Inorganic phosphate induces spore morphogenesis and enterotoxin production in the intestinal pathogen Clostridium perfringens

Identification of orphan histidine kinases that impact sporulation and enterotoxin production by Clostridium perfringens type F strain SM101 in a pathophysiologically-relevant ex vivo mouse intestinal contents model

When causing food poisoning or antibiotic-associated diarrhea, Clostridium perfringens type F strains must sporulate to produce C. perfringens enterotoxin (CPE) in the intestines. C. perfringens is thought to use some of its seven annotated orphan histidine kinases to phosphorylate Spo0A and initiate sporulation and CPE production. We previously demonstrated the CPR0195 orphan kinase, but not the putative CPR1055 orphan kinase, is important when type F strain SM101 initiates sporulation and CPE production in modified Duncan-Strong (MDS) sporulation medium. Since there is no small animal model for C. perfringens sporulation, the current study used diluted mouse intestinal contents (MIC) to develop an ex vivo sporulation model and employed this model to test sporulation and CPE production by SM101 CPR0195 and CPR1055 null mutants in a pathophysiologically-relevant context. Surprisingly, both mutants still sporulated and produced CPE at wild-type levels in MIC. Therefore, five single null mutants were constructed that cannot produce one of the previously-unstudied putative orphan kinases of SM101. Those mutants implicated CPR1316, CPR1493, CPR1953 and CPR1954 in sporulation and CPE production by SM101 MDS cultures. Phosphorylation activity was necessary for CPR1316, CPR1493, CPR1953 and CPR1954 to affect sporulation in those MDS cultures, supporting their identity as kinases. Importantly, only the CPR1953 or CPR1954 null mutants exhibited significantly reduced levels of sporulation and CPE production in MIC cultures. These phenotypes were reversible by complementation. Characterization studies suggested that, in MDS or MIC, the CPR1953 and CPR1954 mutants produce less Spo0A than wild-type SM101. In addition, the CPR1954 mutant exhibited little or no Spo0A phosphorylation in MDS cultures. These studies, i) highlight the importance of using pathophysiologically-relevant models to investigate C. perfringens sporulation and CPE production in a disease context and ii) link the CPR1953 and CPR1954 kinases to C. perfringens sporulation and CPE production in disease-relevant conditions.

Mechanisms and Applications of Bacterial Sporulation and Germination in the Intestine

Recent studies have suggested a major role for endospore forming bacteria within the gut microbiota, not only as pathogens but also as commensal and beneficial members contributing to gut homeostasis. In this review the sporulation processes, spore properties, and germination processes will be explained within the scope of the human gut. Within the gut, spore-forming bacteria are known to interact with the host’s immune system, both in vegetative cell and spore form. Together with the resistant nature of the spore, these characteristics offer potential for spores’ use as delivery vehicles for therapeutics. In the last part of the review, the therapeutic potential of spores as probiotics, vaccine vehicles, and drug delivery systems will be discussed.

Tetanus Toxin Synthesis is Under the Control of A Complex Network of Regulatory Genes in Clostridium tetani

Clostridium tetani produces a potent neurotoxin, the tetanus toxin (TeNT), which is responsible for an often-fatal neurological disease (tetanus) characterized by spastic paralysis. Prevention is efficiently acquired by vaccination with the TeNT toxoid, which is obtained by C.tetani fermentation and subsequent purification and chemical inactivation. C.tetani synthesizes TeNT in a regulated manner. Indeed, the TeNT gene (tent) is mainly expressed in the late exponential and early stationary growth phases. The gene tetR (tetanus regulatory gene), located immediately upstream of tent, encodes an alternative sigma factor which was previously identified as a positive regulator of tent. In addition, the genome of C.tetani encodes more than 127 putative regulators, including 30 two-component systems (TCSs). Here, we investigated the impact of 12 regulators on TeNT synthesis which were selected based on their homology with related regulatory elements involved in toxin production in other clostridial species. Among nine TCSs tested, three of them impact TeNT production, including two positive regulators that indirectly stimulate tent and tetR transcription. One negative regulator was identified that interacts with both tent and tetR promoters. Two other TCSs showed a moderate effect: one binds to the tent promoter and weakly increases the extracellular TeNT level, and another one has a weak inverse effect. In addition, CodY (control of dciA (decoyinine induced operon) Y) but not Spo0A (sporulation stage 0) or the DNA repair protein Mfd (mutation frequency decline) positively controls TeNT synthesis by interacting with the tent promoter. Moreover, we found that inorganic phosphate and carbonate are among the environmental factors that control TeNT production. Our data show that TeNT synthesis is under the control of a complex network of regulators that are largely distinct from those involved in the control of toxin production in Clostridium botulinum or Clostridium difficile.

