The pore-forming protein Cry5B elicits the pathogenicity of Bacillus sp. against Caenorhabditis elegans

C. elegans monitor energy status via the AMPK pathway to trigger innate immune responses against bacterial pathogens

Pathogen recognition and the triggering of host innate immune system are critical to understanding pathogen-host interaction. Cellular surveillance systems have been identified as an important strategy for the identification of microbial infection. In the present study, using Bacillus thuringiensis-Caenorhabditis elegans as a model, we found an approach for surveillance systems to sense pathogens. We report that Bacillus thuringiensis Cry5Ba, a typical pore-forming toxin, caused mitochondrial damage and energy imbalance by triggering potassium ion leakage, instead of directly targeting mitochondria. Interestingly, we find C. elegans can monitor intracellular energy status to trigger innate immune responses via AMP-activated protein kinase (AMPK), secreting multiple effectors to defend against pathogenic attacks. Our study indicates that the imbalance of energy status is a prevalent side effect of pathogen infection. Furthermore, the AMPK-dependent surveillance system may serve as a practicable strategy for the host to recognize and defense against pathogens.

Bacillus thuringiensis toxin Cry5Ba triggers potassium ion leakage, causing mitochondrial damage and energy imbalance. C. elegans can monitor this intracellular energy imbalance via AMP-activated protein kinase to trigger innate immune responses.

Injectable Nanosponge-Loaded Pluronic F127 Hydrogel for Pore-Forming Toxin Neutralization

PURPOSE

Pore-forming toxins (PFTs) perform important functions during bacterial infections. Among various virulence-targeting therapies, nanosponges (NSs) have excellent neutralization effects on multiple PFTs. To enhance treatment efficacy, NSs tend to be incorporated into other biomaterials, such as hydrogels.

METHODS

In the present work, red blood cell (RBC) vesicles were harvested to wrap polymer nanoparticles, leading to the formation of NSs, and the optimal Pluronic F127 hydrogel concentration was determined for gelation. Then, a novel detoxification system was constructed by incorporating NSs into an optimized Pluronic F127 hydrogel (NS-pGel). Next, the system was characterized by rheological and sustained release behavior as well as micromorphology. Then, the in vitro neutralization effect of NS-pGel on various PFTs was examined by a hemolysis protocol. Finally, therapeutic and prophylactic detoxification efficiency was evaluated in a mouse subcutaneous infection model in vivo.

RESULTS

A thermosensitive, injectable detoxification system was successfully constructed by loading NSs into a 30% Pluronic F127 hydrogel. Characterization results demonstrated that the NS-pGel hybrid system sustained an ideal fluidity and viscosity at lower temperatures but exhibited a quick sol-gel transition capacity near body temperature. In addition, this hybrid system had a sustained release behavior accompanied by good biocompatibility and biodegradability. Finally, the NS-pGel system showed neutralization effects similar to those of NSs both in vitro and in vivo, indicating a good preservation of NS functionality.

CONCLUSION

In conclusion, we constructed a novel temperature-sensitive detoxification system with good biocompatibility and biodegradability, which may be applied to the clinical treatment of PFT-induced local lesions and infections.

Proteomic analysis of Caenorhabditis elegans against Salmonella Typhi toxic proteins

Bacterial effector molecules are crucial infectious agents that can cause pathogenesis. In the present study, the pathogenesis of toxic Salmonella enterica serovar Typhi (S. Typhi) proteins on the model host Caenorhabditis elegans was investigated by exploring the host's regulatory proteins during infection through the quantitative proteomics approach. Extracted host proteins were analyzed using two-dimensional gel electrophoresis (2D-GE) and differentially regulated proteins were identified using MALDI TOF/TOF/MS analysis. Of the 150 regulated proteins identified, 95 were downregulated while 55 were upregulated. The interaction network of regulated proteins was predicted using the STRING tool. Most downregulated proteins were involved in muscle contraction, locomotion, energy hydrolysis, lipid synthesis, serine/threonine kinase activity, oxidoreductase activity, and protein unfolding. Upregulated proteins were involved in oxidative stress pathways. Hence, cellular stress generated by S. Typhi proteins in the model host was determined using lipid peroxidation as well as oxidant and antioxidant assays. In addition, candidate proteins identified via extract analysis were validated by western blotting, and the roles of several crucial molecules were analyzed in vivo using transgenic strains (myo-2 and col-19) and mutant (ogt-1) of C. elegans. To the best of our knowledge, this is the first study to report protein regulation in host C. elegans exposed to toxic S. Typhi proteins. It highlights the significance of p38 MAPK and JNK immune pathways.

Take a Walk to the Wild Side of Caenorhabditis elegans-Pathogen Interactions

Microbiomes form intimate functional associations with their hosts. Much has been learned from correlating changes in microbiome composition to host organismal functions. However, in-depth functional studies require the manipulation of microbiome composition coupled with the precise interrogation of organismal physiology-features available in few host study systems. Caenorhabditis elegans has proven to be an excellent genetic model organism to study innate immunity and, more recently, microbiome interactions. The study of C. elegans-pathogen interactions has provided in depth understanding of innate immune pathways, many of which are conserved in other animals. However, many bacteria were chosen for these studies because of their convenience in the lab setting or their implication in human health rather than their native interactions with C. elegans In their natural environment, C. elegans feed on a variety of bacteria found in rotting organic matter, such as rotting fruits, flowers, and stems. Recent work has begun to characterize the native microbiome and has identified a common set of bacteria found in the microbiome of C. elegans While some of these bacteria are beneficial to C. elegans health, others are detrimental, leading to a complex, multifaceted understanding of bacterium-nematode interactions. Current research on nematode-bacterium interactions is focused on these native microbiome components, both their interactions with each other and with C. elegans We will summarize our knowledge of bacterial pathogen-host interactions in C. elegans, as well as recent work on the native microbiome, and explore the incorporation of these bacterium-nematode interactions into studies of innate immunity and pathogenesis.

