The ExsY protein is required for complete formation of the exosporium of Bacillus anthracis

A new fluorescence-based approach for direct visualization of coat formation during sporulation in Bacillus cereus

The human pathogenic bacteria Bacillus cereus, Bacillus anthracis and the entomopathogenic Bacillus thuringiensis form spores encased in a protein coat surrounded by a balloon-like exosporium. These structures mediate spore interactions with its environment, including the host immune system, control the transit of molecules that trigger germination and thus are essential for the spore life cycle. Formation of the coat and exosporium has been traditionally visualized by transmission electronic microscopy on fixed cells. Recently, we showed that assembly of the exosporium can be directly observed in live B. cereus cells by super resolution-structured illumination microscopy (SR-SIM) using the membrane MitoTrackerGreen (MTG) dye. Here, we demonstrate that the different steps of coat formation can also be visualized by SR-SIM using MTG and SNAP-cell TMR-star dyes during B. cereus sporulation. We used these markers to characterize a subpopulation of engulfment-defective B. cereus cells that develops at a suboptimal sporulation temperature. Importantly, we predicted and confirmed that synthesis and accumulation of coat material, as well as synthesis of the σK-dependent protein BxpB, occur in cells arrested during engulfment. These results suggest that, unlike the well-studied model organism Bacillus subtilis, the activity of σK is not strictly linked to the state of forespore development in B. cereus.

Roles and Organization of BxpB (ExsFA) and ExsFB in the Exosporium Outer Basal Layer of Bacillus anthracis

BxpB (also known as ExsFA) and ExsFB are an exosporium basal layer structural protein and a putative interspace protein of Bacillus anthracis that are known to be required for proper incorporation of the BclA collagen-like glycoprotein on the spore surface. Despite extensive similarity of the two proteins, their distribution in the spore is markedly different. We utilized a fluorescent fusion approach to examine features of the two genes that affect spore localization. The timing of expression of the bxpB and exsFB genes and their distinct N-terminal sequences were both found to be important for proper assembly into the exosporium basal layer. Results of this study provided evidence that the BclA nap glycoprotein is not covalently attached to BxpB protein despite the key role that the latter plays in BclA incorporation. Assembly of the BxpB- and ExsFB-containing outer basal layer appears not to be completely abolished in mutants lacking the ExsY and CotY basal layer structural proteins despite these spores lacking a visible exosporium. The BxpB and, to a lesser extent, the ExsFB proteins, were found to be capable of self-assembly in vitro into higher-molecular-weight forms that are stable to boiling in SDS under reducing conditions. IMPORTANCE The genus Bacillus consists of spore-forming bacteria. Some species of this genus, especially those that are pathogens of animals or insects, contain an outermost spore layer called the exosporium. The zoonotic pathogen B. anthracis is an example of this group. The exosporium likely contributes to virulence and environmental persistence of these pathogens. This work provides important new insights into the exosporium assembly process and the interplay between BclA and BxpB in this process.

ExsY, CotY, and CotE Effects on Bacillus anthracis Outer Spore Layer Architecture

Bacillus anthracis, Bacillus cereus, and Bacillus thuringiensis are the major pathogens of the spore-forming genus Bacillus and possess an outer spore layer, the exosporium, not found in many of the nonpathogenic species. The exosporium consists of a basal layer with the ExsY, CotY, and BxpB proteins being the major structural components and an exterior nap layer containing the BclA glycoprotein. During the assembly process, the nascent exosporium basal layer is attached to the spore coat by a protein linker that includes the CotO and CotE proteins. Using transmission electron microscopy, Western blotting, immunofluorescence, and fluorescent fusion protein approaches, we examined the impact of single, double, and triple mutants of the major exosporium proteins on exosporium protein content and distribution. Plasmid-based expression of exsY and cotE resulted in increased production of exosporium lacking spores, and the former also resulted in outer spore coat disruptions. The exosporium bottlecap produced by exsY null spores was found to be more stable than previously reported, and its spore association was partially dependent on CotE. Deletion mutants of five putative spore genes (bas1131, bas1142, bas1143, bas2277, and bas3594) were created and shown not to have obvious effects on spore morphology or BclA and BxpB content. The BclC collagen-like glycoprotein was found to be present in the spore and possibly localized to the interspace region. IMPORTANCE B. anthracis is an important zoonotic animal pathogen causing sporadic outbreaks of anthrax worldwide. Spores are the infectious form of the bacterium and can persist in soil for prolonged periods of time. The outermost B. anthracis spore layer is the exosporium, a protein shell that is the site of interactions with both the soil and with the innate immune system of infected hosts. Although much is known regarding the sporulation process among members of the genus Bacillus, significant gaps in our understanding of the exosporium assembly process exist. This study provides evidence for the properties of key exosporium basal layer structural proteins. The results of this work will guide future studies on exosporium protein-protein interactions during the assembly process.

Localization of the CotY and ExsY proteins to the exosporium basal layer of Bacillus anthracis

Spores are an infectious form of the zoonotic bacterial pathogen, Bacillus anthracis. The outermost spore layer is the exosporium, comprised of a basal layer and an external glycoprotein nap layer. The major structural proteins of the inner basal layer are CotY (at the mother cell central pole or bottlecap) and ExsY around the rest of the spore. The basis for the cap or noncap specificity of the CotY and ExsY proteins is currently unknown. We investigated the role of sequence differences between these proteins in localization during exosporium assembly. We found that sequence differences were less important than the timing of expression of the respective genes in the positioning of these inner basal layer structural proteins. Fusion constructs with the fluorescent protein fused at the N-terminus resulted in poor incorporation whereas fusions at the carboxy terminus of CotY or ExsY resulted in good incorporation. However, complementation studies revealed that fusion constructs, although accurate indicators of protein localization, were not fully functional. A model is presented that explains the localization patterns observed. Bacterial two-hybrid studies in Escherichia coli hosts were used to examine protein-protein interactions with full-length and truncated proteins. The N-terminus amino acid sequences of ExsY and CotY appear to be recognized by spore proteins located in the spore interspace, consistent with interactions seen with ExsY and CotY with the interspace proteins CotE and CotO, known to be involved with exosporium attachment.

Role of novel polysaccharide layers in assembly of the exosporium, the outermost protein layer of the Bacillus anthracis spore

A fundamental question in cell biology is how cells assemble their outer layers. The bacterial endospore is a well-established model for cell layer assembly. However, the assembly of the exosporium, a complex protein shell comprising the outermost layer in the pathogen Bacillus anthracis, remains poorly understood. Exosporium assembly begins with the deposition of proteins at one side of the spore surface, followed by the progressive encirclement of the spore. We seek to resolve a major open question: the mechanism directing exosporium assembly to the spore, and then into a closed shell. We hypothesized that material directly underneath the exosporium (the interspace) directs exosporium assembly to the spore and drives encirclement. In support of this, we show that the interspace possesses at least two distinct layers of polysaccharide. Secondly, we show that putative polysaccharide biosynthetic genes are required for exosporium encirclement, suggesting a direct role for the interspace. These results not only significantly clarify the mechanism of assembly of the exosporium, an especially widespread bacterial outer layer, but also suggest a novel mechanism in which polysaccharide layers drive the assembly of a protein shell.

