The Bacillus subtilis germinant receptor GerA triggers premature germination in response to morphological defects during sporulation

Germination of Bacillus spores by LiCl

Spores of Bacillus subtilis have been found to germinate when incubated with LiCl, but not with other monovalent or divalent metal cations. Bacillus megaterium spores also germinated with LiCl, but B. cereus spores did not. In B. subtilis, the LiCl germination was via the activation of spores' GerA germinant receptor (GR), and in B. megaterium, it was the GerU GR. Notably, LiCl germination was much slower than normal physiological germinant triggered GR germination. In B. subtilis spores, rates of LiCl germination were increased in spores with a more fluid IM and decreased in spores with a less fluid IM. Analyses of the GerA germinant binding site suggested that Li+ could bind in a specific site in the B. subtilis GerAB subunit where normally a Na+ likely binds. Importantly, NaCl strongly inhibited LiCl germination of B. subtilis spores, much more so than the larger cation in KCl, although neither salt inhibited L-alanine germination via the GerA GR. These findings increase the understanding of features of mechanisms of germination of Bacillus spores.IMPORTANCEThe ability of some bacteria to form spores upon nutrient starvation confers properties of metabolic dormancy and enhanced resistance to environmental stressors that would otherwise kill vegetative cells. Since spore-forming bacteria include several notable pathogens and economically significant spoilage organisms, insight into how spores are stimulated to germinate and form new vegetative cells is important. Here, we reveal that relatively high concentrations of the inorganic salt lithium chloride trigger the germination of Bacillus subtilis and Bacillus megaterium spores by stimulating one of the spores of each species cohort of nutrient germinant receptors. This is significant since novel germinants and increased knowledge of the germination process should provide opportunities for improved control of spores in healthcare, food, and environmental sectors.

Complex sporulation-specific expression of transcription termination factor Rho highlights its involvement in Bacillus subtilis cell differentiation

Termination factor Rho, responsible for the main factor-dependent pathway of transcription termination and the major inhibitor of antisense transcription, is an emerging regulator of various physiological processes in microorganisms. In Gram-positive bacterium Bacillus subtilis, Rho is involved in the control of cell adaptation to starvation and, in particular, in the control of sporulation, a complex differentiation program leading to the formation of a highly resistant dormant spore. While the initiation of sporulation requires a decrease in Rho protein levels during the transition to stationary phase, the mechanisms regulating the expression of rho gene throughout the cell cycle remain largely unknown. Here we show that a drop in the activity of the vegetative SigA-dependent rho promoter causes the inhibition of rho expression in stationary phase. However, after the initiation of sporulation, rho gene is specifically reactivated in two compartments of the sporulating cell using distinct mechanisms. In the mother cell, rho expression occurs by read-through transcription initiated at the SigH-dependent promoter of the distal spo0F gene. In the forespore, rho gene is transcribed from the intrinsic promoter recognized by the alternative sigma factor SigF. These regulatory elements ensure the activity of Rho during sporulation, which appears important for the proper formation of spores. We provide experimental evidence that disruption of the spatiotemporal expression of rho during sporulation affects the resistance properties of spores, their morphology, and the ability to return to vegetative growth under favorable growth conditions.

Alanine and glutamate catabolism collaborate to ensure the success of Bacillus subtilis sporulation

Sporulation as a typical bacterial differentiation process has been studied for decades. However, two crucial aspects of sporulation, (i) the energy sources supporting the process, and (ii) the maintenance of spore dormancy throughout sporulation, are scarcely explored. Here, we reported the crucial role of RocG-mediated glutamate catabolism in regulating mother cell lysis, a critical step for sporulation completion of Bacillus subtilis, likely by providing energy metabolite ATP. Notably, rocG overexpression resulted in an excessive ATP accumulation in sporulating cells, leading to adverse effects on future spore properties, e.g. increased germination efficiency, reduced DPA content, and lowered heat resistance. Additionally, we revealed that Ald-mediated alanine metabolism was highly related to the inhibition of premature germination and the maintenance of spore dormancy during sporulation, which might be achieved by decreasing the typical germinant L-alanine concentration in sporulating environment. Our data inferred that sporulation of B. subtilis was a highly orchestrated biological process requiring a delicate balance in diverse metabolic pathways, hence ensuring both the completion of sporulation and production of high-quality spores.