Recombinant Paraprobiotics as a New Paradigm for Treating Gastrointestinal Nematode Parasites of Humans

Gastrointestinal nematodes (GINs) of humans, e.g., hookworms, negatively impact childhood growth, cognition, nutrition, educational attainment, income, productivity, and pregnancy. Hundreds of millions of people are targeted with mass drug administration (MDA) of donated benzimidazole anthelmintics.

Gastrointestinal nematodes (GINs) of humans, e.g., hookworms, negatively impact childhood growth, cognition, nutrition, educational attainment, income, productivity, and pregnancy. Hundreds of millions of people are targeted with mass drug administration (MDA) of donated benzimidazole anthelmintics. However, benzimidazole efficacy against GINs is suboptimal, and reduced/low efficacy has been seen. Developing an anthelmintic for human MDA is daunting: it must be safe, effective, inexpensive, stable without a cold chain, and massively scalable. Bacillus thuringiensis crystal protein 5B (Cry5B) has anthelmintic properties that could fill this void. Here, we developed an active pharmaceutical ingredient (API) containing B. thuringiensis Cry5B compatible with MDA. We expressed Cry5B in asporogenous B. thuringiensis during vegetative phase, forming cytosolic crystals. These bacteria with cytosolic crystals (BaCC) were rendered inviable (inactivated BaCC [IBaCC]) with food-grade essential oils. IBaCC potency was validated in vitro against nematodes. IBaCC was also potent in vivo against human hookworm infections in hamsters. IBaCC production was successfully scaled to 350 liters at a contract manufacturing facility. A simple fit-for-purpose formulation to protect against stomach digestion and powdered IBaCC were successfully made and used against GINs in hamsters and mice. A pilot histopathology study and blood chemistry workup showed that five daily consecutive doses of 200 mg/kg body weight Cry5B IBaCC (the curative single dose is 40 mg/kg) was nontoxic to hamsters and completely safe. IBaCC is a safe, inexpensive, highly effective, easy-to-manufacture, and scalable anthelmintic that is practical for MDA and represents a new paradigm for treating human GINs.

Use of RNAi as a preliminary tool for screening putative receptors of nematicidal toxins from Bacillus thuringiensis

Bacillus thuringiensis is a potential control agent for plant-parasitic nematodes. Nematode intestinal receptors for Cry21-type toxins are poorly known. Therefore, a strategy was tested as a primary screening tool to find possible Cry toxin receptors, using a nematicidal Bt strain and the RNAi technique on Caenorhabditis elegans. Six genes encoding intestinal membrane proteins were selected (abt-4, bre-1, bre-2, bre-3, asps-1, abl-1) as possible targets for Cry proteins. Fractions of each selected gene were amplified by PCR. Amplicons were cloned into the L4440 vector to transform the E. coli HT155 (DE3) strain. Transformed bacteria were used to silence the selected genes using the RNAi feeding method. Nematodes with silenced genes were tested with the Bt strain LBIT-107, which harbors the nematicidal protein Cry21Aa3, among others. Results indicated that nematodes with the silenced abt-4 gene were 69.5% more resistant to the LBIT-107 strain, in general, and 79% to the Cry21Aa3 toxin, specifically.

A unifying hypothesis on the central role of reactive oxygen species in bacterial pathogenesis and host defense in C. elegans

During intestinal infection, microbes induce ROS by various mechanisms in C. elegans. ROS can have beneficial roles, acting as antimicrobials and as signaling molecules that activate cytoprotective pathways. Failure to maintain appropriate levels of ROS causes oxidative stress and cellular damage. This review uses the Damage Response Framework to interpret several recent observations on the relationships between infection, host response, and host damage, with a focus on mechanisms mediated by ROS. We propose a unifying hypothesis that ROS drive a collapse in proteostasis in infected C. elegans, which results in death during unresolved infection. Because the signaling pathways highlighted here are conserved in mammals, the mentioned and future studies can provide new tools of hypothesis generation in human health and disease.

Anthrax toxin component, Protective Antigen, protects insects from bacterial infections

Anthrax is a major zoonotic disease of wildlife, and in places like West Africa, it can be caused by Bacillus anthracis in arid nonsylvatic savannahs, and by B. cereus biovar anthracis (Bcbva) in sylvatic rainforests. Bcbva-caused anthrax has been implicated in as much as 38% of mortality in rainforest ecosystems, where insects can enhance the transmission of anthrax-causing bacteria. While anthrax is well-characterized in mammals, its transmission by insects points to an unidentified anthrax-resistance mechanism in its vectors. In mammals, a secreted anthrax toxin component, 83 kDa Protective Antigen (PA83), binds to cell-surface receptors and is cleaved by furin into an evolutionary-conserved PA20 and a pore-forming PA63 subunits. We show that PA20 increases the resistance of Drosophila flies and Culex mosquitoes to bacterial challenges, without directly affecting the bacterial growth. We further show that the PA83 loop known to be cleaved by furin to release PA20 from PA63 is, in part, responsible for the PA20-mediated protection. We found that PA20 binds directly to the Toll activating peptidoglycan-recognition protein-SA (PGRP-SA) and that the Toll/NF-κB pathway is necessary for the PA20-mediated protection of infected flies. This effect of PA20 on innate immunity may also exist in mammals: we show that PA20 binds to human PGRP-SA ortholog. Moreover, the constitutive activity of Imd/NF-κB pathway in MAPKK Dsor1 mutant flies is sufficient to confer the protection from bacterial infections in a manner that is independent of PA20 treatment. Lastly, Clostridium septicum alpha toxin protects flies from anthrax-causing bacteria, showing that other pathogens may help insects resist anthrax. The mechanism of anthrax resistance in insects has direct implications on insect-mediated anthrax transmission for wildlife management, and with potential for applications, such as reducing the sensitivity of pollinating insects to bacterial pathogens.

