers for the display of recombinant proteins, only one convincing example resulted in the display of functional enzymes. In addition, no examples are available about the use of an inner-coat protein for the display of an active passenger enzyme. In our study, we show that the inner-coat oxalate decarboxylase (OxdD) can expose an endogenous phytase, a commonly used feed enzyme for monogastric animals, in an active form at the spore surface. Importantly, despite the higher abundance of CotG outer-coat protein, an OxdD-Phy fusion was more represented at the spore surface. The potential of OxdD as a carrier protein is further documented through the spore display of a bioactive heterologous passenger, the tetrameric beta-glucuronidase enzyme from Escherichia coli.

A distance-weighted interaction map reveals a previously uncharacterized layer of the Bacillus subtilis spore coat

Bacillus subtilis spores are encased in a protein assembly called the spore coat that is made up of at least 70 different proteins. Conventional electron microscopy shows the coat to be organized into two distinct layers. Because the coat is about as wide as the theoretical limit of light microscopy, quantitatively measuring the localization of individual coat proteins within the coat is challenging. We used fusions of coat proteins to green fluorescent protein to map genetic dependencies for coat assembly and to define three independent subnetworks of coat proteins. To complement the genetic data, we measured coat protein localization at subpixel resolution and integrated these two data sets to produce a distance-weighted genetic interaction map. Using these data, we predict that the coat comprises at least four spatially distinct layers, including a previously uncharacterized glycoprotein outermost layer that we name the spore crust. We found that crust assembly depends on proteins we predicted to localize to the crust. The crust may be conserved in all Bacillus spores and may play critical functions in the environment.

Macromolecules that prefer their membranes curvy

Understanding the mechanisms that underlie the organization of bacterial cells has become a significant challenge in the field of bacterial cytology. Of specific interest are early macromolecular sorting events that establish cellular non-uniformity and provide chemical landmarks for later localization events. In this review, we will examine specific examples of lipids and proteins that appear to exploit differences in membrane curvature to drive their localization to particular regions of a bacterial cell. We will also discuss the physical limits of curvature-mediated localization within bacteria, and the use of modelling to infer biophysical properties of curvature-sensing macromolecules.

The coat morphogenetic protein SpoVID is necessary for spore encasement in Bacillus subtilis

Endospores formed by Bacillus subtilis are encased in a tough protein shell known as the coat, which consists of at least 70 different proteins. We investigated the process of spore coat morphogenesis using a library of 40 coat proteins fused to green fluorescent protein and demonstrate that two successive steps can be distinguished in coat assembly. The first step, initial localization of proteins to the spore surface, is dependent on the coat morphogenetic proteins SpoIVA and SpoVM. The second step, spore encasement, requires a third protein, SpoVID. We show that in spoVID mutant cells, most coat proteins assembled into a cap at one side of the developing spore but failed to migrate around and encase it. We also found that SpoIVA directly interacts with SpoVID. A domain analysis revealed that the N-terminus of SpoVID is required for encasement and is a structural homologue of a virion protein, whereas the C-terminus is necessary for the interaction with SpoIVA. Thus, SpoVM, SpoIVA and SpoVID are recruited to the spore surface in a concerted manner and form a tripartite machine that drives coat formation and spore encasement.

Characterization of a Bacillus anthracis spore coat-surface protein that influences coat-surface morphology

Bacterial spores are encased in a multilayered proteinaceous shell, called the coat. In many Bacillus spp., the coat protects against environmental assault and facilitates germination. In Bacillus anthracis, the spore is the etiological agent of anthrax, and the functions of the coat likely contribute to virulence. Here, we characterize a B. anthracis spore protein, called Cotbeta, which is encoded only in the genomes of the Bacillus cereus group. We found that Cotbeta is synthesized specifically during sporulation and is assembled onto the spore coat surface. Our analysis of a cotbeta null mutant in the Sterne strain reveals that Cotbeta has a role in determining coat-surface morphology but does not detectably affect germination. In the fully virulent Ames strain, a cotbeta null mutation has no effect on virulence in a murine model of B. anthracis infection.

