Role played by exosporium glycoproteins in the surface properties of Bacillus cereus spores and in their adhesion to stainless steel

Sporulation conditions influence the surface and adhesion properties of Bacillus subtilis spores

Spore-forming bacteria of the Bacillus subtilis group are responsible for recurrent contamination of processing lines in the food industry which can lead to food spoilage. The persistence of B. subtilis would be due to the high resistance of spores to extreme environmental condition and their propensity to contaminate surfaces. While it is well known that sporulation conditions modulate spore resistance properties, little is known about their effect on surface and adhesion properties. Here, we studied the impact of 13 sporulation conditions on the surface and adhesion properties of B. subtilis 168 spores. We showed that Ca2+ or Mg2+ depletion, lower oxygen availability, acidic pH as well as oxidative stresses during sporulation lead to the release of more hydrophobic and adherent spores. The consequences of these sporulation conditions on crust composition in carbohydrates and proteins were also evaluated. The crust glycans of spores produced in a sporulation medium depleted in Ca2+ or Mg2+ or oxygen-limited conditions were impaired and contained lower amounts of rhamnose and legionaminic acid. In addition, we showed that lower oxygen availability or addition of hydrogen peroxide during sporulation decreases the relative amount of two crust proteins (CgeA and CotY) and the changes observed in these conditions could be due to transcriptional repression of genes involved in crust synthesis in late stationary phase. The fact that sporulation conditions affect the ease with which spores can contaminate surfaces could explain the frequent and recurrent presence of B. subtilis spores in food processing lines.

Endospore pili: Flexible, stiff, and sticky nanofibers

Species belonging to the Bacillus cereus group form endospores (spores) whose surface is decorated with micrometers-long and nanometers-wide endospore appendages (Enas). The Enas have recently been shown to represent a completely novel class of Gram-positive pili. They exhibit remarkable structural properties making them extremely resilient to proteolytic digestion and solubilization. However, little is known about their functional and biophysical properties. In this work, we apply optical tweezers to manipulate and assess how wild-type and Ena-depleted mutant spores immobilize on a glass surface. Furthermore, we utilize optical tweezers to extend S-Ena fibers to measure their flexibility and tensile stiffness. Finally, by oscillating single spores, we examine how the exosporium and Enas affect spores' hydrodynamic properties. Our results show that S-Enas (μm-long pili) are not as effective as L-Enas in immobilizing spores to glass surfaces but are involved in forming spore-to-spore connections, holding the spores together in a gel-like state. The measurements also show that S-Enas are flexible but tensile stiff fibers, which support structural data suggesting that the quaternary structure is composed of subunits arranged in a complex to produce a bendable fiber (helical turns can tilt against each other) with limited axial fiber extensibility. Finally, the results show that the hydrodynamic drag is 1.5 times higher for wild-type spores expressing S- and L-Enas compared with mutant spores expressing only L-Enas or "bald spores" lacking Ena, and 2 times higher compared with spores of the exosporium-deficient strain. This study unveils novel findings on the biophysics of S- and L-Enas, their role in spore aggregation, binding of spores to glass, and their mechanical behavior upon exposure to drag forces.

New Thoughts on an Old Topic: Secrets of Bacterial Spore Resistance Slowly Being Revealed