Comparative Mortality and Adaptation of a Smurf Assay in two Species of Tenebrionid Beetles Exposed to Bacillus thuringiensis

Bacillus thuringiensis is a spore-forming bacterium which infects insect larvae naturally via the oral route. Its virulence factors interact with the epithelium of the digestive tract of insect larvae, disrupting its function and eventually leading to the death of susceptible hosts. The most cited B. thuringiensis killing mechanism is the extensive damage caused to the insect midgut, leading to its leakage. The mortality caused by B. thuringiensis has been shown to vary between serovars and isolates, as well as between host life stages. Moreover, whether susceptibility to B. thuringiensis-induced gut leakage is generalized to all host species and whether there is individual variation within species is unclear. In this study, we adapted a non-invasive “Smurf” assay from Drosophila melanogaster to two species of tenebrionid beetles: The mealworm beetle Tenebrio molitor and the red flour beetle Tribolium castaneum, during exposure to B. thuringiensis. We highlight a differential mortality between two age/size classes of T. molitor larvae, as well as different killing dynamics between B. thuringiensis var. tenebrionis and var. tolworthi in T. castaneum. The Smurf assay did not reveal a high occurrence of extensive gut disintegration in both host species upon ingestion during B. thuringiensis exposure.

Avoidance behavior independent of innate-immune signaling seen in Caenorhabditis elegans challenged with Bacillus anthracis

Small organisms, like the nematode C. elegans, are emerging as insightful models in which to study host/pathogen interactions and the evolving interplay between host defenses and microbial offenses. In C. elegans the innate immune response has been shown to be connected to the DAF-2 insulin/insulin-like growth factor 1 (IGF-1) signal pathway, a critical transduction pathway that mediates stress response in the worms via the DAF-16 FOXO/forkhead transcription factor. Our studies of the C. elegans' phenotypes that are associated with behavioral innate immune response (avoidance behavior) and IGF-1 signaling perturbations (lifespan effects) led us to question the cause of the avoidance behavior observed when C. elegans are challenged with B. anthracis. While worms indeed avoid B. anthracis, and this behavior seems to be partly tied to IGF-1 signaling, the bacteria have neither nematocidal nor visible pathogenic effects on the worms. In fact, worms fed B. anthracis alone exhibit extended lifespans. We demonstrate that the extended lifespan phenotype seen in worms fed B. anthracis is likely the result of calorie restriction, and that worms do not eat B. anthracis even when avoidance behaviors have been suppressed. We further demonstrate a large time lag between the onset of avoidance behavior (which occurs upon contact with B. anthracis), and the induction of IGF-1 signaling (which occurs much later) in worms fed B. anthracis. Taken together, our data demonstrate behavioral avoidance that does not appear to be linked to a measurable immune response. We propose that, in some situations, avoidance behaviors categorized as immunological might be more accurately described as broad foraging behaviors induced in worms presented with a non-preferred food choice, or with a food choice that is either difficult or impossible for the worms to ingest.

Galleria mellonella as an Infection Model for Bacillus anthracis Sterne

Understanding bacterial virulence provides insight into the molecular basis behind infection and could identify new drug targets. However, assessing potential virulence determinants relies on testing in an animal model. The mouse is a well-validated model but it is constrained by the ethical and logistical challenges of using vertebrate animals. Recently the larva of the greater wax moth Galleria mellonella has been explored as a possible infection model for a number of pathogens. In this study, we developed G. mellonella as an infection model for Bacillus anthracis Sterne. We first validated two different infection assays, a survival assay and a competition assay, using mutants containing disruptions in known B. anthracis virulence genes. We next tested the utility of G. mellonella to assess the virulence of transposon mutants with unknown mutations that had increased susceptibility to hydrogen peroxide in in vitro assays. One of these transposon mutants also displayed significantly decreased virulence in G. mellonella. Further investigation revealed that this mutant had a disruption in the petrobactin biosynthesis operon (asbABCDEF), which has been previously implicated in both virulence and defense against oxidative stress. We conclude that G. mellonella can detect attenuated virulence of B. anthracis Sterne in a manner consistent with that of mammalian infection models. Therefore, G. mellonella could serve as a useful alternative to vertebrate testing, especially for early assessments of potential virulence genes when use of a mammalian model may not be ethical or practical.

Bacillus thuringiensis Spores and Vegetative Bacteria: Infection Capacity and Role of the Virulence Regulon PlcR Following Intrahaemocoel Injection of Galleria mellonella

Bacillus thuringiensis is an invertebrate pathogen that produces insecticidal crystal toxins acting on the intestinal barrier. In the Galleria mellonella larvae infection model, toxins from the PlcR virulence regulon contribute to pathogenicity by the oral route. While B. thuringiensis is principally an oral pathogen, bacteria may also reach the insect haemocoel following injury of the cuticle. Here, we address the question of spore virulence as compared to vegetative cells when the wild-type Bt407cry- strain and its isogenic ∆plcR mutant are inoculated directly into G. mellonella haemocoel. Mortality dose-response curves were constructed at 25 and 37 °C using spores or vegetative cell inocula, and the 50% lethal dose (LD₅₀) in all infection conditions was determined after 48 h of infection. Our findings show that (i) the LD₅₀ is lower for spores than for vegetative cells for both strains, while the temperature has no significant influence, and (ii) the ∆plcR mutant is four to six times less virulent than the wild-type strain in all infection conditions. Our results suggest that the environmental resistant spores are the most infecting form in haemocoel and that the PlcR virulence regulon plays an important role in toxicity when reaching the haemocoel from the cuticle and not only following ingestion.