L-Alanine promotes anti-infectious properties of Bacillus subtilis S-2 spores via the germination receptor gerAA

Bacillus species, which have two cell-type forms (vegetative cells and spores), demonstrate a variety of probiotic functions in animal feed additives and human nutrition. We previously found that the probiotic effect of Bacillus subtilis S-2 spores with high germination response to L-alanine was specifically enhanced by the L-alanine pretreatment. The germination response of Bacillus is highly associated with the germination receptors of spores. However, how L-alanine-induced germination of spores exerts anti-infectious effect in epithelial cells remains unclear. In this study, we constructed the mutant strain of B. subtilis S-2 with germination receptor gerAA knockout to further explore the role of spore germination in resisting pathogen infection to cells. The differential probiotic effects of B. subtilis S-2 and S-2ΔgerAA spores pretreated with L-alanine were evaluated in intestinal porcine epithelial cells (IPEC-J2) or Caco2 cells infected with enterotoxigenic Escherichia coli (ETEC) or following IL-1β stimulation. The results showed that the germination response of the S-2ΔgerAA spores to L-alanine was significantly reduced. Compared with the S-2ΔgerAA spores, the L-alanine-induced germination of B. subtilis S-2 spores significantly increased the activity of anti-adhesion of ETEC to IPEC-J2 cells and reduced the expression of inflammatory factors and cell receptors. L-alanine induction also significantly promoted the expression of autophagy-related proteins in the B. subtilis S-2 spores. These findings demonstrate that the gerAA germination receptor is essential for the probiotic function of Bacillus spores and that L-alanine treatment promotes the anti-infectious properties of the germinated spores in porcine intestinal epithelial IPEC-J2 cells. The result suggests the importance of germination receptor gerAA in helping spore germination and enhancing anti-infectious activity. The findings in the study benefit to screening of potential Bacillus probiotics and increasing probiotic efficacy induced by L-alanine as an adjuvant.

Catabolism of germinant amino acids is required to prevent premature spore germination in Bacillus subtilis

Spores of Bacillus subtilis germinate in response to specific germinant molecules that are recognized by receptors in the spore envelope. Germinants signal to the dormant spore that the environment can support vegetative growth, so many germinants, such as alanine and valine, are also essential metabolites. As such, they are also required to build the spore. Here we show that these germinants cause premature germination if they are still present at the latter stages of spore formation and beyond, but that B. subtilis metabolism is configured to prevent this: alanine and valine are catabolized and cleared from wild-type cultures even when alternative carbon and nitrogen sources are present. Alanine and valine accumulate in the spent media of mutants that are unable to catabolize these amino acids, and premature germination is pervasive. Premature germination does not occur if the germinant receptor that responds to alanine and valine is eliminated, or if wild-type strains that are able to catabolize and clear alanine and valine are also present in coculture. Our findings demonstrate that spore-forming bacteria must fine-tune the concentration of any metabolite that can also function as a germinant to a level that is high enough to allow for spore development to proceed, but not so high as to promote premature germination. These results indicate that germinant selection and metabolism are tightly linked, and suggest that germinant receptors evolve in tandem with the catabolic priorities of the spore-forming bacterium.

IMPORTANCE

Many bacterial species produce dormant cells called endospores, which are not killed by antibiotics or common disinfection practices. Endospores pose critical challenges in the food industry, where endospore contaminations cause food spoilage, and in hospitals, where infections by pathogenic endospore formers threaten the life of millions every year. Endospores lose their resistance properties and can be killed easily when they germinate and exit dormancy. We have discovered that the enzymes that break down the amino acids alanine and valine are critical for the production of stable endospores. If these enzymes are absent, endospores germinate as they are formed or shortly thereafter in response to alanine, which can initiate the germination of many different species' endospores, or to valine. By blocking the activity of alanine dehydrogenase, the enzyme that breaks down alanine and is not present in mammals, it may be possible to inactivate endospores by triggering premature and unproductive germination.