Learning from Nature: Bacterial Spores as a Target for Current Technologies in Medicine (Review)

The capability of some representatives of Clostridium spp. and Bacillus spp. genera to form spores in extreme external conditions long ago became a subject of medico-biological investigations. Bacterial spores represent dormant cellular forms of gram-positive bacteria possessing a high potential of stability and the capability to endure extreme conditions of their habitat. Owing to these properties, bacterial spores are recognized as the most stable systems on the planet, and spore-forming microorganisms became widely spread in various ecosystems.

Spore-forming bacteria have been attracted increased interest for years due to their epidemiological danger. Bacterial spores may be in the quiescent state for dozens or hundreds of years but after they appear in the favorable conditions of a human or animal organism, they turn into vegetative forms causing an infectious process. The greatest threat among the pathogenic spore-forming bacteria is posed by the causative agents of anthrax (B. anthracis), food toxicoinfection (B. cereus), pseudomembranous colitis (C. difficile), botulism (C. botulinum), gas gangrene (C. perfringens).

For the effective prevention of severe infectious diseases first of all it is necessary to study the molecular structure of bacterial spores and the biochemical mechanisms of sporulation and to develop innovative methods of detection and disinfection of dormant cells. There is another side of the problem: the necessity to investigate exo- and endospores from the standpoint of obtaining similar artificially synthesized models in order to use them in the latest medical technologies for the development of thermostable vaccines, delivery of biologically active substances to the tissues and intracellular structures. In recent years, bacterial spores have become an interesting object for the exploration from the point of view of a new paradigm of unicellular microbiology in order to study microbial heterogeneity by means of the modern analytical tools.

The Morphogenetic Protein CotE Positions Exosporium Proteins CotY and ExsY during Sporulation of Bacillus cereus

The exosporium is the outermost spore layer of some Bacillus and Clostridium species and related organisms. It mediates the interactions of spores with their environment, modulates spore adhesion and germination, and has been implicated in pathogenesis. In Bacillus cereus, the exosporium consists of a crystalline basal layer, formed mainly by the two cysteine-rich proteins CotY and ExsY, surrounded by a hairy nap composed of glycoproteins. The morphogenetic protein CotE is necessary for the integrity of the B. cereus exosporium, but how CotE directs exosporium assembly remains unknown. Here, we used super-resolution fluorescence microscopy to follow the localization of SNAP-tagged CotE, CotY, and ExsY during B. cereus sporulation and evidenced the interdependencies among these proteins. Complexes of CotE, CotY, and ExsY are present at all sporulation stages, and the three proteins follow similar localization patterns during endospore formation that are reminiscent of the localization pattern of Bacillus subtilis CotE. We show that B. cereus CotE guides the formation of one cap at both forespore poles by positioning CotY and then guides forespore encasement by ExsY, thereby promoting exosporium elongation. By these two actions, CotE ensures the formation of a complete exosporium. Importantly, we demonstrate that the assembly of the exosporium is not a unidirectional process, as previously proposed, but occurs through the formation of two caps, as observed during B. subtilis coat morphogenesis, suggesting that a general principle governs the assembly of the spore surface layers of Bacillaceae.

IMPORTANCE Spores of Bacillaceae are enveloped in an outermost glycoprotein layer. In the B. cereus group, encompassing the Bacillus anthracis and B. cereus pathogens, this layer is easily recognizable by a characteristic balloon-like appearance and separation from the underlying coat by an interspace. In spite of its importance for the environmental interactions of spores, including those with host cells, the mechanism of assembly of the exosporium is poorly understood. We used super-resolution fluorescence microscopy to directly visualize the formation of the exosporium during the sporulation of B. cereus, and we studied the localization and interdependencies of proteins essential for exosporium morphogenesis. We discovered that these proteins form a morphogenetic scaffold before a complete exosporium or coat is detectable. We describe how the different proteins localize to the scaffold and how they subsequently assemble around the spore, and we present a model for the assembly of the exosporium.

Henriques A. A protein phosphorylation module patterns the Bacillus subtilis spore outer coat

Assembly of the Bacillus subtilis spore coat involves over 80 proteins which self-organize into a basal layer, a lamellar inner coat, a striated electrodense outer coat and a more external crust. CotB is an abundant component of the outer coat. The C-terminal moiety of CotB, SKRB , formed by serine-rich repeats, is polyphosphorylated by the Ser/Thr kinase CotH. We show that another coat protein, CotG, with a central serine-repeat region, SKRG , interacts with the C-terminal moiety of CotB and promotes its phosphorylation by CotH in vivo and in a heterologous system. CotG itself is phosphorylated by CotH but phosphorylation is enhanced in the absence of CotB. Spores of a strain producing an inactive form of CotH, like those formed by a cotG deletion mutant, lack the pattern of electrondense outer coat striations, but retain the crust. In contrast, deletion of the SKRB region, has no major impact on outer coat structure. Thus, phosphorylation of CotG by CotH is a key factor establishing the structure of the outer coat. The presence of the cotB/cotH/cotG cluster in several species closely related to B. subtilis hints at the importance of this protein phosphorylation module in the morphogenesis of the spore surface layers.

The Clostridioides difficile Cysteine-Rich Exosporium Morphogenetic Protein, CdeC, Exhibits Self-Assembly Properties That Lead to Organized Inclusion Bodies in Escherichia coli

The endospore of Clostridioides difficile is the vehicle for transmission and persistence of the pathogen, and, specifically, the exosporium is the first contact between the host and the spore. The underlying mechanisms that govern exosporium assembly in C. difficile remain understudied, in part due to difficulties in obtaining pure soluble recombinant proteins of the C. difficile exosporium. Understanding the exosporium assembly’s molecular bases may be essential to developing new therapies against C. difficile infection.

Clostridioides difficile is an obligately anaerobic, spore-forming, Gram-positive pathogenic bacterium that is considered the leading cause of nosocomial diarrhea worldwide. Recent studies have attempted to understand the biology of the outermost layer of C. difficile spores, the exosporium, which is believed to contribute to early interactions with the host. The fundamental role of the cysteine-rich proteins CdeC and CdeM has been described. However, the molecular details behind the mechanism of exosporium assembly are missing. The underlying mechanisms that govern exosporium assembly in C. difficile remain poorly studied, in part due to difficulties in obtaining pure soluble recombinant proteins of the C. difficile exosporium. In this work, we observed that CdeC was able to form organized inclusion bodies (IBs) in Escherichia coli filled with lamella-like structures separated by an interspace of 5 to 15 nm; however, CdeC expression in an E. coli strain with a more oxidative environment led to the loss of the lamella-like organization of CdeC IBs. Additionally, dithiothreitol (DTT) treatment of CdeC inclusion bodies released monomeric soluble forms of CdeC. Deletions in different portions of CdeC did not affect CdeC’s ability to aggregate and form oligomers stable under denaturation conditions but affected CdeC’s self-assembly properties. Overall, these observations have important implications in further studies elucidating the role of CdeC in the exosporium assembly of C. difficile spores.

IMPORTANCE The endospore of Clostridioides difficile is the vehicle for transmission and persistence of the pathogen, and, specifically, the exosporium is the first contact between the host and the spore. The underlying mechanisms that govern exosporium assembly in C. difficile remain understudied, in part due to difficulties in obtaining pure soluble recombinant proteins of the C. difficile exosporium. Understanding the exosporium assembly’s molecular bases may be essential to developing new therapies against C. difficile infection.