The intestinal intermediate filament network responds to and protects against microbial insults and toxins

The enrichment of intermediate filaments in the apical cytoplasm of intestinal cells is evolutionarily conserved, forming a sheath that is anchored to apical junctions and positioned below the microvillar brush border, which suggests a protective intracellular barrier function. To test this, we used Caenorhabditis elegans, the intestinal cells of which are endowed with a particularly dense intermediate filament-rich layer that is referred to as the endotube. We found alterations in endotube structure and intermediate filament expression upon infection with nematicidal B. thuringiensis or treatment with its major pore-forming toxin crystal protein Cry5B. Endotube impairment due to defined genetic mutations of intermediate filaments and their regulators results in increased Cry5B sensitivity as evidenced by elevated larval arrest, prolonged time of larval development and reduced survival. Phenotype severity reflects the extent of endotube alterations and correlates with reduced rescue upon toxin removal. The results provide in vivo evidence for a major protective role of a properly configured intermediate filament network as an intracellular barrier in intestinal cells. This notion is further supported by increased sensitivity of endotube mutants to oxidative and osmotic stress.

Whole-Genome Analysis of Bacillus thuringiensis Revealing Partial Genes as a Source of Novel Cry Toxins

Despite the successful application of crystal proteins (Cry) from Bacillus thuringiensis as biological control agents against insects, there is an increasing demand to identify new Cry toxins having higher toxicity and broad-spectrum activity against insects and plant-parasitic nematodes. To find novel Cry toxins, we screened 100 whole-genome sequences of B. thuringiensis Surprisingly, in addition to full Cry toxins, we found partial sequences, such as typical N-terminal or C-terminal regions with conserved domains, widely distributed among 20 strains of B. thuringiensis In order to further elucidate the functions of partial genes, here, we selected a partial sequence from strain C15, having 28% similarity with the N terminus of Cry5Ba and lacking a typical C terminus, and denoted it Cry5B-like N terminus. This fragment when coexpressed as a fusion protein with the C terminus of Cry5Ba (N-C fusion protein) produces pyramidal crystals. A recombinant N-C fusion protein having a 50% lethal concentration (LC₅₀) of 23.7 μg/ml severely affected the life span, growth, and survival rate of nematodes. Light microscopy showed damage to the intestine of nematodes, confirming the pathogenicity of the N-C fusion protein. Last, the green fluorescent protein (GFP)-labeled mutant Caenorhabditis elegans FT63 showed significant damage to the intestine upon feeding N-C fusion toxin compared to the control. These results imply that partial genes can be a source of new Cry toxins, and further understanding about functions of partial cry genes can help in the study of the evolutionary strategy of B. thuringiensis to produce the multidomain toxins.IMPORTANCE Genomic analysis revealed that coding sequences for N termini and C termini of crystal proteins are widely distributed in B. thuringiensis We found Cry5B-like N terminus, lacking typical C terminus, was unable to be expressed in wild-type strain C15. However, its fusion with the C terminus of Cry5Ba not only was successfully expressed but also exhibited activity against the nematodes. This study provides insight into a potential source for novel Cry toxins.

Nematicidal Activity of Cry1Ea11 from Bacillus thuringiensis BRC-XQ12 Against the Pine Wood Nematode (Bursaphelenchus xylophilus)

The nematicidal activity of 92 Bacillus thuringiensis strains against the pine wood nematode Bursaphelenchus xylophilus, one of the world's top 10 plant-parasitic nematodes, was determined. The insecticidal crystal proteins (ICPs) from Bacillus thuringiensis BRC-XQ12 were the most toxic to Bursaphelenchus xylophilus, with a lethal concentration 50 (LC₅₀) of 32.13 μg/ml. Because the ICPs expressed by Bacillus thuringiensis BRC-XQ12 were closest to Cry1Ea6 and B. thuringiensis BRC-XQ12 contained four kinds of cry1 subgenes (cry1Aa, cry1Cb, cry1Ea, and cry1Ia), Cry1Ea was most likely to be the key active component against the nematode. The 3,516-bp cry1Ea11 gene from BRC-XQ12, as designated by the B. thuringiensis δ-endotoxin nomenclature committee, was expressed in Escherichia coli. Purified Cry1Ea11 showed an LC₅₀ of 32.53 and 23.23 μg/ml at 24 and 48 h, with corresponding virulence equations of Y = 32.15X + 1.38 (R² = 0.9951) and Y = 34.29X + 3.16 (R² = 0.9792), respectively. In order to detect the pathway of B. thuringiensis Cry1Ea11 into Bursaphelenchus xylophilus, the nematode was fed with NHS-rhodamine-labeled GST-Cry1Ea11. The results of confocal laser-scanning microscopy showed that the 159-kDa GST-Cry1Ea11 could be detected in the stylet and the esophageal lumen of the pine wood nematode, indicating that GST-Cry1Ea11 could enter into the nematode through the stylet. As far as we know, no Cry1 proteins have been shown to have activity against plant-parasitic nematodes before. These results demonstrate that Cry1Ea11 is a promising nematicidal protein for controlling pine wilt disease rendered by B. xylophilus, further dramatically broadening the spectrum of Bacillus thuringiensis ICPs.