The quest for bacterial survival is exemplified by spores formed by some Firmicutes members. They turn up everywhere one looks, and their ubiquity reflects adaptations to the stresses bacteria face. Spores are impactful in public health, food safety, and biowarfare. Heat resistance is the hallmark of spores and is countered principally by a mineralized gel-like protoplast, termed the spore core, with reduced water which minimizes macromolecular movement/denaturation/aggregation. Dry heat, however, introduces mutations into spore DNA. Spores have countermeasures to extreme conditions that are multifactorial, but the fact that spore DNA is in a crystalline-like nucleoid in the spore core, likely due to DNA saturation with small acid-soluble spore proteins (SASPs), suggests that reduced macromolecular motion is also critical in spore dry heat resistance. SASPs are also central in the radiation resistance characteristic of spores, where the contributions of four spore features-SASP; Ca2+, with pyridine-2,6-dicarboxylic acid (CaDPA); photoproduct lyase; and low water content-minimize DNA damage. Notably, the spore environment steers UV photochemistry toward a product that germinated spores can repair without significant mutagenesis. This resistance extends to chemicals and macromolecules that could damage spores. Macromolecules are excluded by the spore coat which impedes the passage of moieties of ≥10 kDa. Additionally, damaging chemicals may be degraded or neutralized by coat enzymes/proteins. However, the principal protective mechanism here is the inner membrane, a compressed structure lacking lipid fluidity and presenting a barrier to the diffusion of chemicals into the spore core; SASP saturation of DNA also protects against genotoxic chemicals. Spores are also resistant to other stresses, including high pressure and abrasion. Regardless, overarching mechanisms associated with resistance seem to revolve around reduced molecular motion, a fine balance between rigidity and flexibility, and perhaps efficient repair.

Sporulation Strategies and Potential Role of the Exosporium in Survival and Persistence of Clostridium botulinum

Clostridium botulinum produces the botulinum neurotoxin that causes botulism, a rare but potentially lethal paralysis. Endospores play an important role in the survival, transmission, and pathogenesis of C. botulinum. C. botulinum strains are very diverse, both genetically and ecologically. Group I strains are terrestrial, mesophilic, and produce highly heat-resistant spores, while Group II strains can be terrestrial (type B) or aquatic (type E) and are generally psychrotrophic and produce spores of moderate heat resistance. Group III strains are either terrestrial or aquatic, mesophilic or slightly thermophilic, and the heat resistance properties of their spores are poorly characterized. Here, we analyzed the sporulation dynamics in population, spore morphology, and other spore properties of 10 C. botulinum strains belonging to Groups I–III. We propose two distinct sporulation strategies used by C. botulinum Groups I–III strains, report their spore properties, and suggest a putative role for the exosporium in conferring high heat resistance. Strains within each physiological group produced spores with similar characteristics, likely reflecting adaptation to respective environmental habitats. Our work provides new information on the spores and on the population and single-cell level strategies in the sporulation of C. botulinum.

Identification of Adhesins in Plant Beneficial Rhizobacteria Bacillus velezensis SQR9 and Their Effect on Root Colonization

Probiotic Bacillus colonization of plant root surfaces has been reported to improve its beneficial effect. Chemotaxis, adhesion, aggregation, and biofilm formation are the four steps of root colonization by plant growth-promoting rhizobacteria (PGPRs). Compared with the other three well-studied processes, adhesion of PGPRs is less known. In this study, using mutant strains deleted for potential adhesin genes in PGPR strain Bacillus velezensis SQR9, adherence to both cucumber root surface and abiotic surface by those strains was evaluated. Results showed that deletion mutations ΔlytB, ΔV529_10500, ΔfliD, ΔyhaN, and ΔsacB reduced the adhesion to root surfaces, while, among them, only ΔfliD had significant defects in adhesion to abiotic surfaces (glass and polystyrene). In addition, B. velevzensis SQR9 mutants defective in adhesion to root surfaces showed a deficiency in rhizosphere colonization. Among the encoded proteins, FliD and YhaN played vital roles in root adhesion. This research systematically explored the potential adhesins in a well-studied PGPR strain and also indicated that adhesion progress was required for root colonization, which will help to enhance rhizosphere colonization and beneficial function of PGPRs in agricultural production.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Enigmatic Pilus-Like Endospore Appendages of Bacillus cereus Group Species