Production of a polyclonal antibody against inosine-uridine preferring nucleoside hydrolase of Acanthamoeba castellanii and its access to diagnosis of Acanthamoeba keratitis

Acanthamoeba keratitis (AK) is a rare disease but its prevalence throughout the globe continues to grow, primarily due to increased contact lens usage. Since early-stage symptoms associated with AK closely resemble those from other corneal infections, accurate diagnosis is difficult and this often results in delayed treatment and exacerbation of the disease, which can lead to permanent visual impairment. Accordingly, developing a rapid Acanthamoeba–specific diagnostic method is highly desired. In the present study, a rapid and differential method for AK diagnosis was developed using the secretory proteins derived from the pathogenic Acanthamoeba. Among the vast quantities of proteins secreted by the pathogenic Acanthamoeba, an open reading frame of the inosine-uridine preferring nucleoside hydrolase (IPNH) gene was obtained. After expressing and purifying the IPNH protein using the pGEX 4T-3 vector system, mice were immunized with the purified proteins for polyclonal antibody generation. Western blot was performed using protein lysates of the human corneal cell, non-pathogenic amoeba, pathogenic amoeba, and clinical amoeba isolate along with lysates from other causes of keratitis such as Staphylococcus aureus, Pseudomonas aeruginosa, and Fusarium solani to confirm Acanthamoeba-specificity. Western blot using the polyclonal IPNH antibody revealed that IPNH was Acanthamoeba-specific since these proteins were only observed in lysates of Acanthamoeba origin or its culture media. Our findings indicate that the IPNH antibody of Acanthamoeba may serve as a potential agent for rapid and differential AK diagnosis.

Sporulation and Germination in Clostridial Pathogens

As obligate anaerobes, clostridial pathogens depend on their metabolically dormant, oxygen-tolerant spore form to transmit disease. However, the molecular mechanisms by which those spores germinate to initiate infection and then form new spores to transmit infection remain poorly understood. While sporulation and germination have been well characterized in Bacillus subtilis and Bacillus anthracis, striking differences in the regulation of these processes have been observed between the bacilli and the clostridia, with even some conserved proteins exhibiting differences in their requirements and functions. Here, we review our current understanding of how clostridial pathogens, specifically Clostridium perfringens, Clostridium botulinum, and Clostridioides difficile, induce sporulation in response to environmental cues, assemble resistant spores, and germinate metabolically dormant spores in response to environmental cues. We also discuss the direct relationship between toxin production and spore formation in these pathogens.

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.

Orthologues of Bacillus subtilis Spore Crust Proteins Have a Structural Role in the Bacillus megaterium QM B1551 Spore Exosporium

The exosporium of Bacillus megaterium QM B1551 spores is morphologically distinct from exosporia observed for the spores of many other species. Previous work has demonstrated that unidentified genes carried on one of the large indigenous plasmids are required for the assembly of the Bacillus megaterium exosporium. Here, we provide evidence that pBM600-encoded orthologues of the Bacillus subtilis CotW and CotX proteins, which form the crust layer in spores of that species, are structural components of the Bacillus megaterium QM B1551 spore exosporium. The introduction of plasmid-borne cotW and orthologous cotX genes to the PV361 strain, which lacks all indigenous plasmids and produces spores that are devoid of an exosporium, results in the development of spores with a rudimentary exosporium-type structure. Additionally, purified recombinant CotW protein is shown to assemble at the air-water interface to form thin sheets of material, which is consistent with the idea that this protein may form a basal layer in the Bacillus megaterium QM B1551 exosporium.IMPORTANCE When starved of nutrients, some bacterial species develop metabolically dormant spores that can persist in a viable state in the environment for several years. The outermost layers of spores are of particular interest since (i) these represent the primary site for interaction with the environment and (ii) the protein constituents may have biotechnological applications. The outermost layer, or exosporium, in Bacillus megaterium QM B1551 spores is of interest, as it is morphologically distinct from the exosporia of spores of the pathogenic Bacillus cereus family. In this work, we provide evidence that structurally important protein constituents of the Bacillus megaterium exosporium are different from those in the Bacillus cereus family. We also show that one of these proteins, when purified, can assemble to form sheets of exosporium-like material. This is significant, as it indicates that spore-forming bacteria employ different proteins and mechanisms of assembly to construct their external layers.

Clostridium difficile exosporium cysteine-rich proteins are essential for the morphogenesis of the exosporium layer, spore resistance, and affect C. difficile pathogenesis

Clostridium difficile is a Gram-positive spore-former bacterium and the leading cause of nosocomial antibiotic-associated diarrhea that can culminate in fatal colitis. During the infection, C. difficile produces metabolically dormant spores, which persist in the host and can cause recurrence of the infection. The surface of C. difficile spores seems to be the key in spore-host interactions and persistence. The proteome of the outermost exosporium layer of C. difficile spores has been determined, identifying two cysteine-rich exosporium proteins, CdeC and CdeM. In this work, we explore the contribution of both cysteine-rich proteins in exosporium integrity, spore biology and pathogenesis. Using targeted mutagenesis coupled with transmission electron microscopy we demonstrate that both cysteine rich proteins, CdeC and CdeM, are morphogenetic factors of the exosporium layer of C. difficile spores. Notably, cdeC, but not cdeM spores, exhibited defective spore coat, and were more sensitive to ethanol, heat and phagocytic cells. In a healthy colonic mucosa (mouse ileal loop assay), cdeC and cdeM spore adherence was lower than that of wild-type spores; while in a mouse model of recurrence of the disease, cdeC mutant exhibited an increased infection and persistence during recurrence. In a competitive infection mouse model, cdeC mutant had increased fitness over wild-type. Through complementation analysis with FLAG fusion of known exosporium and coat proteins, we demonstrate that CdeC and CdeM are required for the recruitment of several exosporium proteins to the surface of C. difficile spores. CdeC appears to be conserved exclusively in related Peptostreptococcaeace family members, while CdeM is unique to C. difficile. Our results sheds light on how CdeC and CdeM affect the biology of C. difficile spores and the assembly of the exosporium layer and, demonstrate that CdeC affect C. difficile pathogenesis.

We discovered a mechanism of assembly of the outer most layer of Clostridium difficile spores, the exosporium. While CdeC is conserved in several Peptostreptococcaeace family members, CdeM is unique to C. difficile. We show that two proteins that are rich in cysteine amino acid residues, CdeC and CdeM, are essential for the recruitment of additional spore coat and exosporium proteins. The absence of CdeC, had profound implications in the correct spore coat assembly which were related to decreased spore resistant properties that are relevant for in vivo infection such as lysozyme resistance, macrophage infection. Notably, the absence of either cysteine rich proteins leads to a decrease in spore adherence of C. difficile spores to healthy colonic mucosa; but only the absence of CdeC affected in vivo competitive fitness in a mouse model, recurrence of the disease in a mouse model of recurrent infection. Considering the importance of the outer layers of C. difficile spores in spore-host interactions, our findings have broad implications on the biology of C. difficile spores and to C. difficile pathogenesis.