A Proteomic Analysis Provides Novel Insights into the Stress Responses of Caenorhabditis elegans towards Nematicidal Cry6A Toxin from Bacillus thuringiensis

Cry6A represents a novel family of nematicidal crystal proteins from Bacillus thuringiensis. It has distinctive architecture as well as mechanism of action from Cry5B, a highly focused family of nematicidal crystal proteins, and even from other insecticidal crystal proteins containing the conserved three-domain. However, how nematode defends against Cry6A toxin remains obscure. In this study, the global defense pattern of Caenorhabditis elegans against Cry6Aa2 toxin was investigated by proteomic analysis. In response to Cry6Aa2, 12 proteins with significantly altered abundances were observed from worms, participating in innate immune defense, insulin-like receptor (ILR) signaling pathway, energy metabolism, and muscle assembly. The differentially expressed proteins (DEPs) functioning in diverse biological processes suggest that a variety of defense responses participate in the stress responses of C. elegans to Cry6Aa2. The functional verifications of DEPs suggest that ILR signaling pathway, DIM-1, galectin LEC-6 all are the factors of defense responses to Cry6Aa2. Moreover, Cry6Aa2 also involves in accelerating the metabolic energy production which fulfills the energy demand for the immune responses. In brief, our findings illustrate the global pattern of defense responses of nematode against Cry6A for the first time, and provide a novel insight into the mechanism through which worms respond to Cry6A.

Bacillus: A Biological Tool for Crop Improvement through Bio-Molecular Changes in Adverse Environments

Crop productivity is affected by environmental and genetic factors. Microbes that are beneficial to plants are used to enhance the crop yield and are alternatives to chemical fertilizers and pesticides. Pseudomonas and Bacillus species are the predominant plant growth-promoting bacteria. The spore-forming ability of Bacillus is distinguished from that of Pseudomonas. Members of this genus also survive for a long time under unfavorable environmental conditions. Bacillus spp. secrete several metabolites that trigger plant growth and prevent pathogen infection. Limited studies have been conducted to understand the physiological changes that occur in crops in response to Bacillus spp. to provide protection against adverse environmental conditions. This review describes the current understanding of Bacillus-induced physiological changes in plants as an adaptation to abiotic and biotic stresses. During water scarcity, salinity and heavy metal accumulate in soil, Bacillus spp. produce exopolysaccharides and siderophores, which prevent the movement of toxic ions and adjust the ionic balance and water transport in plant tissues while controlling the pathogenic microbial population. In addition, the synthesis of indole-3-acetic acid, gibberellic acid and1-aminocyclopropane-1-carboxylate (ACC) deaminase by Bacillus regulates the intracellular phytohormone metabolism and increases plant stress tolerance. Cell-wall-degrading substances, such as chitosanase, protease, cellulase, glucanase, lipopeptides and hydrogen cyanide from Bacillus spp. damage the pathogenic bacteria, fungi, nematodes, viruses and pests to control their populations in plants and agricultural lands. The normal plant metabolism is affected by unfavorable environmental stimuli, which suppress crop growth and yield. Abiotic and biotic stress factors that have detrimental effects on crops are mitigated by Bacillus-induced physiological changes, including the regulation of water transport, nutrient up-take and the activation of the antioxidant and defense systems. Bacillus association stimulates plant immunity against stresses by altering stress-responsive genes, proteins, phytohormones and related metabolites. This review describes the beneficial effect of Bacillus spp. on crop plants, which improves plant productivity under unfavorable climatic conditions, and the current understanding of the mitigation mechanism of Bacillus spp. in stress-tolerant and/or stress-resistant plants.

A novel metalloproteinase virulence factor is involved in Bacillus thuringiensis pathogenesis in nematodes and insects

The Gram-positive soil bacterium Bacillus thuringiensis has been developed as the leading microbial insecticide for years. The pathogenesis of B. thuringiensis requires common extracellular factors that depend on the PlcR regulon, which regulates a large number of virulence factors; however, the precise role of many of these proteins is not known. In this study, we describe the complete lifecycle of a nematicidal B. thuringiensis strain in the free living nematode Caenorhabditis elegans using in vitro and in vivo molecular techniques to follow host and bacterial effectors during the infection process. We then focus on the metalloproteinase ColB, a collagenase, which was found highly important for destruction of the intestine thereby facilitates the adaptation and colonization of B. thuringiensis in C. elegans. In vivo green fluorescent protein (GFP) reporter-gene studies showed that ColB expression is highly induced and regulated by the global activator PlcR. Finally, we demonstrated that ColB also takes part in B. thuringiensis virulence in an insect model following injection and oral infection. Indeed, addition of purified ColB accelerates the action of Cry toxin proteins in insects, too. These results give novel insights into host adaptation for B. thuringiensis and other B. cereus group bacteria and highlight the role of collagenase metalloproteases to synergize infection process.

High instability of a nematicidal Cry toxin plasmid in Bacillus thuringiensis

In bacterial pathogens, virulence factors are often carried on plasmids and other mobile genetic elements, and as such, plasmid evolution is central in understanding pathogenicity. Bacillus thuringiensis is an invertebrate pathogen that uses plasmid-encoded crystal (Cry) toxins to establish infections inside the host. Our study aimed to quantify stability of two Cry toxin-encoding plasmids, BTI_23p and BTI_16p, under standard laboratory culturing conditions. These two plasmids are part of the genome of the B. thuringiensis strain MYBT18679, which is of particular interest because of its high pathogenicity towards nematodes. One of the plasmids, BTI_23p, was found to be highly unstable, with substantial loss occurring within a single growth cycle. Nevertheless, longer term experimental evolution in the absence of a host revealed maintenance of the plasmid at low levels in the bacterial populations. BTI_23p encodes two nematicidal Cry toxins, Cry21Aa2 and Cry14Aa1. Consistent with previous findings, loss of the plasmid abolished pathogenicity towards the nematode Caenorhabditis elegans, which could be rescued by addition of Cry21Aa2-expressing Escherichia coli. These results implicate BTI_23p as a plasmid that is required for successful infection, yet unstable when present at high frequency in the population, consistent with the role of Cry toxins as public goods.