The Impact of pH on Clostridioides difficile Sporulation and Physiology

Clostridioides difficile is a pathogenic bacterium that infects the human colon to cause diarrheal disease. Growth of the bacterium is known to be dependent on certain bile acids, oxygen levels, and nutrient availability in the intestine, but how the environmental pH can influence C. difficile is mostly unknown. Previous studies indicated that C. difficile modulates the intestinal pH, and prospective cohort studies have found a strong association between a more alkaline fecal pH and C. difficile infection. Based on these data, we hypothesized that C. difficile physiology can be affected by various pH conditions. In this study, we investigated the impact of a range of pH conditions on C. difficile to assess potential effects on growth, sporulation, motility, and toxin production in the strains 630Δerm and R20291. We observed pH-dependent differences in sporulation rate, spore morphology, and viability. Sporulation frequency was lowest under acidic conditions, and differences in cell morphology were apparent at low pH. In alkaline environments, C. difficile sporulation was greater for strain 630Δerm, whereas R20291 produced relatively high levels of spores in a broad range of pH conditions. Rapid changes in pH during exponential growth impacted sporulation similarly among the strains. Furthermore, we observed an increase in C. difficile motility with increases in pH, and strain-dependent differences in toxin production under acidic conditions. The data demonstrate that pH is an important parameter that affects C. difficile physiology and may reveal relevant insights into the growth and dissemination of this pathogen.IMPORTANCE Clostridioides difficile is an anaerobic bacterium that causes gastrointestinal disease. C. difficile forms dormant spores which can survive harsh environmental conditions, allowing their spread to new hosts. In this study, we determine how intestinally relevant pH conditions impact C. difficile physiology in the two divergent strains, 630Δerm and R20291. Our data demonstrate that low pH conditions reduce C. difficile growth, sporulation, and motility. However, toxin production and spore morphology were differentially impacted in the two strains at low pH. In addition, we observed that alkaline environments reduce C. difficile growth, but increase cell motility. When pH was adjusted rapidly during growth, we observed similar impacts on both strains. This study provides new insights into the phenotypic diversity of C. difficile grown under diverse pH conditions present in the intestinal tract, and demonstrates similarities and differences in the pH responses of different C. difficile isolates.

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.

Enterotoxic Clostridia: Clostridium perfringens Enteric Diseases

In humans and livestock, Clostridium perfringens is an important cause of intestinal infections that manifest as enteritis, enterocolitis, or enterotoxemia. This virulence is largely related to the toxin-producing ability of C. perfringens. This article primarily focuses on the C. perfringens type F strains that cause a very common type of human food poisoning and many cases of nonfoodborne human gastrointestinal diseases. The enteric virulence of type F strains is dependent on their ability to produce C. perfringens enterotoxin (CPE). CPE has a unique amino acid sequence but belongs structurally to the aerolysin pore-forming toxin family. The action of CPE begins with binding of the toxin to claudin receptors, followed by oligomerization of the bound toxin into a prepore on the host membrane surface. Each CPE molecule in the prepore then extends a beta-hairpin to form, collectively, a beta-barrel membrane pore that kills cells by increasing calcium influx. The cpe gene is typically encoded on the chromosome of type F food poisoning strains but is encoded by conjugative plasmids in nonfoodborne human gastrointestinal disease type F strains. During disease, CPE is produced when C. perfringens sporulates in the intestines. Beyond type F strains, C. perfringens type C strains producing beta-toxin and type A strains producing a toxin named CPILE or BEC have been associated with human intestinal infections. C. perfringens is also an important cause of enteritis, enterocolitis, and enterotoxemia in livestock and poultry due to intestinal growth and toxin production.

Regulatory Targets of the Response Regulator RR_1586 from Clostridioides difficile Identified Using a Bacterial One-Hybrid Screen