Clostridium sordellii outer spore proteins maintain spore structural integrity and promote bacterial clearance from the gastrointestinal tract

Bacterial spores play an important role in disease initiation, transmission and persistence. In some species, the exosporium forms the outermost structure of the spore and provides the first point of contact between the spore and the environment. The exosporium may also be involved in spore adherence, protection and germination. Clostridium sordellii is a highly lethal, spore forming pathogen that causes soft-tissue infections, enteritis and toxic-shock syndrome. Despite the importance of C. sordellii spores in disease, spore proteins from this bacterium have not been defined or interrogated functionally. In this study, we identified the C. sordellii outer spore proteome and two of the identified proteins, CsA and CsB, were characterised using a genetic and phenotypic approach. Both proteins were essential for the correct formation and positioning of the C. sordellii spore coat and exosporium. The absence of CsA reduced sporulation levels and increased spore sensitivity to heat, sodium hydroxide and hydrochloric acid. By comparison, CsB was required for normal levels of spore adherence to cervical, but not vaginal, cells, with csB mutant spores having increased adherence properties. The establishment of a mouse infection model of the gastrointestinal tract for C. sordellii allowed the role of CsA and CsB to be interrogated in an infected host. Following the oral administration of spores to mice, the wild-type strain efficiently colonized the gastrointestinal tract, with the peak of bacterial numbers occurring at one day post-infection. Colonization was reduced by two logs at four days post-infection. By comparison, mice infected with the csB mutant did not show a reduction in bacterial numbers. We conclude that C. sordellii outer spore proteins are important for the structural and functional integrity of spores. Furthermore, outer spore proteins are required for wild-type levels of colonization during infection, possibly as a result of the role that the proteins play in spore structure and morphology.

Surviving Between Hosts: Sporulation and Transmission

To survive adverse conditions, some bacterial species are capable of developing into a cell type, the "spore," which exhibits minimal metabolic activity and remains viable in the presence of multiple environmental challenges. For some pathogenic bacteria, this developmental state serves as a means of survival during transmission from one host to another. Spores are the highly infectious form of these bacteria. Upon entrance into a host, specific signals facilitate germination into metabolically active replicating organisms, resulting in disease pathogenesis. In this article, we will review spore structure and function in well-studied pathogens of two genera, Bacillus and Clostridium, focusing on Bacillus anthracis and Clostridium difficile, and explore current data regarding the lifestyles of these bacteria outside the host and transmission from one host to another.

Assembly of the outermost spore layer: pieces of the puzzle are coming together

Certain endospore-forming soil dwelling bacteria are important human, animal or insect pathogens. These organisms produce spores containing an outer layer, the exosporium. The exosporium is the site of interactions between the spore and the soil environment and between the spore and the infected host during the initial stages of infection. The composition and assembly process of the exosporium are poorly understood. This is partly due to the extreme stability of the exosporium that has proven to be refractive to existing methods to deconstruct the intact structure into its component parts. Although more than 20 proteins have been identified as exosporium-associated, their abundance, relationship to other proteins and the processes by which they are assembled to create the exosporium are largely unknown. In this issue of Molecular Microbiology, Terry, Jiang, and colleagues in Per Bullough's laboratory show that the ExsY protein is a major structural protein of the exosporium basal layer of B. cereus family spores and that it can self-assemble into complex structures that possess many of the structural features characteristic of the exosporium basal layer. The authors refined a model for exosporium assembly. Their findings may have implications for exosporium formation in other spore forming bacteria, including Clostridium species.

Molecular tiling on the surface of a bacterial spore - the exosporium of the Bacillus anthracis/cereus/thuringiensis group

Bacteria of the genera Bacillus and Clostridium form highly resistant spores, which in the case of some pathogens act as the infectious agents. An exosporium forms the outermost layer of some spores; it plays roles in protection, adhesion, dissemination, host targeting in pathogens and germination control. The exosporium of the Bacillus cereus group, including the anthrax pathogen, contains a 2D‐crystalline basal layer, overlaid by a hairy nap. BclA and related proteins form the hairy nap, and require ExsFA (BxpB) for their localization on the basal layer. Until now, the identity of the main structural protein components of the basal layer was unknown. We demonstrate here that ExsY forms one of the essential components. Through heterologous expression in Escherichia coli, we also demonstrate that ExsY can self‐assemble into ordered 2D arrays that mimic the structure of the exosporium basal layer. Self‐assembly is likely to play an important role in the construction of the exosporium. The ExsY array is stable to heat and chemical denaturants, forming a robust layer that would contribute to overall spore resistance. Our structural analysis also provides novel insight into the location of other molecular components anchored onto the exosporium, such as BclA and ExsFA.

The Bacillus anthracis Exosporium: What's the Big "Hairy" Deal?

In some Bacillus species, including Bacillus subtilis, the coat is the outermost layer of the spore. In others, such as the Bacillus cereus family, there is an additional layer that envelops the coat, called the exosporium. In the case of Bacillus anthracis, a series of fine hair-like projections, also referred to as a "hairy" nap, extends from the exosporium basal layer. The exact role of the exosporium in B. anthracis, or for any of the Bacillus species possessing this structure, remains unclear. However, it has been assumed that the exosporium would play some role in infection for B. anthracis, because it is the outermost structure of the spore and would make initial contact with host and immune cells during infection. Therefore, the exosporium has been a topic of great interest, and over the past decade much progress has been made to understand its composition, biosynthesis, and potential roles. Several key aspects of this spore structure, however, are still debated and remain undetermined. Although insights have been gained on the interaction of exosporium with the host during infection, the exact role and significance of this complex structure remain to be determined. Furthermore, because the exosporium is a highly antigenic structure, future strategies for the next-generation anthrax vaccine should pursue its inclusion as a component to provide protection against the spore itself during the initial stages of anthrax.

Characterization of the spore surface and exosporium proteins of Clostridium sporogenes; implications for Clostridium botulinum group I strains

Clostridium sporogenes is a non-pathogenic close relative and surrogate for Group I (proteolytic) neurotoxin-producing Clostridium botulinum strains. The exosporium, the sac-like outermost layer of spores of these species, is likely to contribute to adhesion, dissemination, and virulence. A paracrystalline array, hairy nap, and several appendages were detected in the exosporium of C. sporogenes strain NCIMB 701792 by EM and AFM. The protein composition of purified exosporium was explored by LC-MS/MS of tryptic peptides from major individual SDS-PAGE-separated protein bands, and from bulk exosporium. Two high molecular weight protein bands both contained the same protein with a collagen-like repeat domain, the probable constituent of the hairy nap, as well as cysteine-rich proteins CsxA and CsxB. A third cysteine-rich protein (CsxC) was also identified. These three proteins are also encoded in C. botulinum Prevot 594, and homologues (75-100% amino acid identity) are encoded in many other Group I strains. This work provides the first insight into the likely composition and organization of the exosporium of Group I C. botulinum spores.