The endospores (spores) of many Bacillus cereus sensu lato species are decorated with multiple hair/pilus-like appendages. Although they have been observed for more than 50 years, all efforts to characterize these fibers in detail have failed until now, largely due to their extraordinary resilience to proteolytic digestion and chemical solubilization. A recent structural analysis of B. cereus endospore appendages (Enas) using cryo-electron microscopy has revealed the structure of two distinct fiber morphologies: the longer and more abundant "Staggered-type" (S-Ena) and the shorter "Ladder-like" type (L-Ena), which further enabled the identification of the genes encoding the S-Ena. Ena homologs are widely and uniquely distributed among B. cereus sensu lato species, suggesting that appendages play important functional roles in these species. The discovery of ena genes is expected to facilitate functional studies involving Ena-depleted mutant spores to explore the role of Enas in the interaction between spores and their environment. Given the importance of B. cereus spores for the food industry and in medicine, there is a need for a better understanding of their biological functions and physicochemical properties. In this review, we discuss the current understanding of the Ena structure and the potential roles these remarkable fibers may play in the adhesion of spores to biotic and abiotic surfaces, aggregation, and biofilm formation.

Convergent evolution of diverse Bacillus anthracis outbreak strains toward altered surface oligosaccharides that modulate anthrax pathogenesis

Bacillus anthracis, a spore-forming gram-positive bacterium, causes anthrax. The external surface of the exosporium is coated with glycosylated proteins. The sugar additions are capped with the unique monosaccharide anthrose. The West African Group (WAG) B. anthracis have mutations rendering them anthrose deficient. Through genome sequencing, we identified 2 different large chromosomal deletions within the anthrose biosynthetic operon of B. anthracis strains from Chile and Poland. In silico analysis identified an anthrose-deficient strain in the anthrax outbreak among European heroin users. Anthrose-deficient strains are no longer restricted to West Africa so the role of anthrose in physiology and pathogenesis was investigated in B. anthracis Sterne. Loss of anthrose delayed spore germination and enhanced sporulation. Spores without anthrose were phagocytized at higher rates than spores with anthrose, indicating that anthrose may serve an antiphagocytic function on the spore surface. The anthrose mutant had half the LD50 and decreased time to death (TTD) of wild type and complement B. anthracis Sterne in the A/J mouse model. Following infection, anthrose mutant bacteria were more abundant in the spleen, indicating enhanced dissemination of Sterne anthrose mutant. At low sample sizes in the A/J mouse model, the mortality of ΔantC-infected mice challenged by intranasal or subcutaneous routes was 20% greater than wild type. Competitive index (CI) studies indicated that spores without anthrose disseminated to organs more extensively than a complemented mutant. Death process modeling using mouse mortality dynamics suggested that larger sample sizes would lead to significantly higher deaths in anthrose-negative infected animals. The model was tested by infecting Galleria mellonella with spores and confirmed the anthrose mutant was significantly more lethal. Vaccination studies in the A/J mouse model showed that the human vaccine protected against high-dose challenges of the nonencapsulated Sterne-based anthrose mutant. This work begins to identify the physiologic and pathogenic consequences of convergent anthrose mutations in B. anthracis.

Laboratory results and mathematical modeling of spore surface interactions in stormwater runoff

Development of numerical models to predict stormwater-mediated transport of pathogenic spores in the environment depends on an understanding of adhesion forces that dictate detachment after rain events. Zeta potential values were measured in the laboratory for Bacillus globigii and Bacillus thuringiensis kurstaki, two common surrogates used to represent Bacillus anthracis, in synthetic baseline ultrapure water and laboratory prepared stormwater. Zeta potential curves were also determined for materials representative of urban infrastructure (concrete and asphalt). These data were used to predict the interaction energy between the spores and urban materials using Derjaguin-Landau-Verwey-Overbeek (DLVO) modeling. B. globigii and B. thuringiensis kurstaki sourced from Yakibou Inc., were found to have similar zeta potential curves, whereas spores sourced from the U.S. military's Dugway laboratory were found to diverge. In the ultrapure water, the modeling results use the laboratory data to demonstrate that the energy barriers between the spores and the urban materials were tunable through compression of the electrical double layer of the spores via changes of ionic strength and pH of the water. In the runoff water, charge neutralization dominated surface processes. The cations, metals, and natural organic matter (NOM) in the runoff water contributed to equalizing the zeta potential values for Dugway B. globigii and B. thuringiensis kurstaki, and drastically modified the surface of the concrete and asphalt. All DLVO energy curves using the runoff water were repulsive. The highest energy barrier predicted in this study was for Dugway B. globigii spores interacting with a concrete surface in runoff water, suggesting that this would be the most challenging combination to detach through water-based decontamination.