A two-domain protein triggers heat shock pathway and necrosis pathway both in model plant and nematode

The entomopathogen Bacillus thuringiensis is equipped with multiple virulent factors. The genome sequence of B. thuringiensis YBT1520 revealed the presence of a two-domain protein named Nel which is composed of a necrosis-inducing phytophthora protein 1-like domain found in phytopathogens and a ricin B-like lectin domain. The merging of two distantly related domains is relatively rare. Nel induced necrosis and pathogen-triggered immunity (PTI) on model plants. The Nel also exhibited inhibition activity to nematode. Microscopic observation showed that the toxicity of Nel to nematodes targets the intestine. Quantitative proteomics revealed that Nel stimulated the host defence. The Nel thus possesses dual roles, as both toxin and elicitor. Remarkably, the Nel protein triggered a similar response, induction of the heat shock pathway and the necrosis pathway, in both model plants and nematodes. The unusual ability of Nel to function across kingdom suggests a highly conserved mechanism in eukaryotes that predates the divergence of plants and animal. It is also speculated that the two-domain protein is the result of horizontal gene transfer (HGT) between phytopathogens and entomopathogens. Our results provide an example that HGT occurs between members of different species or even genera with lower frequency are particularly important for evolution of new bacterial pathogen lineages with new virulence. Bacillus thuringiensis occupies the same ecological niches, plant and soil, as phytopathogens, providing the opportunity for gene exchange.

Genomic and transcriptomic insights into the efficient entomopathogenicity of Bacillus thuringiensis

Bacillus thuringiensis has been globally used as a microbial pesticide for over 70 years. However, information regarding its various adaptions and virulence factors and their roles in the entomopathogenic process remains limited. In this work, we present the complete genomes of two industrially patented Bacillus thuringiensis strains (HD-1 and YBT-1520). A comparative genomic analysis showed a larger and more complicated genome constitution that included novel insecticidal toxicity-related genes (ITRGs). All of the putative ITRGs were summarized according to the steps of infection. A comparative genomic analysis showed that highly toxic strains contained significantly more ITRGs, thereby providing additional strategies for infection, immune evasion, and cadaver utilization. Furthermore, a comparative transcriptomic analysis suggested that a high expression of these ITRGs was a key factor in efficient entomopathogenicity. We identified an active extra urease synthesis system in the highly toxic strains that may aid B. thuringiensis survival in insects (similar to previous results with well-known pathogens). Taken together, these results explain the efficient entomopathogenicity of B. thuringiensis. It provides novel insights into the strategies used by B. thuringiensis to resist and overcome host immune defenses and helps identify novel toxicity factors.

The diverse nematicidal properties and biocontrol efficacy of Bacillus thuringiensis Cry6A against the root-knot nematode Meloidogyne hapla

Cry6A toxin from Bacillus thuringiensis is a representative nematicidal crystal protein with a variety of nematicidal properties to free-living nematode Caenorhabditis elegans. Cry6A shares very low homology and different structure with Cry5B, another representative nematicidal crystal protein, and probably acts in a distinct pathway. All these strongly indicate that Cry6A toxin is likely a potent candidate for nematicide. The present study dealt with global investigation to determine the detrimental impacts of Cry6Aa2 toxin on Meloidogyne hapla, a root-knot nematode, and evaluated its biocontrol efficacy in pot experiment. Obtained results indicated that Cry6Aa2 toxin exhibits obvious toxicity to second-stage juvenile of M. hapla, and significantly inhibits egg hatch, motility, and penetration to host plant. Pot experiment suggested that soil drenching with spore-crystal mixture of Cry6Aa2 can clearly lighten the disease of root-knot nematode, including reduction of galling index and egg masses on host plant root, decreasing final population of nematode in soil. Moreover, application of Cry6Aa2 can obviously promote plant growth. These results demonstrated that Cry6Aa2 toxin is a promising nematicidal agent, and possesses great potential in plant-parasitic nematode management and construction of transgenic crop with constant resistance to nematode.

Quantitative proteome analysis of Caenorhabditis elegans upon exposure to nematicidal Bacillus thuringiensis

Caenorhabditis elegans can be infected by a plethora of pathogens, most of them are also pathogenic for humans. Consequently, the nematode has emerged as a powerful surrogate host to model microbial human infectious diseases in a non-vertebrate, for the study of innate immunity and host-pathogen interactions. Signaling cascades are well investigated that face bacterial or fungal pathogens. We analyzed the downstream processes of these cascades, i.e. the differential expression of effector and regulatory molecules due to a microbial challenge with a pathogenic strain of the bacterium Bacillus thuringiensis (Bt) in comparison to a non-pathogenic Bt strain. The protein abundance profile of the nematode was studied by quantitative proteomics using iTRAQ labeling and 2D-LC-MS analysis. We developed (i) a novel method for the preparation of defined C. elegans samples; (ii) a pooling strategy for fractions in 2D-LC separation schemes; and (iii) an isobaric labeling scheme reducing the number of necessary LC-MS experiments. More than 3,600 proteins were quantified, 288 of which showed altered abundances, implicating protein classes such as lectins, lysozymes, and transthyretin-like proteins to be involved in the nematode innate immune defense. A number of gene products previously only identified by transcriptomic profiling could be verified at the protein level. Moreover, several other protein classes such as proteases, proteins related to autophagy and apoptosis, structural proteins, and proteins involved in chromatin organization were detected. The results provide an overview of the physiological response towards a pathogen at protein level in the important model organism C. elegans, giving insights into highly complex host-pathogen interactions.