The Exosporium Layer of Bacterial Spores: a Connection to the Environment and the Infected Host

Much of what we know regarding bacterial spore structure and function has been learned from studies of the genetically well-characterized bacterium Bacillus subtilis. Molecular aspects of spore structure, assembly, and function are well defined. However, certain bacteria produce spores with an outer spore layer, the exosporium, which is not present on B. subtilis spores. Our understanding of the composition and biological functions of the exosporium layer is much more limited than that of other aspects of the spore. Because the bacterial spore surface is important for the spore's interactions with the environment, as well as being the site of interaction of the spore with the host's innate immune system in the case of spore-forming bacterial pathogens, the exosporium is worthy of continued investigation. Recent exosporium studies have focused largely on members of the Bacillus cereus family, principally Bacillus anthracis and Bacillus cereus. Our understanding of the composition of the exosporium, the pathway of its assembly, and its role in spore biology is now coming into sharper focus. This review expands on a 2007 review of spore surface layers which provided an excellent conceptual framework of exosporium structure and function (A. O. Henriques and C. P. Moran, Jr., Annu Rev Microbiol 61:555-588, 2007, http://dx.doi.org/10.1146/annurev.micro.61.080706.093224). That review began a process of considering outer spore layers as an integrated, multilayered structure rather than simply regarding the outer spore components as independent parts.

The Regulation of Exosporium-Related Genes in Bacillus thuringiensis

Bacillus anthracis, Bacillus cereus, and Bacillus thuringiensis (Bt) are spore-forming members of the Bacillus cereus group. Spores of B. cereus group species are encircled by exosporium, which is composed of an external hair-like nap and a paracrystalline basal layer. Despite the extensive studies on the structure of the exosporium-related proteins, little is known about the transcription and regulation of exosporium gene expression in the B. cereus group. Herein, we studied the regulation of several exosporium-related genes in Bt. A SigK consensus sequence is present upstream of genes encoding hair-like nap proteins (bclA and bclB), basal layer proteins (bxpA, bxpB, cotB, and exsY ), and inosine hydrolase (iunH). Mutation of sigK decreased the transcriptional activities of all these genes, indicating that the transcription of these genes is controlled by SigK. Furthermore, mutation of gerE decreased the transcriptional activities of bclB, bxpB, cotB, and iunH but increased the expression of bxpA, and GerE binds to the promoters of bclB, bxpB, cotB, bxpA, and iunH. These results suggest that GerE directly regulates the transcription of these genes, increasing the expression of bclB, bxpB, cotB, and iunH and decreasing that of bxpA. These findings provide insight into the exosporium assembly process at the transcriptional level.

Sporulation Temperature Reveals a Requirement for CotE in the Assembly of both the Coat and Exosporium Layers of Bacillus cereus Spores

The Bacillus cereus spore surface layers consist of a coat surrounded by an exosporium. We investigated the interplay between the sporulation temperature and the CotE morphogenetic protein in the assembly of the surface layers of B. cereus ATCC 14579 spores and on the resulting spore properties. The cotE deletion affects the coat and exosporium composition of the spores formed both at the suboptimal temperature of 20°C and at the optimal growth temperature of 37°C. Transmission electron microscopy revealed that ΔcotE spores had a fragmented and detached exosporium when formed at 37°C. However, when produced at 20°C, ΔcotE spores showed defects in both coat and exosporium attachment and were susceptible to lysozyme and mutanolysin. Thus, CotE has a role in the assembly of both the coat and exosporium, which is more important during sporulation at 20°C. CotE was more represented in extracts from spores formed at 20°C than at 37°C, suggesting that increased synthesis of the protein is required to maintain proper assembly of spore surface layers at the former temperature. ΔcotE spores formed at either sporulation temperature were impaired in inosine-triggered germination and resistance to UV-C and H2O2 and were less hydrophobic than wild-type (WT) spores but had a higher resistance to wet heat. While underscoring the role of CotE in the assembly of B. cereus spore surface layers, our study also suggests a contribution of the protein to functional properties of additional spore structures. Moreover, it also suggests a complex relationship between the function of a spore morphogenetic protein and environmental factors such as the temperature during spore formation.

Collagen-like proteins (ClpA, ClpB, ClpC, and ClpD) are required for biofilm formation and adhesion to plant roots by Bacillus amyloliquefaciens FZB42

The genes of collagen-like proteins (CLPs) have been identified in a broad range of bacteria, including some human pathogens. They are important for biofilm formation and bacterial adhesion to host cells in some human pathogenic bacteria, including several Bacillus spp. strains. Interestingly, some bacterial CLP-encoding genes (clps) have also been found in non-human pathogenic strains such as B. cereus and B. amyloliquefaciens, which are types of plant-growth promoting rhizobacteria (PGPR). In this study, we investigated a putative cluster of clps in B. amyloliquefaciens strain FZB42 and a collagen-related structural motif containing glycine-X-threonine repeats was found in the genes RBAM_007740, RBAM_007750, RBAM_007760, and RBAM_007770. Interestingly, biofilm formation was disrupted when these genes were inactivated separately. Scanning electron microscopy and hydrophobicity value detection were used to assess the bacterial cell shape morphology and cell surface architecture of clps mutant cells. The results showed that the CLPs appeared to have roles in bacterial autoaggregation, as well as adherence to the surface of abiotic materials and the roots of Arabidopsis thaliana. Thus, we suggest that the CLPs located in the outer layer of the bacterial cell (including the cell wall, outer membrane, flagella, or other associated structures) play important roles in biofilm formation and bacteria-plant interactions. This is the first study to analyze the function of a collagen-like motif-containing protein in a PGPR bacterium. Knocking out each clp gene produced distinctive morphological phenotypes, which demonstrated that each product may play specific roles in biofilm formation. Our in silico analysis suggested that these four tandemly ranked genes might not belong to an operon, but further studies are required at the molecular level to test this hypothesis. These results provide insights into the functions of clps during interactions between bacteria and plants.

The regulated synthesis of a Bacillus anthracis spore coat protein that affects spore surface properties

AIMS

Examine the regulation of a spore coat protein and the effects on spore properties.

METHODS AND RESULTS

A c. 23 kDa band in coat/exosporial extracts of Bacillus anthracis Sterne spores varied in amount depending upon the conditions of sporulation. It was identified by MALDI as a likely orthologue of ExsB of Bacillus cereus. Little if any was present in an exosporial preparation with a location to the inner coat/cortex region established by spore fractionation and immunogold labelling of electron micrograph sections. Because of its predominant location in the inner coat, it has been renamed Cotγ. It was relatively deficient in spores produced at 37°C and when acidic fermentation products were produced a difference attributable to transcriptional regulation. The deficiency or absence of Cotγ resulted in a less robust exosporium positioned more closely to the coat. These spores were less hydrophobic and germinated somewhat more rapidly. Hydrophobicity and appearance were rescued in the deletion strain by introduction of the cotγ gene.

CONCLUSIONS

The deficiency or lack of a protein largely found in the inner coat altered spore hydrophobicity and surface appearance.

SIGNIFICANCE AND IMPACT OF THE STUDY

The regulated synthesis of Cotγ may be a paradigm for other spore coat proteins with unknown functions that modulate spore properties in response to environmental conditions.