Role of spore coat proteins in the resistance of Bacillus subtilis spores to Caenorhabditis elegans predation

Bacterial spores are resistant to a wide range of chemical and physical insults that are normally lethal for the vegetative form of the bacterium. While the integrity of the protein coat of the spore is crucial for spore survival in vitro, far less is known about how the coat provides protection in vivo against predation by ecologically relevant hosts. In particular, assays had characterized the in vitro resistance of spores to peptidoglycan-hydrolyzing enzymes like lysozyme that are also important effectors of innate immunity in a wide variety of hosts. Here, we use the bacteriovorous nematode Caenorhabditis elegans, a likely predator of Bacillus spores in the wild, to characterize the role of the spore coat in an ecologically relevant spore-host interaction. We found that ingested wild-type Bacillus subtilis spores were resistant to worm digestion, whereas vegetative forms of the bacterium were efficiently digested by the nematode. Using B. subtilis strains carrying mutations in spore coat genes, we observed a correlation between the degree of alteration of the spore coat assembly and the susceptibility to the worm degradation. Surprisingly, we found that the spores that were resistant to lysozyme in vitro can be sensitive to C. elegans digestion depending on the extent of the spore coat structure modifications.

Structure, assembly, and function of the spore surface layers

Endospores formed by Bacillus, Clostridia, and related genera are encased in a protein shell called the coat. In many species, including B. subtilis, the coat is the outermost spore structure, and in other species, such as the pathogenic organisms B. anthracis and B. cereus, the spore is encased in an additional layer called the exosporium. Both the coat and the exosporium have roles in protection of the spore and in its environmental interactions. Assembly of both structures is a function of the mother cell, one of two cellular compartments of the developing sporangium. Studies in B. subtilis have revealed that the timing of coat protein production, the guiding role of a small group of morphogenetic proteins, and several types of posttranslational modifications are essential for the fidelity of the assembly process. Assembly of the exosporium requires a set of novel proteins as well as homologues of proteins found in the outermost layers of the coat and of some of the coat morphogenetic factors, suggesting that the exosporium is a more specialized structure of a multifunctional coat. These and other insights into the molecular details of spore surface morphogenesis provide avenues for exploitation of the spore surface layers in applications for biotechnology and medicine.

CotC-CotU heterodimerization during assembly of the Bacillus subtilis spore coat

We report evidence that CotC and CotU, two previously identified components of the Bacillus subtilis spore coat, are produced concurrently in the mother cell chamber of the sporulating cell under the control of sigmaK and GerE and immediately assembled around the forming spore. In the coat, the two proteins interact to form a coat component of 23 kDa. The CotU-CotC interaction was not detected in two heterologous hosts, suggesting that it occurs only in B. subtilis. Monomeric forms of both CotU and CotC failed to be assembled at the surface of the developing spore and accumulated in the mother cell compartment of cells mutant for cotE. In contrast, neither CotU nor CotC accumulated in the mother cell compartment of cells mutant for cotH. These results suggest that CotH is required to protect both CotU and CotC in the mother cell compartment of the sporangium and that CotE is needed to allow their assembly and subsequent interaction at the spore surface.

Analysis of interactions between nutrient germinant receptors and SpoVA proteins ofBacillus subtilisspores

Yeast two-hybrid and Far Western analyses were used to detect interactions between Bacillus subtilis spores' nutrient germinant receptor proteins and proteins encoded by the spoVA operon, all of which are involved in spore germination and located in the spores' inner membrane. These analyses indicated that two subunits of the GerA nutrient germinant receptor interact, consistent with previous genetic data, and that some GerA proteins interact with SpoVAD and some with SpoVAE. SpoVA proteins appear to be involved in the release of the spore's dipicolinic acid during spore germination, an event triggered by the binding of nutrient germinants to their receptors. Consequently, these new findings suggest that nutrient germinant receptors physically contact SpoVA proteins, and presumably this is a route for signal transduction during spore germination.

Non-uniform assembly of the Bacillus anthracis exosporium and a bottle cap model for spore germination and outgrowth

Spores of Bacillus anthracis are enclosed by an exosporium composed of a basal layer and an external hair-like nap. The nap is formed by a collagen-like glycoprotein called BclA, while the basal layer contains many different proteins, one of which is a spore-specific alanine racemase (Alr). In this study, we employed fluorescence microscopy and a fluorescently labelled anti-Alr monoclonal antibody (mAb) to examine the distribution of Alr within the exosporium. Binding of the mAb occurred over approximately three-quarters of the exosporium but not in a cap-like region at one end of the spore, indicating the absence or inaccessibility of Alr in this region. We also determined that the cap-like region, or cap, corresponds to the first part of the exosporium assembled within the mother cell during sporulation and the only part of the exosporium assembled in a DeltaexsY mutant strain of B. anthracis. Our results provide the first direct evidence that exosporium assembly is a non-uniform process and suggest that exosporium formation is discontinuous. Finally, we demonstrated that during spore germination and outgrowth, the outgrowing cell always escapes from its exosporium shell by popping through the cap, suggesting that the cap is designed to facilitate the emergence of the outgrowing cell.