Bacterial spore germination receptors are nutrient-gated ion channels

Bacterial spores resist antibiotics and sterilization and can remain metabolically inactive for decades, but they can rapidly germinate and resume growth in response to nutrients. Broadly conserved receptors embedded in the spore membrane detect nutrients, but how spores transduce these signals remains unclear. Here, we found that these receptors form oligomeric membrane channels. Mutations predicted to widen the channel initiated germination in the absence of nutrients, whereas those that narrow it prevented ion release and germination in response to nutrients. Expressing receptors with widened channels during vegetative growth caused loss of membrane potential and cell death, whereas the addition of germinants to cells expressing wild-type receptors triggered membrane depolarization. Therefore, germinant receptors act as nutrient-gated ion channels such that ion release initiates exit from dormancy.

Glutamate catabolism during sporulation determines the success of the future spore germination

Bacterial spores can preserve cellular dormancy for years, but still hold the remarkable ability to revive and recommence life. This cellular awakening begins with a rapid and irreversible event termed germination; however, the metabolic determinants required for its success have been hardly explored. Here, we show that at the onset of the process of sporulation, the metabolic enzyme RocG catabolizes glutamate, facilitating ATP production in the spore progenitor cell, and subsequently influencing the eventual spore ATP reservoir. Mutants displaying low RocG levels generate low ATP-containing spores that exhibit severe germination deficiency. Importantly, this phenotype could be complemented by expressing RocG at a specific window of time during the initiation of sporulation. Thus, we propose that despite its low abundance in dormant spores, ATP energizes spore germination, and its production, fueled by RocG, is coupled with the initial developmental phase of spore formation.

The SpoVA membrane complex is required for dipicolinic acid import during sporulation and export during germination

In response to starvation, endospore-forming bacteria differentiate into stress-resistant spores that can remain dormant for years yet rapidly germinate and resume growth in response to nutrients. The small molecule dipicolinic acid (DPA) plays a central role in both the stress resistance of the dormant spore and its exit from dormancy during germination. The spoVA locus is required for DPA import during sporulation and has been implicated in its export during germination, but the molecular bases are unclear. Here, we define the minimal set of proteins encoded in the Bacillus subtilis spoVA operon required for DPA import and demonstrate that these proteins form a membrane complex. Structural modeling of these components combined with mutagenesis and in vivo analysis reveal that the C and Eb subunits form a membrane channel, while the D subunit functions as a cytoplasmic plug. We show that point mutations that impair the interactions between D and the C-Eb membrane complex reduce the efficiency of DPA import during sporulation and reciprocally accelerate DPA release during germination. Our data support a model in which DPA transport into spores involves cycles of unplugging and then replugging the C-Eb membrane channel, while nutrient detection during germination triggers DPA release by unplugging it.

A security check that monitors cell morphogenesis

Bacterial endospore formation depends on a series of checkpoints to ensure the fidelity of this critical developmental process. Delerue et al. have uncovered a novel checkpoint in Bacillus subtilis that senses defects in the assembly of static polymers in one compartment and arrests the assembly of peptidoglycan in another compartment.

Genetic Evidence for Signal Transduction within the Bacillus subtilis GerA Germinant Receptor

Bacterial spores can rapidly exit dormancy through the process of germination. This process begins with the activation of nutrient receptors embedded in the spore membrane. The prototypical germinant receptor in Bacillus subtilis responds to l-alanine and is thought to be a complex of proteins encoded by the genes in the gerA operon: gerAA, gerAB, and gerAC. The GerAB subunit has recently been shown to function as the nutrient sensor, but beyond contributing to complex stability, no additional functions have been attributed to the other two subunits. Here, we investigate the role of GerAA. We resurrect a previously characterized allele of gerA (termed gerA*) that carries a mutation in gerAA and show that it constitutively activates germination even in the presence of a wild-type copy of gerA. Using an enrichment strategy to screen for suppressors of gerA*, we identified mutations in all three gerA genes that restore a functional receptor. Characterization of two distinct gerAB suppressors revealed that one (gerAB[E105K]) reduces the GerA complex's ability to respond to l-alanine, while another (gerAB[F259S]) disrupts the germinant signal downstream of l-alanine recognition. These data argue against models in which GerAA is directly or indirectly involved in germinant sensing. Rather, our data suggest that GerAA is responsible for transducing the nutrient signal sensed by GerAB. While the steps downstream of gerAA have yet to be uncovered, these results validate the use of a dominant-negative genetic approach in elucidating the gerA signal transduction pathway. IMPORTANCE Endospore formers are a broad group of bacteria that can enter dormancy upon starvation and exit dormancy upon sensing the return of nutrients. How dormant spores sense and respond to these nutrients is poorly understood. Here, we identify a key step in the signal transduction pathway that is activated after spores detect the amino acid l-alanine. We present a model that provides a more complete picture of this process that is critical for allowing dormant spores to germinate and resume growth.