Potential biological targets ofBacillus anthracisin anti-infective approaches against the threat of bioterrorism

The terrorist attacks of 2001 involving anthrax underscore the imperative that safe and effective medical countermeasures should be readily available. Vaccination appears to be the most effective form of mass protection against a biological attack, but the current vaccines have drawbacks that justify the enormous amount of effort currently being put into developing more effective vaccines and other treatment modalities. After providing a comprehensive overview of the organism Bacillus anthracis as a biological weapon and its pathogenicity, this review briefly summarizes the current knowledge vital to the management of anthrax disease. This knowledge has been acquired since 2001 as a result of the progress on anthrax research and focuses on the possible development of improved human anti-infective strategies targeting B. anthracis spore components, as well as strategies based on host-pathogen interactions.

Involvement of alanine racemase in germination of Bacillus cereus spores lacking an intact exosporium

The L-alanine mediated germination of food isolated Bacillus cereus DSA 1 spores, which lacked an intact exosporium, increased in the presence of D-cycloserine (DCS), which is an alanine racemase (Alr) inhibitor, reflecting the activity of the Alr enzyme, capable of converting L-alanine to the germination inhibitor D-alanine. Proteomic analysis of the alkaline extracts of the spore proteins, which include exosporium and coat proteins, confirmed that Alr was present in the B. cereus DSA 1 spores and matched to that encoded by B. cereus ATCC 14579, whose spore germination was strongly affected by the block of conversion of L- to D-alanine. Unlike ATCC 14579 spores, L-alanine germination of B. cereus DSA 1 spores was not affected by the preincubation with DCS, suggesting a lack of restriction in the reactant accessibility.

D-cycloserine or similar physiochemical compounds may be uniquely suited for use in Bacillus anthracis spore decontamination strategies

AIMS

As observed in the aftermath of the anthrax attacks of 2001, decontamination and remediation of a site contaminated by the accidental or intentional release of Bacillus anthracis spores is difficult, costly and potentially damaging to the environment. The identification of novel strategies that neutralize the threat of spores while minimizing environmental damage remains a high priority. We investigated the efficacy of d-cycloserine (DCS), an antibiotic and inhibitor of the spore-associated enzyme (alanine racemase) responsible for converting l-alanine to d-alanine, as a spore germination enhancer and antimicrobial agent.

METHODS AND RESULTS

We characterized the impact of DCS exposure on both germinating spores and vegetative cells of fully virulent B. anthracis by evaluating spore germination kinetics, determining the minimum inhibitory concentrations (MICs) required to affect growth of the bacteria and performing macrophage viability assays. DCS enhanced germination induced by l-alanine and also efficiently killed the newly germinated spores. Furthermore, DCS proved nontoxic to macrophages at concentrations that provided protection from the killing effects of spores. Similar tests were conducted with Bacillus thuringiensis (subspecies kurstaki and Al Hakam) to determine its potential as a possible surrogate for B. anthracis field trials. Bacillus thuringiensis spores responded in a similar manner to B. anthracis spores when exposed to DCS.

CONCLUSIONS

These results further support that DCS augments the germination response of spores in the presence of l-alanine but also reveal that DCS is bactericidal towards germinating spores.

SIGNIFICANCE AND IMPACT OF THE STUDY

DCS (or similar compounds) may be uniquely suited for use as part of decontamination strategies by augmenting the induction of spore germination and then rendering the germinated spores nonviable.

A genetic approach for the identification of exosporium assembly determinants of Bacillus anthracis

The exosporium is the outermost layer of spores of the zoonotic pathogen Bacillus anthracis. The composition of the exosporium and its functions are only partly understood. Because this outer spore layer is refractive to traditional biochemical analysis, a genetic approach is needed in order to define the proteins which comprise this important spore layer and its assembly pathway. We have created a novel genetic screening system for the identification and isolation of mutants with defects in exosporium assembly during B. anthracis spore maturation. The system is based on the targeting sequence of the BclA exosporium nap layer glycoprotein and a fluorescent reporter. By utilizing this screening system and gene inactivation with Tn916, several novel putative exosporium-associated determinants were identified. A sampling of the mutants obtained was further characterized, confirming their exosporium defect and validating the utility of this screen to identify novel spore determinants in the genome of this pathogen.

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.

Assembly of the BclB glycoprotein into the exosporium and evidence for its role in the formation of the exosporium 'cap' structure in Bacillus anthracis

The outermost layer of the Bacillus anthracis spore consists of an exosporium comprised of an outer hair-like nap layer and an internal basal layer. A major component of the hair-like nap is the glycosylated collagen-like protein BclA. A second collagen-like protein, BclB, is also present in the exosporium. BclB possesses an N-terminal sequence that targets it to the exosporium and is similar in sequence to a cognate targeting region in BclA. BclB lacks, however, sequence similarity to the region of BclA thought to mediate attachment to the basal layer via covalent interactions with the basal layer protein BxpB. Here we demonstrate that BxpB is critical for correct localization of BclB during spore formation and that the N-terminal domains of the BclA and BclB proteins compete for BxpB-controlled assembly sites. We found that BclB is located principally in a region of the exosporium that excludes a short arc on one side of the exosporium (the so-called bottle-cap region). We also found that in bclB mutant spores, the distribution of exosporium proteins CotY and BxpB is altered, suggesting that BclB has roles in exosporium assembly. In bclB mutant spores, the distance between the exosporium and the coat, the interspace, is reduced.

Characterization of a spore-specific protein of the Bacillus cereus group

Bc1245 is a monocistronic chromosomal gene of Bacillus cereus ATCC 14579 encoding a putative protein of 143 amino acids identified in this study to have a spore-related function in B. cereus. Bc1245 is highly conserved in the genome of members of the B. cereus group, indicating an important function of the gene in this group of bacteria. Quantitative PCR revealed that bc1245 is transcribed late in sporulation (upon formation of phase-bright spores) and at the same time as the mother cell-specific transcription factor σ(K) . The σ(K) regulon includes structural components of the spore (such as coat proteins), and it is therefore plausible that bc1245 might encode a structural outer spore protein. This was confirmed by detection of BC1245 in exosporium extracts from B. cereus by immunoblotting against BC1245 antiserum.

Surface architecture of endospores of the Bacillus cereus/anthracis/thuringiensis family at the subnanometer scale

Bacteria of the Bacillus cereus family form highly resistant spores, which in the case of the pathogen B. anthracis act as the agents of infection. The outermost layer, the exosporium, enveloping spores of the B. cereus family as well as a number of Clostridia, plays roles in spore adhesion, dissemination, targeting, and germination control. We have analyzed two naturally crystalline layers associated with the exosporium, one representing the "basal" layer to which the outermost spore layer ("hairy nap") is attached, and the other likely representing a subsurface ("parasporal") layer. We have used electron cryomicroscopy at a resolution of 0.8-0.6 nm and circular dichroism spectroscopic measurements to reveal a highly α-helical structure for both layers. The helices are assembled into 2D arrays of "cups" or "crowns." High-resolution atomic force microscopy of the outermost layer showed that the open ends of these cups face the external environment and the highly immunogenic collagen-like fibrils of the hairy nap (BclA) are attached to this surface. Based on our findings, we present a molecular model for the spore surface and propose how this surface can act as a semipermeable barrier and a matrix for binding of molecules involved in defense, germination control, and other interactions of the spore with the environment.