Linking Geospatial and Laboratory Sciences to Define Mechanisms behind Landscape Level Drivers of Anthrax Outbreaks

Background: A seasonal predictor of anthrax outbreaks is rainfall, which may be approximated by NDVI using remote sensing. How rainfall or vegetative green-up influences bacterial physiology or microecology to drive anthrax outbreaks is not known. Methods: Rainfall and NDVI dependency of anthrax epizootics was demonstrated with global and local phenological analysis. Growth analysis of B. anthracis in response to pH and calcium gradients was carried out. The influence of pH and calcium levels on expression of toxin and sporulation related proteins in broth culture models was characterized using engineered B. anthracis luminescent reporter strains. Results: Short-term bacterial growth and longer-term bacterial survival were altered by pH and calcium. These conditions also played a major role in pagA and sspB promoter-driven luminescent expression in B. anthracis. Conclusions: Rainfall induced cycling of pH and calcium in soils plays a plausible role in amplifying spore load and persistence in endemic anthrax zones. Observed evidence of B. anthracis favoring soil alkalinity and high soil calcium levels in the environment were linked to physiological conditions that promote bacterial growth, survival, toxin secretion and spore formation; illustrating the utility of bringing laboratory-based (controlled) microbiology experiments into the fold of zoonotic disease ecology.

Contributions of crust proteins to spore surface properties in Bacillus subtilis

Surface properties, such as adhesion and hydrophobicity, constrain dispersal of bacterial spores in the environment. In Bacillus subtilis, these properties are influenced by the outermost layer of the spore, the crust. Previous work has shown that two clusters, cotVWXYZ and cgeAB, encode the protein components of the crust. Here, we characterize the respective roles of these genes in surface properties using Bacterial Adherence to Hydrocarbons assays, negative staining of polysaccharides by India ink and Transmission Electron Microscopy. We showed that inactivation of crust genes caused increases in spore relative hydrophobicity, disrupted the spore polysaccharide layer, and impaired crust structure and attachment to the rest of the coat. We also found that cotO, previously identified for its role in outer coat formation, is necessary for proper encasement of the spore by the crust. In parallel, we conducted fluorescence microscopy experiments to determine the full network of genetic dependencies for subcellular localization of crust proteins. We determined that CotZ is required for the localization of most crust proteins, while CgeA is at the bottom of the genetic interaction hierarchy.

Assessing the Impact of Germination and Sporulation Conditions on the Adhesion of Bacillus Spores to Glass and Stainless Steel by Fluid Dynamic Gauging

The adhesion of spores of 3 Bacillus species with distinctive morphologies to stainless steel and borosilicate glass was studied using the fluid dynamic gauging technique. Marked differences were observed between different species of spores, and also between spores of the same species prepared under different sporulation conditions. Spores of the food-borne pathogen B. cereus were demonstrated to be capable of withstanding shear stresses greater than 1500 Pa when adhered to stainless steel, in contrast to spores of Bacillus subtilis and Bacillus megaterium, which detached in response to lower shear stress. An extended DLVO model was shown to be capable of predicting the relative differences in spore adhesion between spores of different species and different culture conditions, but did not predict absolute values of force of adhesion well. Applying the model to germinating spores showed a significant reduction in adhesion force shortly after triggering germination, indicating a potential strategy to achieve enhanced removal of spores from surfaces in response to shear stress, such as during cleaning-in-place procedures.

PRACTICAL APPLICATION

Spore-forming bacteria are a concern to the food industry because they have the potential to cause food-borne illness and product spoilage, while being strongly adhesive to processing surfaces and resistant to cleaning-in-place procedures. This work is of significance to the food processors and manufacturers because it offers insight to the properties of spore adhesion and identifies a potential strategy to facilitate the removal of spores during cleaning procedures.