Bacterial developmental checkpoint that directly monitors cell surface morphogenesis

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

The bacterial tyrosine kinase system CpsBCD governs the length of capsule polymers

Many pathogenic bacteria are encased in a layer of capsular polysaccharide (CPS). This layer is important for virulence by masking surface antigens, preventing opsonophagocytosis, and avoiding mucus entrapment. The bacterial tyrosine kinase (BY-kinase) regulates capsule synthesis and helps bacterial pathogens to survive different host niches. BY-kinases autophosphorylate at the C-terminal tyrosine residues upon external stimuli, but the role of phosphorylation is still unclear. Here, we report that the BY-kinase CpsCD is required for growth in Streptococcus pneumoniae Cells lacking a functional cpsC or cpsD accumulated low molecular weight CPS and lysed because of the lethal sequestration of the lipid carrier undecaprenyl phosphate, resulting in inhibition of peptidoglycan (PG) synthesis. CpsC interacts with CpsD and the polymerase CpsH. CpsD phosphorylation reduces the length of CPS polymers presumably by controlling the activity of CpsC. Finally, pulse-chase experiments reveal the spatiotemporal coordination between CPS and PG synthesis. This coordination is dependent on CpsC and CpsD. Together, our study provides evidence that BY-kinases regulate capsule polymer length by fine-tuning CpsC activity through autophosphorylation.

Identification of a Novel Regulator of Clostridioides difficile Cortex Formation

Clostridioides difficile is a leading cause of health care-associated infections worldwide. These infections are transmitted by C. difficile′s metabolically dormant, aerotolerant spore form. Functional spore formation depends on the assembly of two protective layers, a thick layer of modified peptidoglycan known as the cortex layer and a multilayered proteinaceous meshwork known as the coat. We previously identified two spore morphogenetic proteins, SpoIVA and SipL, that are essential for recruiting coat proteins to the developing forespore and making functional spores. While SpoIVA and SipL directly interact, the identities of the proteins they recruit to the forespore remained unknown. Here, we used mass spectrometry-based affinity proteomics to identify proteins that interact with the SpoIVA-SipL complex. These analyses identified the Peptostreptococcaceae family-specific, sporulation-induced bitopic membrane protein CD3457 (renamed SpoVQ) as a protein that interacts with SipL and SpoIVA. Loss of SpoVQ decreased heat-resistant spore formation by ∼5-fold and reduced cortex thickness ∼2-fold; the thinner cortex layer of ΔspoVQ spores correlated with higher levels of spontaneous germination (i.e., in the absence of germinant). Notably, loss of SpoVQ in either spoIVA or sipL mutants prevented cortex synthesis altogether and greatly impaired the localization of a SipL-mCherry fusion protein around the forespore. Thus, SpoVQ is a novel regulator of C. difficile cortex synthesis that appears to link cortex and coat formation. The identification of SpoVQ as a spore morphogenetic protein further highlights how Peptostreptococcaceae family-specific mechanisms control spore formation in C. difficile.

IMPORTANCE The Centers for Disease Control has designated Clostridioides difficile as an urgent threat because of its intrinsic antibiotic resistance. C. difficile persists in the presence of antibiotics in part because it makes metabolically dormant spores. While recent work has shown that preventing the formation of infectious spores can reduce C. difficile disease recurrence, more selective antisporulation therapies are needed. The identification of spore morphogenetic factors specific to C. difficile would facilitate the development of such therapies. In this study, we identified SpoVQ (CD3457) as a spore morphogenetic protein specific to the Peptostreptococcaceae family that regulates the formation of C. difficile’s protective spore cortex layer. SpoVQ acts in concert with the known spore coat morphogenetic factors, SpoIVA and SipL, to link formation of the protective coat and cortex layers. These data reveal a novel pathway that could be targeted to prevent the formation of infectious C. difficile spores.