YwdL in Bacillus cereus: its role in germination and exosporium structure

In members of the Bacillus cereus group the outermost layer of the spore is the exosporium, which interacts with hosts and the environment. Efforts have been made to identify proteins of the exosporium but only a few have so far been characterised and their role in determining spore architecture and spore function is still poorly understood. We have characterised the exosporium protein, YwdL. ΔywdL spores have a more fragile exosporium, subject to damage on repeated freeze-thawing, although there is no evidence of altered resistance properties, and coats appear intact. Immunogold labelling and Western blotting with anti-YwdL antibodies identified YwdL to be located exclusively on the inner surface of the exosporium of B. cereus and B. thuringiensis. We conclude that YwdL is important for formation of a robust exosporium but is not required to maintain the crystalline assembly within the basal layer or for attachment of the hairy nap structure. ΔywdL spores are unable to germinate in response to CaDPA, and have altered germination properties, a phenotype that confirms the expected defect in localization of the cortex lytic enzyme CwlJ in the coat.

Localization and assembly of the novel exosporium protein BetA of Bacillus anthracis

The exosporium of Bacillus anthracis is comprised of two distinct layers: a basal layer and a hair-like nap that covers the basal layer. The hair-like nap contains the glycoproteins BclA and, most likely, BclB. BclA and BclB are directed to assemble into the exosporium by motifs in their N-terminal domains. Here, we identify a previously uncharacterized putative gene encoding this motif, which we have named betA (Bacillus exosporium-targeted protein of B. anthracis). Like bclA, betA encodes a putative collagenlike repeat region. betA is present in several genomes of exosporium-producing Bacillus species but, so far, not in any others. Using fluorescence microscopic localization of a BetA-enhanced green fluorescent protein (eGFP) fusion protein and immunofluorescence microscopy with anti-BetA antibodies, we showed that BetA resides in the exosporium basal layer, likely underneath BclA. BetA assembles at the spore surface at around hour 5 of sporulation and under the control of BxpB, similar to the control of deposition of BclA. We suggest a model in which BclA and BetA are incorporated into the exosporium by a mechanism that depends on their similar N termini. These data suggest that BetA is a member of a growing family of exosporium proteins that assemble under the control of targeting sequences in their N termini.

Current physical and SDS extraction methods do not efficiently remove exosporium proteins from Bacillus anthracis spores

Biochemical studies of the outermost spore layers of the Bacillus cereus family are hindered by difficulties in efficient dispersal of the external spore layers and difficulties in dissociating protein complexes that comprise the exosporium layer. Detergent and physical methods have been utilized to disrupt the exosporium layer. Herein we compare commonly used SDS extraction buffers used to extract spore proteins and demonstrate the incomplete extractability of the exosporium layer by these methods. Sonication and bead beating methods for exosporium layer removal were also examined. A combination of genetic and physical methods is the most effective for isolating proteins found in the spore exosporium.

The co-dependence of BxpB/ExsFA and BclA for proper incorporation into the exosporium of Bacillus anthracis

The outermost layer of the Bacillus anthracis spore consists of an exosporium comprised of two distinct layers, an outer hair-like nap layer and an internal basal layer. The hair-like nap is primarily comprised of the glycosylated collagen-like protein BclA. BclA is found in a trimeric form in close association with many other exosporium proteins in high-molecular weight complexes. We previously had characterized an N-terminal sequence of BclA that is sufficient for incorporation into the exosporium. Here we utilized site-directed mutagenesis to identify BclA residues critical to two steps in this process, positioning of the protein at the site of the developing exosporium basal layer and stable incorporation which includes a proteolytic cleavage of BclA after residue 19. The BxpB (ExsFA) protein is known to be important for proper incorporation of BclA onto the exosporium. BxpB and BclA were found to be expressed at the same time in sporulating cells of B. anthracis and immediately colocalize to high-molecular weight complexes. The BxpB protein was found to be in close proximity to the BclA NTD. BxpB and BclA are co-dependent for exosporium incorporation, with the BclA NTD being sufficient to deliver BxpB to the exosporium.

A novel spore protein, ExsM, regulates formation of the exosporium in Bacillus cereus and Bacillus anthracis and affects spore size and shape

Bacillus cereus spores are assembled with a series of concentric layers that protect them from a wide range of environmental stresses. The outermost layer, or exosporium, is a bag-like structure that interacts with the environment and is composed of more than 20 proteins and glycoproteins. Here, we identified a new spore protein, ExsM, from a beta-mercaptoethanol extract of B. cereus ATCC 4342 spores. Subcellular localization of an ExsM-green fluorescent protein (GFP) protein revealed a dynamic pattern of fluorescence that follows the site of formation of the exosporium around the forespore. Under scanning electron microscopy, exsM null mutant spores were smaller and rounder than wild-type spores, which had an extended exosporium (spore length for the wt, 2.40 +/- 0.56 microm, versus that for the exsM mutant, 1.66 +/- 0.38 microm [P < 0.001]). Thin-section electron microscopy revealed that exsM mutant spores were encased by a double-layer exosporium, both layers of which were composed of a basal layer and a hair-like nap. Mutant exsM spores were more resistant to lysozyme treatment and germinated with higher efficiency than wild-type spores, and they had a delay in outgrowth. Insertional mutagenesis of exsM in Bacillus anthracis DeltaSterne resulted in a partial second exosporium and in smaller spores. In all, these findings suggest that ExsM plays a critical role in the formation of the exosporium.

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.

ExsB, an unusually highly phosphorylated protein required for the stable attachment of the exosporium of Bacillus anthracis

The outermost layer of the Bacillus anthracis spore, the exosporium, is composed of a paracrystalline basal layer and an external hair-like nap. The nap is formed from a single collagen-like glycoprotein, while the basal layer contains many different proteins, including a 186-amino acid protein called ExsB. In this study, we discovered that ExsB is unusually highly phosphorylated, with at least 14 of its 19 threonine residues modified. The phosphorylated threonines are included in seven contiguous approximately 12-residue imperfect repeats, which presumably contain kinase recognition sequences. We demonstrated that a B. anthracis DeltaexsB mutant unable to synthesize ExsB produced spores with an exosporium that was readily sloughed, indicating that ExsB was required for stable exosporium attachment. This unstable exosporium also lacked the enzyme alanine racemase, which is normally tightly associated with the exosporium. Additionally, purified DeltaexsB spores lacking a visible exosporium were devoid of most exosporium proteins but, surprisingly, retained the putative exosporium proteins BxpC and CotB-1. Finally, we showed that transcription of the exsB gene occurred only during the late stages of sporulation, and we used an active and phosphorylated ExsB-EGFP fusion protein to monitor ExsB localization to wild-type and DeltabxpB mutant exosporia.