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.

A four-gene operon in Bacillus cereus produces two rare spore-decorating sugars

Bacterial glycan structures on cell surfaces are critical for cell-cell recognition and adhesion and in host-pathogen interactions. Accordingly, unraveling the sugar composition of bacterial cell surfaces can shed light on bacterial growth and pathogenesis. Here, we found that two rare sugars with a 3-C-methyl-6-deoxyhexose structure were linked to spore glycans in Bacillus cereus ATCC 14579 and ATCC 10876. Moreover, we identified a four-gene operon in B. cereus ATCC 14579 that encodes proteins with the following sequential enzyme activities as determined by mass spectrometry and one- and two-dimensional NMR methods: CTP:glucose-1-phosphate cytidylyltransferase, CDP-Glc 4,6-dehydratase, NADH-dependent SAM:C-methyltransferase, and NADPH-dependent CDP-3-C-methyl-6-deoxyhexose 4-reductase. The last enzyme predominantly yielded CDP-3-C-methyl-6-deoxygulose (CDP-cereose) and likely generated a 4-epimer CDP-3-C-methyl-6-deoxyallose (CDP-cillose). Some members of the B. cereus sensu lato group produce CDP-3-C-methyl-6-deoxy sugars for the formation of cereose-containing glycans on spores, whereas others such as Bacillus anthracis do not. Gene knockouts of the Bacillus C-methyltransferase and the 4-reductase confirmed their involvement in the formation of cereose-containing glycan on B. cereus spores. We also found that cereose represented 0.2-1% spore dry weight. Moreover, mutants lacking cereose germinated faster than the wild type, yet the mutants exhibited no changes in sporulation or spore resistance to heat. The findings reported here may provide new insights into the roles of the uncommon 3-C-methyl-6-deoxy sugars in cell-surface recognition and host-pathogen interactions of the genus Bacillus.

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.

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.

Glycosylation of BclA Glycoprotein from Bacillus cereus and Bacillus anthracis Exosporium Is Domain-specific

The spores of the Bacillus cereus group (B. cereus, Bacillus anthracis, and Bacillus thuringiensis) are surrounded by a paracrystalline flexible yet resistant layer called exosporium that plays a major role in spore adhesion and virulence. The major constituent of its hairlike surface, the trimerized glycoprotein BclA, is attached to the basal layer through an N-terminal domain. It is then followed by a repetitive collagen-like neck bearing a globular head (C-terminal domain) that promotes glycoprotein trimerization. The collagen-like region of B. anthracis is known to be densely substituted by unusual O-glycans that may be used for developing species-specific diagnostics of B. anthracis spores and thus targeted therapeutic interventions. In the present study, we have explored the species and domain specificity of BclA glycosylation within the B. cereus group. First, we have established that the collagen-like regions of both B. anthracis and B. cereus are similarly substituted by short O-glycans that bear the species-specific deoxyhexose residues anthrose and the newly observed cereose, respectively. Second we have discovered that the C-terminal globular domains of BclA from both species are substituted by polysaccharide-like O-linked glycans whose structures are also species-specific. The presence of large carbohydrate polymers covering the surface of Bacillus spores may have a profound impact on the way that spores regulate their interactions with biotic and abiotic surfaces and represents potential new diagnostic targets.

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.

Single cell profiling of surface carbohydrates on Bacillus cereus

Cell surface carbohydrates are important to various bacterial activities and functions. It is well known that different types of Bacillus display heterogeneity of surface carbohydrate compositions, but detection of their presence, quantitation and estimation of variation at the single cell level have not been previously solved. Here, using atomic force microscopy (AFM)-based recognition force mapping coupled with lectin probes, the specific carbohydrate distributions of N-acetylglucosamine and mannose/glucose were detected, mapped and quantified on single B. cereus surfaces at the nanoscale across the entire cell. Further, the changes of the surface carbohydrate compositions from the vegetative cell to spore were shown. These results demonstrate AFM-based 'recognition force mapping' as a versatile platform to quantitatively detect and spatially map key bacterial surface biomarkers (such as carbohydrate compositions), and monitor in situ changes in surface biochemical properties during intracellular activities at the single cell level.