The Clostridioides difficile R20291 genome encodes 57 response regulator proteins that, as part of two-component signaling pathways, regulate adaptation to environmental conditions. Genomic and transcriptomic studies in C. difficile have been limited, due to technical challenges, to the analysis of either high-throughput screens or high-priority targets, such as primary regulators of toxins or spore biology. We present the use of several technically accessible and generally applicable techniques to elucidate the putative regulatory targets of a response regulator, RR_1586, involved in sporulation of the hypervirulent C. difficile strain R20291. A DNA-binding specificity motif for RR_1586 was determined using a bacterial one-hybrid assay originally developed for Drosophila transcription factors. Comparative bioinformatics approaches identified and in vitro experiments confirmed RR_1586 binding sites upstream of putative target genes, including those that encode phosphate ion transporters, spermidine/putrescine biosynthesis and transport pathways, ABC type transport systems, known regulators of sporulation, and genes encoding spore structural proteins. Representative examples of these regulatory interactions have been tested and confirmed in Escherichia coli-based reporter assays. Finally, evidence of possible regulatory mechanisms is also presented. A working model includes self-regulation by RR_1586 and phosphorylation-dependent and -independent DNA binding at low- and high-fidelity binding sites, respectively. Broad application of this and similar approaches is anticipated to be an important catalyst for the study of gene regulation by two-component systems from pathogenic or technically challenging bacteria.IMPORTANCEClostridioides difficile spores survive under harsh conditions and can germinate into actively dividing cells capable of causing disease. An understanding of the regulatory networks controlling sporulation and germination in C. difficile could be exploited for therapeutic advantage. However, such studies are hindered by the challenges of working with an anaerobic pathogen recalcitrant to genetic manipulation. Although two-component response regulators can be identified from genetic sequences, identification of their downstream regulatory networks requires further development. This work integrates experimental and bioinformatic approaches, which provide practical advantages over traditional transcriptomic analyses, to identify the putative regulon of the C. difficile response regulator RR_1586 by first screening for protein-DNA interactions in E. coli and then predicting regulatory outputs in C. difficile.

An enhanced green fluorescence protein (EGFP)-based reporter assay for quantitative detection of sporulation in Clostridium perfringens SM101

Clostridium perfringens type F is a spore-forming anaerobe that causes bacterial food-borne illness in humans. The disease develops when ingested vegetative cells reach the intestinal tract and begin to form spores that produce the diarrheagenic C. perfringens enterotoxin (CPE). Given that CPE production is regulated by the master regulator of sporulation (transcription factor Spo0A), the identification of sporulation-inducing factors in the intestine is relevant to better understanding of the disease. To examine these factors, we established assays to quantify C. perfringens sporulation stage under microscopy by using two fluorescent reporters, namely, Evoglow-Bs2 and CpEGFP. When the reporter genes were placed under control of the cpe promoter, both protein products were expressed specifically during sporulation. However, the intensity of the anaerobic reporter Evoglow-Bs2 was weak and rapidly photobleached during microscopic observation. Alternatively, CpEGFP, a canonical green fluorescence protein with optimized codon usage for Clostridium species, was readily detectable in the mother-cell compartment of most bacteria at early stages of sporulation. Additionally, CpEGFP expression predicted final spore yield and was quantifiable in 96-well plates using fluorescence plate reader. These results indicate that CpEGFP can be used to analyze the sporulation of C. perfringens and has a potential application in the large-scale screening of sporulation-regulating biomolecules.

NanR Regulates Sporulation and Enterotoxin Production by Clostridium perfringens Type F Strain F4969

Clostridium perfringens type F strains, which produce C. perfringens enterotoxin (CPE), are a major cause of gastrointestinal infections, including the second most prevalent bacterial foodborne illness and 5 to 10% cases of antibiotic-associated diarrhea. Virulence of type F strains is primarily ascribable to CPE, which is synthesized only during sporulation. Many type F strains also produce NanI sialidase and carry a nan operon that likely facilitates uptake and metabolism of sialic acid liberated from glycoconjugates by NanI. During vegetative growth of type F strain F4969, NanR can regulate expression of nanI Given their importance for type F disease, the current study investigated whether NanR can also influence sporulation and CPE production when F4969 or isogenic derivatives are cultured in modified Duncan-Strong sporulation (MDS) medium. An isogenic F4969 nanR null mutant displayed much less sporulation and CPE production but more NanI production than wild-type F4969, indicating that NanR positively regulates sporulation and CPE production but represses NanI production in MDS. Results for the nanR mutant also demonstrated that NanR regulates expression of the nan operon. A nanI nanR double null mutant mirrored the outcome of the nanR null mutant strain but with a stronger inhibition of sporulation and CPE production, even after overnight incubation. Coupled with results using a nanI null mutant, which had no impairment of sporulation or CPE production, NanR appears to carefully modulate the availability of NanI, nan operon-encoded proteins and sialic acid to provide sufficient nutrients to sustain sporulation and CPE production when F4969 is cultured in MDS medium.