Prevalence of Bacillus anthracis-like organisms and bacteriophages in the intestinal tract of the earthworm Eisenia fetida

Stable infection of Bacillus anthracis laboratory strains with environmental bacteriophages confers survival phenotypes in soil and earthworm intestinal niches (R. Schuch and V. A. Fischetti, PLoS One 4:e6532, 2009). Here, the natural occurrence of two such B. anthracis-infective bacteriophages, Wip1 and Wip4, was examined in the intestines of Eisenia fetida earthworms as part of a 6-year longitudinal study at a Pennsylvania forest site. The Wip1 tectivirus was initially dominant before being supplanted by the Wip4 siphovirus, which was then dominant for the next 3 years. In a host range analysis of a wide-ranging group of Bacillus species and related organisms, Wip1 and Wip4 were both infective only toward B. anthracis and certain B. cereus strains. The natural host of Wip4 remained constant for 3 years and was a B. cereus strain that expressed a B. anthracis-like surface polysaccharide at septal positions on the cell surface. Next, a novel metagenomic approach was used to determine the extent to which such B. cereus- and B. anthracis-like strains are found in worms from two geographical locations. Three different enrichment strategies were used for metagenomic DNA isolation, based either on the ability of B. cereus sensu lato to form heat-resistant spores, the sensitivity of B. anthracis to the PlyG lysin, or the selective amplification of environmental phages cocultured with B. anthracis. Findings from this work indicate that B. cereus sensu lato and its phages are common inhabitants of earthworm intestines.

Sequence motifs and proteolytic cleavage of the collagen-like glycoprotein BclA required for its attachment to the exosporium of Bacillus anthracis

Bacillus anthracis spores are enclosed by an exosporium comprised of a basal layer and an external hair-like nap. The filaments of the nap are composed of trimers of the collagen-like glycoprotein BclA. The attachment of essentially all BclA trimers to the exosporium requires the basal layer protein BxpB, and both proteins are included in stable high-molecular-mass exosporium complexes. BclA contains a proteolytically processed 38-residue amino-terminal domain (NTD) that is essential for basal-layer attachment. In this report, we identify three NTD submotifs (SM1a, SM1b, and SM2, located within residues 21 to 33) that are important for BclA attachment and demonstrate that residue A20, the amino-terminal residue of processed BclA, is not required for attachment. We show that the shortest NTD of BclA-or of a recombinant protein-sufficient for high-level basal-layer attachment is a 10-residue motif consisting of an initiating methionine, an apparently arbitrary second residue, SM1a or SM1b, and SM2. We also demonstrate that cleavage of the BclA NTD is necessary for efficient attachment to the basal layer and that the site of cleavage is somewhat flexible, at least in certain mutant NTDs. Finally, we propose a mechanism for BclA attachment and discuss the possibility that analogous mechanisms are involved in the attachment of many different collagen-like proteins of B. anthracis and closely related Bacillus species.

Processing, assembly and localization of a Bacillus anthracis spore protein

All Bacillus spores are encased in macromolecular shells. One of these is a proteinacious shell called the coat that, in Bacillus subtilis, provides critical protective functions. The Bacillus anthracis spore is the infectious particle for the disease anthrax. Therefore, the coat is of particular interest because it may provide essential protective functions required for the appearance of anthrax. Here, we analyse a protein component of the spore outer layers that was previously designated BxpA. Our data indicate that a significant amount of BxpA is located below the spore coat and associated with the cortex. By SDS-PAGE, BxpA migrates as a 9 kDa species when extracted from Sterne strain spores, and as 11 and 14 kDa species from Ames strain spores, even though it has predicted masses of 27 and 29 kDa, respectively, in these two strains. We investigated the possibility that BxpA is subject to post-translational processing as previously suggested. In B. subtilis, a subset of coat proteins is proteolysed or cross-linked by the spore proteins YabG or Tgl, respectively. To investigate the possibility that similar processing occurs in B. anthracis, we generated mutations in the yabG or tgl genes in the Sterne and Ames strains and analysed the consequences for BxpA assembly by SDS-PAGE. We found that in a tgl mutant of B. anthracis, the apparent mass of BxpA increased. This is consistent with the possibility that Tgl directs the cross-linking of BxpA into a form that normally does not enter the gel. Unexpectedly, the apparent mass of BxpA also increased in a yabG mutant, suggesting a relatively complex role for proteolysis in spore protein maturation in B. anthracis. These data reveal a previously unobserved event in spore protein maturation in B. anthracis. We speculate that proteolysis and cross-linking are ubiquitous spore assembly mechanisms throughout the genus Bacillus.

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.

Localization and assembly of proteins comprising the outer structures of the Bacillus anthracis spore

Bacterial spores possess a series of concentrically arranged protective structures that contribute to dormancy, survival and, ultimately, germination. One of these structures, the coat, is present in all spores. In Bacillus anthracis, however, the spore is surrounded by an additional, poorly understood, morphologically complex structure called the exosporium. Here, we characterize three previously discovered exosporium proteins called ExsFA (also known as BxpB), ExsFB (a highly related paralogue of exsFA/bxpB) and IunH (similar to an inosine-uridine-preferring nucleoside hydrolase). We show that in the absence of ExsFA/BxpB, the exosporium protein BclA accumulates asymmetrically to the forespore pole closest to the midpoint of the sporangium (i.e. the mother-cell-proximal pole of the forespore), instead of uniformly encircling the exosporium. ExsFA/BxpB may also have a role in coat assembly, as mutant spore surfaces lack ridges seen in wild-type spores and have a bumpy appearance. ExsFA/BxpB also has a modest but readily detected effect on germination. Nonetheless, an exsFA/bxpB mutant strain is fully virulent in both intramuscular and aerosol challenge models in Guinea pigs. We show that the pattern of localization of ExsFA/BxpB-GFP is a ring, consistent with a location for this protein in the basal layer of the exosporium. In contrast, ExsFB-GFP fluorescence is a solid oval, suggesting a distinct subcellular location for ExsFB-GFP. We also used these fusion proteins to monitor changes in the subcellular locations of these proteins during sporulation. Early in sporulation, both fusions were present throughout the mother cell cytoplasm. As sporulation progressed, GFP fluorescence moved from the mother cell cytoplasm to the forespore surface and formed either a ring of fluorescence, in the case of ExsFA/BxpB, or a solid oval of fluorescence, in the case of ExsFB. IunH-GFP also resulted in a solid oval of fluorescence. We suggest the interpretation that at least some ExsFB-GFP and IunH-GFP resides in the region between the coat and the exosporium, called the interspace.

The spore-specific alanine racemase of Bacillus anthracis and its role in suppressing germination during spore development

Spores of Bacillus anthracis are enclosed by an exosporium composed of a basal layer and an external hair-like nap. The nap is apparently formed by a single glycoprotein, while the basal layer contains many different structural proteins and several enzymes. One of the enzymes is Alr, an alanine racemase capable of converting the spore germinant l-alanine to the germination inhibitor d-alanine. Unlike other characterized exosporium proteins, Alr is nonuniformly distributed in the exosporium and might have a second spore location. In this study, we demonstrated that expression of the alr gene, which encodes Alr, is restricted to sporulating cells and that the bulk of alr transcription and Alr synthesis occurs during the late stages of sporulation. We also mapped two alr promoters that are differentially active during sporulation and might be involved in the atypical localization of Alr. Finally, we constructed a Deltaalr mutant of B. anthracis that lacks Alr and examined the properties of the spores produced by this strain. Mature Deltaalr spores germinate more efficiently in the presence of l-alanine, presumably because of their inability to convert exogenous l-alanine to d-alanine, but they respond normally to other germinants. Surprisingly, the production of mature spores by the Deltaalr mutant is defective because approximately one-half of the nascent spores germinate and lose their resistance properties before they are released from the mother cell. This phenotype suggests that an important function of Alr is to produce D-alanine during the late stages of sporulation to suppress premature germination of the developing spore.