Apertures in the Clostridium sporogenes spore coat and exosporium align to facilitate emergence of the vegetative cell

Clostridium sporogenes forms highly heat resistant endospores, enabling this bacterium to survive adverse conditions. Subsequently, spores may germinate, giving rise to vegetative cells that multiply and lead to food spoilage. Electron microscopy was used to visualise changes in spore structures during germination, emergence and outgrowth. C. sporogenes spores were surrounded by an exosporium that was oval in shape and typically 3 μm in length. An aperture of 0.3–0.4 μm was observed at one end of the exosporium. The rupture of the spore coats occurs adjacent to the opening in the exosporium. The germinated cell emerges through this hole in the spore coat and then through the pre-existing aperture in the exosporium, before eventually being released, leaving behind a largely intact exosporium with an enlarged aperture (0.7–1.0 μm) and coat shell. The formation of this aperture, its function and its alignment with the spore coat is discussed.

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Electron microscopy was used to study structures of Clostridium sporogenes spores.

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Spore exosporia possessed either a terminal aperture or a lipped protrusion.

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Cells emerged through the preformed aperture or sporiduct of the spore.

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Apertures in the spore coat and exosporium were aligned, and may be predetermined.

The effect of selected factors on the survival of Bacillus cereus in the human gastrointestinal tract

Bacillus cereus is a Gram-positive bacterium widely distributed in soil and vegetation. This bacterial species can also contaminate raw or processed foods. Pathogenic B. cereus strains can cause a range of infections in humans, as well as food poisoning of an emetic (intoxication) or diarrheal type (toxico-infection). Toxico-infections are due to the action of the Hbl toxin, Nhe toxin, and cytotoxin K produced by the microorganism in the gastrointestinal tract. This occurs once the spores or vegetative B. cereus cells survive the pH barrier of the stomach and reach the small intestine where they produce toxins in sufficient amounts. This article discusses the effect of various factors on the survival of B. cereus in the gastrointestinal tract, including low pH and the presence of digestive enzymes in the stomach, bile salts in the small intestine, and indigenous microflora in the lower parts of the gastrointestinal tract. Additional aspects also reported to affect B. cereus survival and virulence in the gastrointestinal tract include the interaction of the spores and vegetative cells with enterocytes. In vitro studies revealed that both vegetative B. cereus and spores can survive in the gastrointestinal tract suggesting that the biological form of the microorganism may have less influence on the occurrence of the symptoms of infection than was once believed. It is most likely the interaction between the pathogen and enterocytes that is necessary for the diarrheal form of B. cereus food poisoning to develop. The adhesion of B. cereus to the intestinal epithelium allows the bacterium to grow and produce enterotoxins in the proximity of the epithelium. Recent studies suggest that the human intestinal microbiota inhibits the growth of vegetative B. cereus cells considerably.

Endospore surface properties of commonly used Bacillus anthracis surrogates vary in aqueous solution

The hydrophobic character and electrophoretic mobility (EPM) of microorganisms are vital aspects of understanding their interactions with the environment. These properties are fundamental in fate-and-transport, physiological, and virulence studies, and thus integral in surrogate selection. Hydrophobic and electrostatic forces are significant contributors to particle and microorganism mobility in the environment. Herein, the surface properties of commonly used Bacillus anthracis surrogate endospores were tested under comparable conditions with respect to culture, endospore purification, buffer type and strength. Additionally, data is presented of endospores suspended in dechlorinated tap water to evaluate the surrogates in regard to a breach of water infrastructure security. The surface properties of B. anthracis were found to be the most hydrophobic and least electronegative among the six Bacillus species tested across buffer strength. The effect of EPM on hydrophobicity varies in a species-specific manner. This study demonstrates that surrogate surface properties differ and care must be taken when choosing the most suitable surrogate. Moreover, it is shown that Bacillus thuringensis best represents Bacillus anthracis-Sterne with respect to both EPM and hydrophobicity across all test buffers.