A novel lipolytic enzyme, YcsK (LipC), located in the spore coat of Bacillus subtilis, is involved in spore germination

The predicted amino acid sequence of Bacillus subtilis ycsK exhibits similarity to the GDSL family of lipolytic enzymes. Northern blot analysis showed that ycsK mRNA was first detected from 4 h after the onset of sporulation and that transcription of ycsK was dependent on SigK and GerE. The fluorescence of the YcsK-green fluorescent protein fusion protein produced in sporulating cells was detectable in the mother cell but not in the forespore compartment under fluorescence microscopy, and the fusion protein was localized around the developing spores dependent on CotE, SafA, and SpoVID. Inactivation of the ycsK gene by insertion of an erythromycin resistance gene did not affect vegetative growth or spore resistance to heat, lysozyme, or chloroform. The germination of ycsK spores in a mixture of L-asparagine, D-glucose, D-fructose, and potassium chloride and LB medium was also the same as that of wild-type spores, but the mutant spores were defective in L-alanine-stimulated germination. In addition, zymogram analysis demonstrated that the YcsK protein heterologously expressed in Escherichia coli showed lipolytic activity. We therefore propose that ycsK should be renamed lipC. This is the first study of a bacterial spore germination-related lipase.

Interaction between coat morphogenetic proteins SafA and SpoVID

Morphogenetic proteins such as SpoVID and SafA govern assembly of the Bacillus subtilis endospore coat by guiding the various protein structural components to the surface of the developing spore. Previously, a screen for peptides able to interact with SpoVID led to the identification of a PYYH motif present in the C-terminal half of the SafA protein and to the subsequent demonstration that SpoVID and SafA directly interact. spoVID and safA spores show deficiencies in coat assembly and are lysozyme susceptible. Both proteins, orthologs of which are found in all Bacillus species, have LysM domains for peptidoglycan binding and localize to the cortex-coat interface. Here, we show that the interaction between SafA and SpoVID involves the PYYH motif (region B) but also a 13-amino-acid region (region A) just downstream of the N-terminal LysM domain of SafA. We show that deletion of region B does not block the interaction of SafA with SpoVID, nor does it bring about spore susceptibility to lysozyme. Nevertheless, it appears to reduce the interaction and affects the complex. In contrast, lesions in region A impaired the interaction of SafA with SpoVID in vitro and, while not affecting the accumulation of SafA in vivo, interfered with the localization of SafA around the developing spore, causing aberrant assembly of the coat and lysozyme sensitivity. A peptide corresponding to region A interacts with SpoVID, suggesting that residues within this region directly contact SpoVID. Since region A is highly conserved among SafA orthologs, this motif may be an important determinant of coat assembly in the group of Bacillus spore formers.

Characterization of the Bacillus subtilis spore morphogenetic coat protein CotO

Bacillus spores are protected by a structurally and biochemically complex protein shell composed of over 50 polypeptide species, called the coat. Coat assembly in Bacillus subtilis serves as a relatively tractable model for the study of the formation of more complex macromolecular structures and organelles. It is also a critical model for the discovery of strategies to decontaminate B. anthracis spores. In B. subtilis, a subset of coat proteins is known to have important roles in assembly. Here we show that the recently identified B. subtilis coat protein CotO (YjbX) has an especially important morphogenetic role. We used electron and atomic force microscopy to show that CotO controls assembly of the coat layers and coat surface topography as well as biochemical and cell-biological analyses to identify coat proteins whose assembly is CotO dependent. cotO spores are defective in germination and partially sensitive to lysozyme. As a whole, these phenotypes resemble those resulting from a mutation in the coat protein gene cotH. Nonetheless, the roles of CotH and CotO and the proteins whose assembly they direct are not identical. Based on fluorescence and electron microscopy, we suggest that CotO resides in the outer coat (although not on the coat surface). We propose that CotO and CotH participate in a late phase of coat assembly. We further speculate that an important role of these proteins is ensuring that polymerization of the outer coat layers occurs in such a manner that contiguous shells, and not unproductive aggregates, are formed.