Effects of Bile Acids and Nisin on the Production of Enterotoxin by Clostridium perfringens in a Nutrient-Rich Medium

Clostridium perfringens is the second most common cause of bacterial foodborne illness in the United States, with nearly a million cases each year. C. perfringens enterotoxin (CPE), produced during sporulation, damages intestinal epithelial cells by pore formation, which results in watery diarrhea. The effects of low concentrations of nisin and bile acids on sporulation and toxin production were investigated in C. perfringens SM101, which carries an enterotoxin gene on the chromosome, in a nutrient-rich medium. Bile acids and nisin increased production of enterotoxin in cultures; bile acids had the highest effect. Both compounds stimulated the transcription of enterotoxin and sporulation-related genes and production of spores during the early growth phase. They also delayed spore outgrowth and nisin was more inhibitory. Bile acids and nisin enhanced enterotoxin production in some but not all other C. perfringens isolates tested. Low concentrations of bile acids and nisin may act as a stress signal for the initiation of sporulation and the early transcription of sporulation-related genes in some strains of C. perfringens, which may result in increased strain-specific production of enterotoxin in those strains. This is the first report showing that nisin and bile acids stimulated the transcription of enterotoxin and sporulation-related genes in a nutrient-rich bacterial culture medium.

Clostridium perfringens Sporulation and Sporulation-Associated Toxin Production

The ability of Clostridium perfringens to form spores plays a key role during the transmission of this Gram-positive bacterium to cause disease. Of particular note, the spores produced by food poisoning strains are often exceptionally resistant to food environment stresses such as heat, cold, and preservatives, which likely facilitates their survival in temperature-abused foods. The exceptional resistance properties of spores made by most type A food poisoning strains and some type C foodborne disease strains involve their production of a variant small acid-soluble protein-4 that binds more tightly to spore DNA than to the small acid-soluble protein-4 made by most other C. perfringens strains. Sporulation and germination by C. perfringens and Bacillus spp. share both similarities and differences. Finally, sporulation is essential for production of C. perfringens enterotoxin, which is responsible for the symptoms of C. perfringens type A food poisoning, the second most common bacterial foodborne disease in the United States. During this foodborne disease, C. perfringens is ingested with food and then, by using sporulation-specific alternate sigma factors, this bacterium sporulates and produces the enterotoxin in the intestines.

Nitrate salts suppress sporulation and production of enterotoxin in Clostridium perfringens strain NCTC8239

Clostridium perfringens type A is a common source of food-borne illness in humans. Ingested vegetative cells sporulate in the small intestinal tract and in the process produce C. perfringens enterotoxin (CPE). Although sporulation plays a critical role in the pathogenesis of food-borne illness, the molecules triggering/inhibiting sporulation are still largely unknown. It has previously been reported by our group that sporulation is induced in C. perfringens strain NCTC8239 co-cultured with Caco-2 cells in Dulbecco's Modified Eagle Medium (DMEM). In contrast, an equivalent amount of spores was not observed when bacteria were co-cultured in Roswell Park Memorial Institute-1640 medium (RPMI). In the present study it was found that, when these two media are mixed, RPMI inhibits sporulation and CPE production induced in DMEM. When a component of RPMI was added to DMEM, it was found that calcium nitrate (Ca[NO₃ ]₂ ) significantly inhibits sporulation and CPE production. The number of spores increased when Ca(NO₃ )₂ -deficient RPMI was used. The other nitrate salts significantly suppressed sporulation, whereas the calcium salts used did not. qPCR revealed that nitrate salts increased expression of bacterial nitrate/nitrite reductase. Furthermore, it was found that nitrite and nitric oxide suppress sporulation. In the sporulation stages, Ca(NO₃ )₂ down-regulated the genes controlled by Spo0A, a master regulator of sporulation, but not spo0A itself. Collectively, these results indicate that nitrate salts suppress sporulation and CPE production by down-regulating Spo0A-regulated genes in C. perfringens strain NCTC8239. Nitrate reduction may be associated with inhibition of sporulation.

Transcriptional Profile during Deoxycholate-Induced Sporulation in a Clostridium perfringens Isolate Causing Foodborne Illness

Clostridium perfringens type A is a common source of foodborne illness (FBI) in humans. Vegetative cells sporulate in the small intestinal tract and produce the major pathogenic factor C. perfringens enterotoxin. Although sporulation plays a critical role in the pathogenesis of FBI, the mechanisms inducing sporulation remain unclear. Bile salts were shown previously to induce sporulation, and we confirmed deoxycholate (DCA)-induced sporulation in C. perfringens strain NCTC8239 cocultured with human intestinal epithelial Caco-2 cells. In the present study, we performed transcriptome analyses of strain NCTC8239 in order to elucidate the mechanism underlying DCA-induced sporulation. Of the 2,761 genes analyzed, 333 were up- or downregulated during DCA-induced sporulation and included genes for cell division, nutrient metabolism, signal transduction, and defense mechanisms. In contrast, the virulence-associated transcriptional regulators (the VirR/VirS system, the agr system, codY, and abrB) were not activated by DCA. DCA markedly increased the expression of signaling molecules controlled by Spo0A, the master regulator of the sporulation process, whereas the expression of spo0A itself was not altered in the presence or absence of DCA. The phosphorylation of Spo0A was enhanced in the presence of DCA. Collectively, these results demonstrated that DCA induced sporulation, at least partially, by facilitating the phosphorylation of Spo0A and activating Spo0A-regulated genes in strain NCTC8239 while altering the expression of various genes.