BIOLOGICAL SIGNIFICANCE

This study identified system-wide effects of Bt intoxication on C. elegans at protein level, expanding the catalogue of immune effectors potentially acting towards the pathogen, and provide verification for numerous gene products implicated in previous transcriptomic studies. The data present evidence in support of both a general defense response as well as a specific reaction against the Bt toxin within the nematode. The described findings will also contribute to a deeper understanding of host-microbe interaction in other organisms, including humans, and may provide key information that touches far reaching aspects of coevolutionary processes.

Antinematode activity of Violacein and the role of the insulin/IGF-1 pathway in controlling violacein sensitivity in Caenorhabditis elegans

The purple pigment violacein is well known for its numerous biological activities including antibacterial, antiviral, antiprotozoan, and antitumor effects. In the current study we identify violacein as the antinematode agent produced by the marine bacterium Microbulbifer sp. D250, thereby extending the target range of this small molecule. Heterologous expression of the violacein biosynthetic pathway in E. coli and experiments using pure violacein demonstrated that this secondary metabolite facilitates bacterial accumulation in the nematode intestine, which is accompanied by tissue damage and apoptosis. Nematodes such as Caenorhabditis elegans utilise a well-defined innate immune system to defend against pathogens. Using C. elegans as a model we demonstrate the DAF-2/DAF-16 insulin/IGF-1 signalling (IIS) component of the innate immune pathway modulates sensitivity to violacein-mediated killing. Further analysis shows that resistance to violacein can occur due to a loss of DAF-2 function and/or an increased function of DAF-16 controlled genes involved in antimicrobial production (spp-1) and detoxification (sod-3). These data suggest that violacein is a novel candidate antinematode agent and that the IIS pathway is also involved in the defence against metabolites from non-pathogenic bacteria.

Division of labour and terminal differentiation in a novel Bacillus thuringiensis strain

A major challenge in bacterial developmental biology has been to understand the mechanisms underlying cell fate decisions. Some differentiated cell types display cooperative behaviour. Cooperation is one of the greatest mysteries of evolutionary biology and microbes have been considered as an excellent system for experimentally testing evolution theories. Bacillus thuringiensis (Bt) is a spore-forming bacterium, which is genetically closely related to B. anthracis, the agent of anthrax, and to B. cereus, an opportunistic human pathogen. The defining feature that distinguishes Bt from its relatives is its ability to produce crystal inclusions in the sporulating cells. These toxins are solubilized after ingestion and are cooperative public goods in insect hosts. In this study, we describe a Bt strain LM1212 that presents the unique ability to terminally differentiate into crystal producers and spore formers. Transcriptional analysis based on lacZ and gfp reporter genes suggested that this phenotype is the consequence of a new type of cell differentiation associated with a novel regulation mode of cry gene expression. The differentiating crystal-producer phenotype has higher spore productivity than a typical Bt strain and is better able to compete with Cry toxin null 'cheaters'. Potentially, this division of labour provides additional fitness benefits in terms of spore viability or durability of Cry toxin.

Bacillus thuringiensis DB27 produces two novel protoxins, Cry21Fa1 and Cry21Ha1, which act synergistically against nematodes

Bacillus thuringiensis has been widely used as a biopesticide, primarily for the control of insect pests, but some B. thuringiensis strains specifically target nematodes. However, nematicidal virulence factors of B. thuringiensis are poorly investigated. Here, we describe virulence factors of nematicidal B. thuringiensis DB27 using Caenorhabditis elegans as a model. We show that B. thuringiensis DB27 kills a number of free-living and animal-parasitic nematodes via intestinal damage. Its virulence factors are plasmid-encoded Cry protoxins, since plasmid-cured derivatives do not produce Cry proteins and are not toxic to nematodes. Whole-genome sequencing of B. thuringiensis DB27 revealed multiple potential nematicidal factors, including several Cry-like proteins encoded by different plasmids. Two of these proteins appear to be novel and show high similarity to Cry21Ba1. Named Cry21Fa1 and Cry21Ha1, they were expressed in Escherichia coli and fed to C. elegans, resulting in intoxication, intestinal damage, and death of nematodes. Interestingly, the effects of the two protoxins on C. elegans are synergistic (synergism factor, 1.8 to 2.5). Using purified proteins, we determined the 50% lethal concentrations (LC50s) for Cry21Fa1 and Cry21Ha1 to be 13.6 μg/ml and 23.9 μg/ml, respectively, which are comparable to the LC50 of nematicidal Cry5B. Finally, we found that signaling pathways which protect C. elegans against Cry5B toxin are also required for protection against Cry21Fa1. Thus, B. thuringiensis DB27 produces novel nematicidal protoxins Cry21Fa1 and Cry21Ha1 with synergistic action, which highlights the importance of naturally isolated strains as a source of novel toxins.

Novel role for the yceGH tellurite resistance genes in the pathogenesis of Bacillus anthracis

Bacillus anthracis, the causative agent of anthrax, relies on multiple virulence factors to subvert the host immune defense. Using Caenorhabditis elegans as an infection model, we screened approximately 5,000 transposon mutants of B. anthracis Sterne for decreased virulence. One of the attenuated mutants resulted in loss of expression of yceG and yceH, the last two genes in a six-gene cluster of tellurite resistance genes. We generated an analogous insertional mutant to confirm the phenotype and characterize the role of yceGH in resistance to host defenses. Loss of yceGH rendered the mutants more sensitive to tellurite toxicity as well as to host defenses such as reactive oxygen species and the cathelicidin family of antimicrobial peptides. Additionally, we see decreased survival in mammalian models of infection, including human whole blood and in mice. We identify a novel role for the yceGH genes in B. anthracis Sterne virulence and suggest that C. elegans is a useful infection model to study anthrax pathogenesis.