Pervasive prophage recombination occurs during evolution of spore-forming Bacilli

Phages are the main source of within-species bacterial diversity and drivers of horizontal gene transfer, but we know little about the mechanisms that drive genetic diversity of these mobile genetic elements (MGEs). Recently, we showed that a sporulation selection regime promotes evolutionary changes within SPβ prophage of Bacillus subtilis, leading to direct antagonistic interactions within the population. Herein, we reveal that under a sporulation selection regime, SPβ recombines with low copy number phi3Ts phage DNA present within the B. subtilis population. Recombination results in a new prophage occupying a different integration site, as well as the spontaneous release of virulent phage hybrids. Analysis of Bacillus sp. strains suggests that SPβ and phi3T belong to a distinct cluster of unusually large phages inserted into sporulation-related genes that are equipped with a spore-related genetic arsenal. Comparison of Bacillus sp. genomes indicates that similar diversification of SPβ-like phages takes place in nature. Our work is a stepping stone toward empirical studies on phage evolution, and understanding the eco-evolutionary relationships between bacteria and their phages. By capturing the first steps of new phage evolution, we reveal striking relationship between survival strategy of bacteria and evolution of their phages.

Bacillus spore germination: Knowns, unknowns and what we need to learn

How might a microbial cell that is entirely metabolically dormant - and which has the ability to remain so for extended periods of time - irreversibly commit itself to resuming vegetative growth within seconds of being exposed to certain amino acids or sugars? That this process takes place in the absence of any detectable ATP or de novo protein synthesis, and relies upon a pre-formed apparatus that is immobilised, respectively, in a semi-crystalline membrane or multi-layered proteinaceous coat, only exacerbates the challenge facing spores of Bacillales species when stimulated to germinate. Whereas the process by which spores are formed in response to nutrient starvation - sporulation - involves the orchestrated interplay between hundreds of distinct proteins, the process by which spores return to life - germination - is a much simpler affair, requiring a handful of receptor and channel proteins complemented with specialized peptidoglycan lysins. Despite this relative simplicity, and research effort spanning many decades, comprehensive understanding of key molecular and biochemical details and, in particular signal transduction mechanisms associated with spore germination, has remained elusive. In this review we provide an up to date overview of the field while identifying what we consider to be the key gaps in knowledge associated with germination of Bacillales spores, suggesting also technical approaches that may provide fresh insight to this unique biological process.

Directed evolution by UV-C treatment of Bacillus cereus spores

Bacterial endospores are exposed to a broad variety of sublethal and lethal stresses in the food production chain. Generally, these stresses will not completely eliminate the existing spore populations, and thus constitute a selection pressure on the spores. One stress that is frequently used in the food production chains to disinfect (food) contact surfaces is UV-C. At a wavelength of 254 nm, UV-C has germicidal properties. The aim of this research is to investigate the impact of UV-C stress on the evolution of endospore recalcitrance and germination in B. cereus. A directed evolution experiment was set up in which B. cereus was repeatedly subjected to a cycle of sporulation, sporicidal UV-C treatment, germination and outgrowth. We show here that three independent lineages of UV-C cycled B. cereus spores reproducibly acquired a 30-fold or higher increase in UV-C resistance at 164 mJ/cm². Surprisingly, the UV-C resistant spores of the clones isolated from each of the lineages also became significantly more sensitive to wet heat as a normally non-lethal heat treatment at 70 °C for 15 min resulted in an average 1.8 log cfu/mL reduction. From time-lapse phase contrast microscopy analysis, UV-C resistant mutant spores also showed a distinctive heterogeneity in refractility and a severe germination defect compared to the wild type. However, UV-C resistance of the corresponding vegetative cells was not altered. In conclusion, this work shows that UV-C resistance of endospores is an adaptive trait that can readily be improved, although at an apparent cost for heat resistance and germination efficiency. As such, these results provide novel insights in the evolvability of, and correlation between, some endospore properties.