A gene encoding a holin-like protein involved in spore morphogenesis and spore germination in Bacillus subtilis

We report here studies of expression and functional analysis of a Bacillus subtilis gene, ywcE, which codes for a product with features of a holin. Primer extension analysis of ywcE transcription revealed that a single transcript accumulated from the onset of sporulation onwards, produced from a sigma(A)-type promoter bearing the TG dinucleotide motif of "extended" -10 promoters. No primer extension product was detected in vivo during growth. However, specific runoff products were produced in vitro from the ywcE promoter by purified sigma(A)-containing RNA polymerase (Esigma(A)), and the in vivo and in vitro transcription start sites were identical. These results suggested that utilization of the ywcE promoter by Esigma(A) during growth was subjected to repression. Studies with a lacZ fusion revealed that the transition-state regulator AbrB repressed the transcription of ywcE during growth. This repression was reversed at the onset of sporulation in a Spo0A-dependent manner, but Spo0A did not appear to contribute otherwise to ywcE transcription. We found ywcE to be required for proper spore morphogenesis. Spores of the ywcE mutant showed a reduced outer coat which lacked the characteristic striated pattern, and the outer coat failed to attach to the underlying inner coat. The mutant spores also accumulated reduced levels of dipicolinic acid. ywcE was also found to be important for spore germination.

TheBacillus subtilisspore coat protein interaction network

Bacterial spores are surrounded by a morphologically complex, mechanically flexible protein coat, which protects the spore from toxic molecules. The interactions among the over 50 proteins that make up the coat remain poorly understood. We have used cell biological and protein biochemical approaches to identify novel coat proteins in Bacillus subtilis and describe the network of their interactions, in order to understand coat assembly and the molecular basis of its protective functions and mechanical properties. Our analysis characterizes the interactions between 32 coat proteins. This detailed view reveals a complex interaction network. A key feature of the network is the importance of a small subset of proteins that direct the assembly of most of the coat. From an analysis of the network topology, we propose a model in which low-affinity interactions are abundant in the coat and account, to a significant degree, for the coat's mechanical properties as well as structural variation between spores.

The ExsA protein of Bacillus cereus is required for assembly of coat and exosporium onto the spore surface

The outermost layer of spores of the Bacillus cereus family is a loose structure known as the exosporium. Spores of a library of Tn917-LTV1 transposon insertion mutants of B. cereus ATCC 10876 were partitioned into hexadecane; a less hydrophobic mutant that was isolated contained an insertion in the exsA promoter region. ExsA is the equivalent of SafA (YrbA) of Bacillus subtilis, which is also implicated in spore coat assembly; the gene organizations around both are identical, and both proteins contain a very conserved N-terminal cortex-binding domain of ca. 50 residues, although the rest of the sequence is much less conserved. In particular, unlike SafA, the ExsA protein contains multiple tandem oligopeptide repeats and is therefore likely to have an extended structure. The exsA gene is expressed in the mother cell during sporulation. Spores of an exsA mutant are extremely permeable to lysozyme and are blocked in late stages of germination, which require coat-associated functions. Two mutants expressing differently truncated versions of ExsA were constructed, and they showed the same gross defects in the attachment of exosporium and spore coat layers. The protein profile of the residual exosporium harvested from spores of the three mutants--two expressing truncated proteins and the mutant with the original transposon insertion in the promoter region--showed some differences from the wild type and from each other, but the major exosporium glycoproteins were retained. The exsA gene is extremely important for the normal assembly and anchoring of both the spore coat and exosporium layers in spores of B. cereus.

Identification of strain specific markers in Bacillus anthracis by random amplification of polymorphic DNA

Classification and differentiation of Bacillus anthracis isolates by genetic markers play an important role in anthrax research. We used a PCR based method--Random Amplification of Polymorphic DNA (RAPD)--to identify genetic markers in B. anthracis strains. Twenty-five differential genetic markers were identified which divided the strains into five different groups. Three selected RAPD-markers were cloned and sequenced. The five RAPD-derived genotypes could be defined by integration of these three markers. This system offers a simple non-expensive method to classify B. anthracis strains in laboratories involved in the research of this bacterium.

Oral priming of mice by recombinant spores of Bacillus subtilis

Recombinant Bacillus subtilis spores were employed as a vaccine delivery system in a heterologous mucosal priming-parenteral boosting vaccination strategy in the mouse model. BALB/c and C57BL/6 mice were orally immunised with recombinant spores expressing tetanus toxin fragment C (TTFC) fused to the spore outer coat protein CotB, and then subcutaneously boosted with soluble TTFC (without adjuvant). Two weeks after boosting, a significantly higher serum TTFC-specific IgG response was stimulated in mice primed with recombinant spores (antibody concentration of 2600 +/- 915 in C57BL/6 and 1200 +/- 370 ng/ml in BALB/c) compared to mice inoculated with wild type spores (650 +/- 250 and 250 +/- 130 ng/ml, respectively). IgG subclass analysis showed a prevalence of IgG1 and IgG2b, indicative of a Th2 type of immune response. Oral administration of recombinant spores stimulated also a significant local TTFC-specific IgA response. These data show that recombinant spores of B. subtilis are able to prime the immune system by the oral route, and that a combined mucosal/parenteral strategy can stimulate both local and systemic antigen-specific immune responses.