IMPORTANCE

Disease caused by Clostridium perfringens type A consistently ranks among the most common bacterial foodborne illnesses in humans in developed countries. The sporulation of C. perfringens in the small intestinal tract is a key event for its pathogenesis, but the factors and underlying mechanisms by which C. perfringens sporulates in vivo currently remain unclear. Bile salts, major components of bile, which is secreted from the liver for the emulsification of lipids, were shown to induce sporulation. However, the mechanisms underlying bile salt-induced sporulation have not yet been clarified. In the present study, we demonstrate that deoxycholate (one of the bile salts) induces sporulation by facilitating the phosphorylation of Spo0A and activating Spo0A-regulated genes using a transcriptome analysis. Thus, this study enhances our understanding of the mechanisms underlying sporulation, particularly that of bile salt-induced sporulation, in C. perfringens.

Clostridium perfringens Enterotoxin: Action, Genetics, and Translational Applications

Clostridium perfringens enterotoxin (CPE) is responsible for causing the gastrointestinal symptoms of several C. perfringens food- and nonfood-borne human gastrointestinal diseases. The enterotoxin gene (cpe) is located on either the chromosome (for most C. perfringens type A food poisoning strains) or large conjugative plasmids (for the remaining type A food poisoning and most, if not all, other CPE-producing strains). In all CPE-positive strains, the cpe gene is strongly associated with insertion sequences that may help to assist its mobilization and spread. During disease, CPE is produced when C. perfringens sporulates in the intestines, a process involving several sporulation-specific alternative sigma factors. The action of CPE starts with its binding to claudin receptors to form a small complex; those small complexes then oligomerize to create a hexameric prepore on the membrane surface. Beta hairpin loops from the CPE molecules in the prepore assemble into a beta barrel that inserts into the membrane to form an active pore that enhances calcium influx, causing cell death. This cell death results in intestinal damage that causes fluid and electrolyte loss. CPE is now being explored for translational applications including cancer therapy/diagnosis, drug delivery, and vaccination.

Optimization of macroelement concentrations, pH and osmolarity for triacylglycerol accumulation in Rhodococcus opacus strain PD630

The refinement of biodiesel or renewable diesel from bacterial lipids has a great potential to make a contribution for energy production in the future. This study provides new data concerning suitable nutrient concentrations for cultivation of the Gram-positive Rhodococcus opacus PD630, which is able to accumulate large amounts of lipids during nitrogen limitation. Enhanced concentrations of magnesium have been shown to increase the final optical density and the lipid content of the cells. Elevated phosphate concentrations slowed down the onset of the accumulation phase, without a clear effect on the final optical density and the cell's lipid content. A robust growth of R. opacus was possible in the presence of ammonium concentrations of up to 1.4 g l(-1) and sucrose concentrations of up to 240 g l(-1), with an optimum regarding growth and lipid storage observed in the range of 0.2 to 0.4 g l(-1) ammonium and 20 to 40 g l(-1) sucrose, respectively. Moreover, R. opacus showed tolerance to high salt concentrations.

Phosphate limitation induces sporulation in the chytridiomycete Blastocladiella emersonii

The cell cycle is controlled by numerous mechanisms that ensure correct cell division. If growth is not possible, cells may eventually promote autophagy, differentiation, or apoptosis. Microorganisms interrupt their growth and differentiate under general nutrient limitation. We analyzed the effects of phosphate limitation on growth and sporulation in the chytridiomycete Blastocladiella emersonii using kinetic data, phase-contrast, and laser confocal microscopy. Under phosphate limitation, zoospores germinated and subsequently formed 2-4 spores, regardless of the nutritional content of the medium. The removal of phosphate at any time during growth induced sporulation of vegetative cells. If phosphate was later added to the same cultures, growth was restored if the cells were not yet committed to sporulation. The cycles of addition and withdrawal of phosphate from growth medium resulted in cycles of germination-growth, germination-sporulation, or germination-growth-sporulation. These results show that phosphate limitation is sufficient to interrupt cell growth and to induce complete sporulation in B. emersonii. We concluded that the determination of growth or sporulation in this microorganism is linked to phosphate availability when other nutrients are not limiting. This result provides a new tool for the dissection of nutrient-energy and signal pathways in cell growth and differentiation.