Bacillus subtilis strain engineered for treatment of soil-transmitted helminth diseases

Soil-transmitted helminths (hookworms, whipworms, and large roundworms) are agents of intestinal roundworm diseases of poverty that infect upwards of 2 billion people worldwide. A great challenge in treating these diseases is the development of anthelmintic therapeutics that are inexpensive, can be produced in great quantity, and are capable of delivery under varied and adverse environmental conditions. A potential solution to this challenge is the use of live bacteria that are acceptable for human consumption, e.g., Bacillus subtilis, and that can be engineered with therapeutic properties. In this study, we expressed the Bacillus thuringiensis anthelmintic protein Cry5B in a bacterial strain that has been used as a model for live bacterial therapy, Bacillus subtilis PY79. PY79 transformed with a Cry5B expression plasmid (PY79-Cry5B) is able to express Cry5B from the endogenous B. thuringiensis cry5B promoter. During sporulation of PY79-Cry5B, Cry5B is packaged as a crystal. Furthermore, Cry5B produced in PY79 is bioactive, with a 50% lethal concentration (LC50) of 4.3 μg/ml against the roundworm Caenorhabditis elegans. PY79-Cry5B was a significantly effective therapeutic in experimental Ancylostoma ceylanicum hookworm infections of hamsters. A single 10-mg/kg (0.071 μmol/kg of body weight) dose of Cry5B administered as a Cry5B-PY79 spore crystal lysate achieved a 93% reduction in hookworm burdens, which is superior on a molar level to reductions seen with clinically used anthelmintics. Given that a bacterial strain such as this one can be produced cheaply in massive quantities, our results demonstrate that the engineering and delivery of live bacterial strains have great potential to treat a significant contributor to poverty worldwide, namely, hookworm disease and other soil-transmitted helminthiasis.

Role of pore-forming toxins in bacterial infectious diseases

Pore-forming toxins (PFTs) are the most common bacterial cytotoxic proteins and are required for virulence in a large number of important pathogens, including Streptococcus pneumoniae, group A and B streptococci, Staphylococcus aureus, Escherichia coli, and Mycobacterium tuberculosis. PFTs generally disrupt host cell membranes, but they can have additional effects independent of pore formation. Substantial effort has been devoted to understanding the molecular mechanisms underlying the functions of certain model PFTs. Likewise, specific host pathways mediating survival and immune responses in the face of toxin-mediated cellular damage have been delineated. However, less is known about the overall functions of PFTs during infection in vivo. This review focuses on common themes in the area of PFT biology, with an emphasis on studies addressing the roles of PFTs in in vivo and ex vivo models of colonization or infection. Common functions of PFTs include disruption of epithelial barrier function and evasion of host immune responses, which contribute to bacterial growth and spreading. The widespread nature of PFTs make this group of toxins an attractive target for the development of new virulence-targeted therapies that may have broad activity against human pathogens.

The red flour beetle as a model for bacterial oral infections

Experimental infection systems are important for studying antagonistic interactions and coevolution between hosts and their pathogens. The red flour beetle Tribolium castaneum and the spore-forming bacterial insect pathogen Bacillus thuringiensis (Bt) are widely used and tractable model organisms. However, they have not been employed yet as an efficient experimental system to study host-pathogen interactions. We used a high throughput oral infection protocol to infect T. castaneum insects with coleopteran specific B. thuringiensis bv. tenebrionis (Btt) bacteria. We found that larval mortality depends on the dietary spore concentration and on the duration of exposure to the spores. Furthermore, differential susceptibility of larvae from different T. castaneum populations indicates that the host genetic background influences infection success. The recovery of high numbers of infectious spores from the cadavers indicates successful replication of bacteria in the host and suggests that Btt could establish infectious cycles in T. castaneum in nature. We were able to transfer plasmids from Btt to a non-pathogenic but genetically well-characterised Bt strain, which was thereafter able to successfully infect T. castaneum, suggesting that factors residing on the plasmids are important for the virulence of Btt. The availability of a genetically accessible strain will provide an ideal model for more in-depth analyses of pathogenicity factors during oral infections. Combined with the availability of the full genome sequence of T. castaneum, this system will enable analyses of host responses during infection, as well as addressing basic questions concerning host-parasite coevolution.

Improvement of crystal solubility and increasing toxicity against Caenorhabditis elegans by asparagine substitution in block 3 of Bacillus thuringiensis crystal protein Cry5Ba

The crystal proteins from Bacillus thuringiensis are widely used for their specific toxicity against insects and nematodes. The highly conserved sequence blocks play an important role in Cry protein stability and flexibility, the basis of toxicity. The block 3 in Cry5Ba subfamily has a shorter sequence (only 12 residues) and more asparagine residues than that of others which harbor about 48 residues but only one asparagine. Based on the theoretical structure model of Cry5Ba, all three asparagines in block 3 are closely located in the interface of putative three domains, implying their probable importance in structure and function. In this study, all three asparagines in Cry5Ba2 block 3 were individually substituted with alanine by site-directed mutagenesis. The wild-type and mutant proteins were overexpressed and crystallized in acrystalliferous B. thuringiensis strain BMB171. However, the crystals formed in one of the mutants, designated N586A, abnormally disappeared and dissolved into the culture supernatant once the sporulation cells lysed, whereas the Cry5Ba crystal and the other mutant crystals were stable. The mutant N586A crystal, isolated from sporulation cells by the ultrasonic process, was found to be easily dissolved at wide range of pH value (5.0 to 10.0). Moreover, the toxicity assays showed that the mutant N586A exhibited nearly 9-fold-higher activity against nematodes and damaged the host's intestine more efficiently than the native Cry5Ba2. These data support the presumption that the amide residue Asn586 at the interface of domains might adversely affect the protein flexibility, solubility and resultant toxicity of Cry5Ba.