Functional relationship between SpoVIF and GerE in gene regulation during sporulation of Bacillus subtilis

The sporulation-specific SpoVIF (YjcC) protein of Bacillus subtilis is essential for the development of heat-resistant spores. The GerE protein, the smallest member of the LuxR-FixJ family, contains a helix-turn-helix (HTH) motif and is involved in the expression of various sporulation-specific genes. In this study, the gene expression and protein composition of sporulating spoVIF-negative cells were analysed. CgeA, CotG and CotS, which are GerE-dependent coat proteins, were not expressed in the spoVIF-negative cells. Northern blotting showed that SpoVIF regulated the transcription of cgeA, cotG and cotS in a manner similar to that of GerE. In spoVIF-negative cells, gerE mRNA was transcribed normally, but immunoblot analysis using anti-GerE antiserum showed that the quantity of GerE protein was considerably less than that in wild-type controls. Using GFP (green fluorescent protein) fusion proteins, the localization of SpoVIF and GerE was observed by fluorescence microscopy. SpoVIF-GFP was detectable in the mother cell compartment, as was GerE-GFP. These results suggest that SpoVIF directly or indirectly controls the function of the GerE protein, and that SpoVIF is required for gene regulation during the latter stages of sporulation.

Assembly of an oxalate decarboxylase produced under sigmaK control into the Bacillus subtilis spore coat

Over 30 polypeptides are synthesized at various times during sporulation in Bacillus subtilis, and they are assembled at the surface of the developing spore to form a multilayer protein structure called the coat. The coat consists of three main layers, an amorphous undercoat close to the underlying spore cortex peptidoglycan, a lamellar inner layer, and an electron-dense striated outer layer. The product of the B. subtilis oxdD gene was previously shown to have oxalate decarboxylase activity when it was produced in Escherichia coli and to be a spore constituent. In this study, we found that OxdD specifically associates with the spore coat structure, and in this paper we describe regulation of its synthesis and assembly. We found that transcription of oxdD is induced during sporulation as a monocistronic unit under the control of sigma(K) and is negatively regulated by GerE. We also found that localization of a functional OxdD-green fluorescent protein (GFP) at the surface of the developing spore depends on the SafA morphogenetic protein, which localizes at the interface between the spore cortex and coat layers. OxdD-GFP localizes around the developing spore in a cotE mutant, which does not assemble the spore outer coat layer, but it does not persist in spores produced by the mutant. Together, the data suggest that OxdD-GFP is targeted to the interior layers of the coat. Additionally, we found that expression of a multicopy allele of oxdD resulted in production of spores with increased levels of OxdD that were able to degrade oxalate but were sensitive to lysozyme.

Interactions among CotB, CotG, and CotH during assembly of the Bacillus subtilis spore coat

Spores formed by wild-type Bacillus subtilis are encased in a multilayered protein structure (called the coat) formed by the ordered assembly of over 30 polypeptides. One polypeptide (CotB) is a surface-exposed coat component that has been used as a vehicle for the display of heterologous antigens at the spore surface. The cotB gene was initially identified by reverse genetics as encoding an abundant coat component. cotB is predicted to code for a 43-kDa polypeptide, but the form that prevails in the spore coat has a molecular mass of about 66 kDa (herein designated CotB-66). Here we show that in good agreement with its predicted size, expression of cotB in Escherichia coli results in the accumulation of a 46-kDa protein (CotB-46). Expression of cotB in sporulating cells of B. subtilis also results in a 46-kDa polypeptide which appears to be rapidly converted into CotB-66. These results suggest that soon after synthesis, CotB undergoes a posttranslational modification. Assembly of CotB-66 has been shown to depend on expression of both the cotH and cotG loci. We found that CotB-46 is the predominant form found in extracts prepared from sporulating cells or in spore coat preparations of cotH or cotG mutants. Therefore, both cotH and cotG are required for the efficient conversion of CotB-46 into CotB-66 but are dispensable for the association of CotB-46 with the spore coat. We also show that CotG does not accumulate in sporulating cells of a cotH mutant, suggesting that CotH (or a CotH-controlled factor) stabilizes the otherwise unstable CotG. Thus, the need for CotH for formation of CotB-66 results in part from its role in the stabilization of CotG. We also found that CotB-46 is present in complexes with CotG at the time when formation of CotB-66 is detected. Moreover, using a yeast two-hybrid system, we found evidence that CotB directly interacts with CotG and that both CotB and CotG self-interact. We suggest that an interaction between CotG and CotB is required for the formation of CotB-66, which may represent a multimeric form of CotB.