Formation and characterization of non-growth states in Clostridium thermocellum: spores and L-forms

Background

Clostridium thermocellum is an anaerobic thermophilic bacterium that exhibits high levels of cellulose solublization and produces ethanol as an end product of its metabolism. Using cellulosic biomass as a feedstock for fuel production is an attractive prospect, however, growth arrest can negatively impact ethanol production by fermentative microorganisms such as C. thermocellum. Understanding conditions that lead to non-growth states in C. thermocellum can positively influence process design and culturing conditions in order to optimize ethanol production in an industrial setting.

Results

We report here that Clostridium thermocellum ATCC 27405 enters non-growth states in response to specific growth conditions. Non-growth states include the formation of spores and a L-form-like state in which the cells cease to grow or produce the normal end products of metabolism. Unlike other sporulating organisms, we did not observe sporulation of C. thermocellum in low carbon or nitrogen environments. However, sporulation did occur in response to transfers between soluble and insoluble substrates, resulting in approximately 7% mature spores. Exposure to oxygen caused a similar sporulation response. Starvation conditions during continuous culture did not result in spore formation, but caused the majority of cells to transition to a L-form state. Both spores and L-forms were determined to be viable. Spores exhibited enhanced survival in response to high temperature and prolonged storage compared to L-forms and vegetative cells. However, L-forms exhibited faster recovery compared to both spores and stationary phase cells when cultured in rich media.

Conclusions

Both spores and L-forms cease to produce ethanol, but provide other advantages for C. thermocellum including enhanced survival for spores and faster recovery for L-forms. Understanding the conditions that give rise to these two different non-growth states, and the implications that each has for enabling or enhancing C. thermocellum survival may promote the efficient cultivation of this organism and aid in its development as an industrial microorganism.

Sugar inhibits the production of the toxins that trigger clostridial gas gangrene

Histotoxic strains of Clostridium perfringens cause human gas gangrene, a devastating infection during which potent tissue-degrading toxins are produced and secreted. Although this pathogen only grows in anaerobic-nutrient-rich habitats such as deep wounds, very little is known regarding how nutritional signals influence gas gangrene-related toxin production. We hypothesize that sugars, which have been used throughout history to prevent wound infection, may represent a nutritional signal against gas gangrene development. Here we demonstrate, for the first time, that sugars (sucrose, glucose) inhibited the production of the main protein toxins, PLC (alpha-toxin) and PFO (theta-toxin), responsible for the onset and progression of gas gangrene. Transcription analysis experiments using plc-gusA and pfoA-gusA reporter fusions as well as RT-PCR analysis of mRNA transcripts confirmed that sugar represses plc and pfoA expression. In contrast an isogenic C. perfringens strain that is defective in CcpA, the master transcription factor involved in carbon catabolite response, was completely resistant to the sugar-mediated inhibition of PLC and PFO toxin production. Furthermore, the production of PLC and PFO toxins in the ccpA mutant strain was several-fold higher than the toxin production found in the wild type strain. Therefore, CcpA is the primary or unique regulatory protein responsible for the carbon catabolite (sugar) repression of toxin production of this pathogen. The present results are analyzed in the context of the role of CcpA for the development and aggressiveness of clostridial gas gangrene and the well-known, although poorly understood, anti-infective and wound healing effects of sugars and related substances.

The Agr-like quorum-sensing system regulates sporulation and production of enterotoxin and beta2 toxin by Clostridium perfringens type A non-food-borne human gastrointestinal disease strain F5603

Clostridium perfringens type A strains producing enterotoxin (CPE) cause one of the most common bacterial food-borne illnesses, as well as many cases of non-food-borne human gastrointestinal disease. Recent studies have shown that an Agr-like quorum-sensing system controls production of chromosomally encoded alpha-toxin and perfringolysin O by C. perfringens, as well as sporulation by Clostridium botulinum and Clostridium sporogenes. The current study explored whether the Agr-like quorum-sensing system also regulates sporulation and production of two plasmid-encoded toxins (CPE and beta2 toxin) that may contribute to the pathogenesis of non-food-borne human gastrointestinal disease strain F5603. An isogenic agrB null mutant was inhibited for production of beta2 toxin during vegetative growth and in sporulating culture, providing the first evidence that, in C. perfringens, this system can control production of plasmid-encoded toxins as well as chromosomally encoded toxins. This mutant also showed reduced production of alpha-toxin and perfringolysin O during vegetative growth. Importantly, when cultured in sporulation medium, the mutant failed to efficiently form spores and was blocked for CPE production. Complementation partially or fully reversed all phenotypic changes in the mutant, confirming that they were specifically due to inactivation of the agr locus. Western blots suggest that this loss of sporulation and sporulation-specific CPE production for the agrB null mutant involves, at least in part, Agr-mediated regulation of production of Spo0A and alternative sigma factors, which are essential for C. perfringens sporulation.