Presence and function of a thick mucous layer rich in polysaccharides around Bacillus subtilis spores

This study was designed to establish the presence and function of the mucous layer surrounding spores of Bacillus subtilis. First, an external layer of variable thickness and regularity was often observed on B. subtilis spores. Further analyses were performed on B. subtilis 98/7 spores surrounded by a thick layer. The mechanical removal of the layer did not affect their resistance to heat or their ability to germinate but rendered the spore less hydrophilic, more adherent to stainless steel, and more resistant to cleaning. This layer was mainly composed of 6-deoxyhexoses, ie rhamnose, 3-O-methyl-rhamnose and quinovose, but also of glucosamine and muramic lactam, known also to be a part of the bacterial peptidoglycan. The specific hydrolysis of the peptidoglycan using lysozyme altered the structure of the required mucous layer and affected the physico-chemical properties of the spores. Such an outermost mucous layer has also been seen on spores of B. licheniformis and B. clausii isolated from food environments.

Bacillus thuringiensis as a surrogate for Bacillus anthracis in aerosol research

Characterization of candidate surrogate spores prior to experimental use is critical to confirm that the surrogate characteristics are as closely similar as possible to those of the pathogenic agent of interest. This review compares the physical properties inherent to spores of Bacillus anthracis (Ba) and Bacillus thuringiensis (Bt) that impact their movement in air and interaction with surfaces, including size, shape, density, surface morphology, structure and hydrophobicity. Also evaluated is the impact of irradiation on the physical properties of both Bacillus species. Many physical features of Bt and Ba have been found to be similar and, while Bt is considered typically non-pathogenic, it is in the B. cereus group, as is Ba. When cultured and sporulated under similar conditions, both microorganisms share a similar cylindrical pellet shape, an aerodynamic diameter of approximately 1 μm (in the respirable size range), have an exosporium with a hairy nap, and have higher relative hydrophobicities than other Bacillus species. While spore size, morphology, and other physical properties can vary among strains of the same species, the variations can be due to growth/sporulation conditions and may, therefore, be controlled. Growth and sporulation conditions are likely among the most important factors that influence the representativeness of one species, or preparation, to another. All Bt spores may, therefore, not be representative of all Ba spores. Irradiated spores do not appear to be a good surrogate to predict the behavior of non-irradiated spores due to structural damage caused by the irradiation. While the use of Bt as a surrogate for Ba in aerosol testing appears to be well supported, this review does not attempt to narrow selection between Bt strains. Comparative studies should be performed to test the hypothesis that viable Ba and Bt spores will behave similarly when suspended in the air (as an aerosol) and to compare the known microscale characteristics versus the macroscale response.

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.

The role of the exosporium in the environmental distribution of anthrax

AIMS

To determine the contribution of the exosporium, the outer layer of the Bacillus anthracis spore, to soil attachment. Persistence of spores in soil and their ability to infect animals has been linked to a range of factors which include the presence of organic material and calcium (OMC), pH > 6.0, temperatures above 15.5°C and cycles of local flooding which are thought to transport buried spores to the surface.

METHODS AND RESULTS

The ability of wild type (exosporium +ve) and sonicated (exosporium -ve) spores to bind to soils which differed in their composition was determined using a flow-through soil column-based method. A statistically significant difference (P < 0.05) in the binding of wild type spores was observed with spores adhering more firmly to the soil with the highest OMC content. We also found that the removal of the exosporium increased the ability of the spore to adhere to both soil types.

CONCLUSION

Structures within the exosporium affected the ability of B. anthracis spores to bind to different soil types. Not surprisingly, wild type spores adhered to soil which has been shown to favour the persistence of the pathogen.

SIGNIFICANCE AND IMPACT OF THE STUDY

The ability to persist in and colonise the soil surface is a key requirement of a pathogen which infects grazing animals. By characterising the process involved, we will be better placed to develop strategies to disrupt the infection cycle.