Arabidopsis thaliana Seedlings Influence Bacillus subtilis Spore Formation

Bacillus subtilis is a Gram-positive plant-growth-promoting rhizobacterium exerting many beneficial effects on plant health. Because they secrete antimicrobial compounds and elicit induced systemic resistance, B. subtilis and phylogenetically related species are of particular interest as antifungals in organic agriculture. These bacteria are also known for their capacity to differentiate phenotypically into endospores able to withstand many environmental stresses. However, although B. subtilis is often inoculated on plants as spores, dynamics of germination and sporulation on roots remain unexplored. Using a hydroponic culture system and a soil system for Arabidopsis thaliana, we observed that B. subtilis spores germinate rapidly on contact with plants. However, the vegetative cells are abundant on roots for only a few days before reversing back to spores. We observed that the germinant receptor GerK and sporulation kinases KinA and KinB identified in vitro control sporulation dynamics on plants. Surprisingly, when plants are inoculated with B. subtilis, free-living cells sporulate more rapidly than plant-associated cells. However, direct contact between plant and bacteria is required for the induction of sporulation in the surrounding B. subtilis. This study has fundamental implications for our understanding of interactions between Bacillus spp. and plants, and particularly for a more efficient usage of B. subtilis as a biofertilizer or biofungicide.

Electrical Polarization Enables Integrative Quality Control during Bacterial Differentiation into Spores

Quality control of offspring is important for the survival of cells. However, the mechanisms by which quality of offspring cells may be checked while running genetic programs of cellular differentiation remain unclear. Here we investigated quality control during sporulating in Bacillus subtilis by combining single-cell time-lapse microscopy, molecular biology, and mathematical modeling. Our results revealed that the quality control via premature germination is coupled with the electrical polarization of outer membranes of developing forespores. The forespores that accumulate fewer cations on their surface are more likely to be aborted. This charge accumulation enables the projection of multi-dimensional information about the external environment and morphological development of the forespore into one-dimensional information of cation accumulation. We thus present a paradigm of cellular regulation by bacterial electrical signaling. Moreover, based on the insight we gain, we propose an electrophysiology-based approach of reducing the yield and quality of Bacillus endospores.

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Quality control during bacterial sporulation is coupled with cation accumulation

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Cation accumulation prevents premature germination

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Cation accumulation integrates information on morphological defects and environments

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Spores are less fit when sporulated with Thioflavin T

A two-step transport pathway allows the mother cell to nurture the developing spore in Bacillus subtilis

One of the hallmarks of bacterial endospore formation is the accumulation of high concentrations of pyridine-2,6-dicarboxylic acid (dipicolinic acid or DPA) in the developing spore. This small molecule comprises 5–15% of the dry weight of dormant spores and plays a central role in resistance to both wet heat and desiccation. DPA is synthesized in the mother cell at a late stage in sporulation and must be translocated across two membranes (the inner and outer forespore membranes) that separate the mother cell and forespore. The enzymes that synthesize DPA and the proteins required to translocate it across the inner forespore membrane were identified over two decades ago but the factors that transport DPA across the outer forespore membrane have remained mysterious. Here, we report that SpoVV (formerly YlbJ) is the missing DPA transporter. SpoVV is produced in the mother cell during the morphological process of engulfment and specifically localizes in the outer forespore membrane. Sporulating cells lacking SpoVV produce spores with low levels of DPA and cells engineered to express SpoVV and the DPA synthase during vegetative growth accumulate high levels of DPA in the culture medium. SpoVV resembles concentrative nucleoside transporters and mutagenesis of residues predicted to form the substrate-binding pocket supports the idea that SpoVV has a similar structure and could therefore function similarly. These findings provide a simple two-step transport mechanism by which the mother cell nurtures the developing spore. DPA produced in the mother cell is first translocated into the intermembrane space by SpoVV and is then imported into the forespore by the SpoVA complex. This pathway is likely to be broadly conserved as DPA synthase, SpoVV, and SpoVA proteins can be found in virtually all endospore forming bacteria.

All pathogenic and non-pathogenic bacteria that differentiate into dormant endospores including Clostridium difficile, Bacillus anthracis, and Bacillus subtilis, contain very high concentrations of the small molecule dipicolinic acid (DPA). This molecule displaces water in the spore core where it plays an integral role in spore resistance and dormancy. DPA and its contribution to spore dehydration were discovered in 1953 but the molecular basis for its accumulation in the spore has remained unclear. The developing endospore resides within a mother cell that assembles protective layers around the spore and nurtures it by providing mother-cell-produced molecules. DPA is produced in the mother cell at a late stage in development and then must be translocated across two membranes into the spore core. Here, we report the discovery of the missing DPA transporter, homologs of which are present in virtually all endospore-forming bacteria. Our data provide evidence for a simple two-step transport pathway in which the mother cell nurtures the developing spore by sequentially moving DPA across the two membranes that surround it.

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

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

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

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