Characterization of the sporulation initiation pathway of Clostridium difficile and its role in toxin production

Clostridium difficile is responsible for significant mortality and morbidity in the hospitalized elderly. C. difficile spores are infectious and are a major factor contributing to nosocomial transmission. The Spo0A response regulator is the master regulator for sporulation initiation and can influence many other cellular processes. Using the ClosTron gene knockout system, we inactivated genes encoding Spo0A and a putative sporulation-associated sensor histidine kinase in C. difficile. Inactivation of spo0A resulted in an asporogeneous phenotype, whereas inactivation of the kinase reduced C. difficile sporulation capacity by 3.5-fold, suggesting that this kinase also has a role in sporulation initiation. Furthermore, inactivation of either spo0A or the kinase resulted in a marked defect in C. difficile toxin production. Therefore, Spo0A and the signaling pathway that modulates its activity appear to be involved in regulation of toxin synthesis in C. difficile. In addition, Spo0A was directly phosphorylated by a putative sporulation-associated kinase, supporting the hypothesis that sporulation initiation in C. difficile is controlled by a two-component signal transduction system rather than a multicomponent phosphorelay. The implications of these findings for C. difficile sporulation, virulence, and transmission are discussed.

Clostridium perfringens sporulation and its relevance to pathogenesis

Enterotoxigenic Clostridium perfringens has been recognized as a ubiquitous human pathogen owing to, at least in part, three important characteristics. First, this bacterium undergoes asymmetric cell division or sporulation under nutritionally deprived conditions. Second, during sporulation growth it produces C. perfringens enterotoxin, an important virulence factor for food poisoning and nonfood-borne gastrointestinal diseases in humans. Third, at the final stage of sporulation, sporulating cells lyse to release mature dormant spores, which are resistant to high heat and can survive in the environment for long periods of time. Here, we present an overview of what is currently known about the mechanisms of sporulation, sporulation-regulated C. perfringens enterotoxin synthesis and spore heat resistance and we discuss their impact on the pathogenesis of C. perfringens.

Characterization of a unique class C acid phosphatase from Clostridium perfringens

Clostridium perfringens is a gram-positive anaerobe and a pathogen of medical importance. The detection of acid phosphatase activity is a powerful diagnostic indicator of the presence of C. perfringens among anaerobic isolates; however, characterization of the enzyme has not previously been reported. Provided here are details of the characterization of a soluble recombinant form of this cell-associated enzyme. The denatured enzyme was approximately 31 kDa and a homodimer in solution. It catalyzed the hydrolysis of several substrates, including para-nitrophenyl phosphate, 4-methylumbelliferyl phosphate, and 3' and 5' nucleoside monophosphates at pH 6. Calculated K(m)s ranged from 0.2 to 0.6 mM with maximum velocity ranging from 0.8 to 1.6 micromol of P(i)/s/mg. Activity was enhanced in the presence of some divalent cations but diminished in the presence of others. Wild-type enzyme was detected in all clinical C. perfringens isolates tested and found to be cell associated. The described enzyme belongs to nonspecific acid phosphatase class C but is devoid of lipid modification commonly attributed to this class.

Development and application of flow-cytometric techniques for analyzing and sorting endospore-forming clostridia

The study of microbial heterogeneity at the single-cell level is a rapidly growing area of research in microbiology and biotechnology due to its significance in pathogenesis, environmental biology, and industrial biotechnologies. However, the tools available for efficiently and precisely probing such heterogeneity are limited for most bacteria. Here we describe the development and application of flow-cytometric (FC) and fluorescence-assisted cell-sorting techniques for the study of endospore-forming bacteria. We show that by combining FC light scattering (LS) with nucleic acid staining, we can discriminate, quantify, and enrich all sporulation-associated morphologies exhibited by the endospore-forming anaerobe Clostridium acetobutylicum. Using FC LS analysis, we quantitatively show that clostridial cultures commonly perform multiple rounds of sporulation and that sporulation is induced earlier by the overexpression of Spo0A, the master regulator of endospore formers. To further demonstrate the power of our approach, we employed FC LS analysis to generate compelling evidence to challenge the long-accepted view in the field that the clostridial cell form is the solvent-forming phenotype.

A critical phosphate concentration in the stationary phase maintains ndh gene expression and aerobic respiratory chain activity in Escherichia coli

Escherichia coli NADH dehydrogenase-2 (NDH-2) is a primary dehydrogenase in aerobic respiration that shows cupric-reductase activity. The enzyme is encoded by ndh, which is highly regulated by global transcription factors. It was described that the gene is expressed in the exponential growth phase and repressed in late stationary phase. We report the maintenance of NDH-2 activity and ndh expression in the stationary phase when cells were grown in media containing at least 37 mM phosphate. Gene regulation was independent of RpoS and other transcription factors described to interact with the ndh promoter. At this critical phosphate concentration, cell viability, oxygen consumption rate, and NADH/NAD+ ratio were maintained in the stationary phase. These physiological parameters gradually changed, but NDH-2 activity remained high for up to 94 h. Phosphate seems to trigger an internal signal in the stationary phase mediated by systems not yet described.

The phosphate regulon and bacterial virulence: a regulatory network connecting phosphate homeostasis and pathogenesis

Bacterial pathogens regulate virulence factor gene expression coordinately in response to environmental stimuli, including nutrient starvation. The phosphate (Pho) regulon plays a key role in phosphate homeostasis. It is controlled by the PhoR/PhoB two-component regulatory system. PhoR is an integral membrane signaling histidine kinase that, through an interaction with the ABC-type phosphate-specific transport (Pst) system and a protein called PhoU, somehow senses environmental inorganic phosphate (P(i)) levels. Under conditions of P(i) limitation (or in the absence of a Pst component or PhoU), PhoR activates its partner response regulator PhoB by phosphorylation, which, in turn, up- or down-regulates target genes. Single-cell profiling of PhoB activation has shown recently that Pho regulon gene expression exhibits a stochastic, "all-or-none" behavior. Recent studies have also shown that the Pho regulon plays a role in the virulence of several bacteria. Here, we present a comprehensive overview of the role of the Pho regulon in bacterial virulence. The Pho regulon is clearly not a simple regulatory circuit for controlling phosphate homeostasis; it is part of a complex network important for both bacterial virulence and stress response.

Carbon catabolite repression of type IV pilus-dependent gliding motility in the anaerobic pathogen Clostridium perfringens

Clostridium perfringens is an anaerobic, gram-positive, spore-forming bacterium responsible for the production of severe histotoxic and gastrointestinal diseases in humans and animals. In silico analysis of the three available genome-sequenced C. perfringens strains (13, SM101, and ATCC13124) revealed that genes that encode flagellar proteins and genes involved in chemotaxis are absent. However, those strains exhibit type IV pilus (TFP)-dependent gliding motility. Since carbon catabolite regulation has been implicated in the control of different bacterial behaviors, we investigated the effects of glucose and other readily metabolized carbohydrates on C. perfringens gliding motility. Our results demonstrate that carbon catabolite regulation constitutes an important physiological regulatory mechanism that reduces the proficiencies of the gliding motilities of a large number of unrelated human- and animal-derived pathogenic C. perfringens strains. Glucose produces a strong dose-dependent inhibition of gliding development without affecting vegetative growth. Maximum gliding inhibition was observed at a glucose concentration (1%) previously reported to also inhibit other important behaviors in C. perfringens, such as spore development. The inhibition of gliding development in the presence of glucose was due, at least in part, to the repression of the genes pilT and pilD, whose products are essential for TFP-dependent gliding proficiency. The inhibitory effects of glucose on pilT and pilD expression were under the control of the key regulatory protein CcpA (catabolite control protein A). The deficiency in CcpA activity of a ccpA knockout C. perfringens mutant strain restored the expressions of pilT and pilD and gliding proficiency in the presence of 1% glucose. The carbon catabolite repression of the gliding motility of the ccpA mutant strain was restored after the introduction of a complementing plasmid harboring a wild-type copy of ccpA. These results point to a central role for CcpA in orchestrating the negative effect of carbon catabolite regulation on C. perfringens gliding motility. Furthermore, we discovered a novel positive role for CcpA in pilT and pilD expression and gliding proficiency in the absence of catabolite regulation. Carbon catabolite repression of gliding motility and the dual role of CcpA, either as repressor or as activator of gliding, are analyzed in the context of the different social behaviors and diseases produced by C. perfringens.

Complementation of a Clostridium perfringens spo0A mutant with wild-type spo0A from other Clostridium species

To evaluate whether C. perfringens can be used as a model organism for studying the sporulation process in other clostridia, C. perfringens spo0A mutant IH101 was complemented with wild-type spo0A from four different Clostridium species. Wild-type spo0A from C. acetobutylicum or C. tetani, but not from C. botulinum or C. difficile, restored sporulation and enterotoxin production in IH101. The ability of spo0A from C. botulinum or C. difficile to complement the lack of spore formation in IH101 might be due, at least in part, to the low levels of spo0A transcription and Spo0A production.