Structure, diversity, and evolution of protein toxins from spore-forming entomopathogenic bacteria

Phylogenetic analysis and homology modelling of a new Cry8A crystal protein expressed in a sporulating soil bacterium

Cry proteins, commonly found in Gram-positive soil bacteria, are used worldwide as aerial sprays or in transgenic plants for controlling crop pest populations and insect vectors. Via PCR analysis, a spore producing soil isolate (BV5) was speculated to encode a Cry gene. Partial nucleotide sequence of the amplified PCR fragment showed homology with the Cry8 genes present in GenBank. A full-length Cry gene was cloned, and the predicted protein sequence grouped the newly isolated Cry protein with other Cry8A present in GenBank with a high possibility of it being a new Cry8. SDS-PAGE and MALDI-TOF mass spectrometry confirmed the expression of a single 135 KDa protein matching uniquely to the putative protein sequence of the BV5 Cry gene. However, bioassay against the coleopteran Anthonomus grandis (Coleopterans are a known Cry8A target), showed no activity. Phylogenetic analysis and homology modelling was performed to characterize the protein structure and function. These analyses suggest a series of mutations in one of the variable loops on the surface of the protein.

Identification of a biomarker for Bacillus thuringiensis strains with high toxicity against Spodoptera frugiperda based on insecticidal gene linkage analysis

BACKGROUND

Bacillus thuringiensis (Bt) is a Gram-positive bacterium that produces various insecticidal proteins used to control insect pests. Spodoptera frugiperda is a global insect pest which causes serious damage to crops, but bio-insecticides currently available to control this pest have limited activity and so new ones are always being sought. In this study we have tested the hypothesis that a biomarker for strain toxicity could be found that would greatly facilitate the identification of new potential products.

RESULTS

Using genomic sequencing data we constructed a linkage network of insecticidal genes from 1957 Bt genomes and found that four gene families, namely cry1A, cry1I, cry2A and vip3A, showed strong linkage. For 95 strains isolated from soil samples we assayed them for toxicity towards S. frugiperda and for the presence of the above gene families. All of the strains that showed high toxicity also contained a member of the vip3A gene family. Two of them were more toxic than a commercially available strain and genomic sequencing identified a number of potentially novel toxin-encoding genes.

CONCLUSIONS

The presence of a vip3A gene in the genome of a Bt strain proved to be a strong indicator of toxicity towards S. frugiperda validating this biomarker approach as a strategy for future discovery programs. © 2024 Society of Chemical Industry.

Graphical models for identifying pore-forming proteins

Pore-forming toxins (PFTs) are proteins that form lesions in biological membranes. Better understanding of the structure and function of these proteins will be beneficial in a number of biotechnological applications, including the development of new pest control methods in agriculture. When searching for new pore formers, existing sequence homology-based methods fail to discover truly novel proteins with low sequence identity to known proteins. Search methodologies based on protein structures would help us move beyond this limitation. As the number of known structures for PFTs is very limited, it's quite challenging to identify new proteins having similar structures using computational approaches like deep learning. In this article, we therefore propose a sample-efficient graphical model, where a protein structure graph is first constructed according to consensus secondary structures. A semi-Markov conditional random fields model is then developed to perform protein sequence segmentation. We demonstrate that our method is able to distinguish structurally similar proteins even in the absence of sequence similarity (pairwise sequence identity < 0.4)-a feat not achievable by traditional approaches like HMMs. To extract proteins of interest from a genome-wide protein database for further study, we also develop an efficient framework for UniRef50 with 43 million proteins.

Cotton plants overexpressing the Bacillus thuringiensis Cry23Aa and Cry37Aa binary-like toxins exhibit high resistance to the cotton boll weevil (Anthonomus grandis)

The cotton boll weevil (CBW, Anthonomus grandis) stands as one of the most significant threats to cotton crops (Gossypium hirsutum). Despite substantial efforts, the development of a commercially viable transgenic cotton event for effective open-field control of CBW has remained elusive. This study describes a detailed characterization of the insecticidal toxins Cry23Aa and Cry37Aa against CBW. Our findings reveal that CBW larvae fed on artificial diets supplemented exclusively with Cry23Aa decreased larval survival by roughly by 69%, while supplementation with Cry37Aa alone displayed no statistical difference compared to the control. However, the combined provision of both toxins in the artificial diet led to mortality rates approaching 100% among CBW larvae (LC50 equal to 0.26 PPM). Additionally, we engineered transgenic cotton plants by introducing cry23Aa and cry37Aa genes under control of the flower bud-specific pGhFS4 and pGhFS1 promoters, respectively. Seven transgenic cotton events expressing high levels of Cry23Aa and Cry37Aa toxins in flower buds were selected for greenhouse bioassays, and the mortality rate of CBW larvae feeding on their T0 and T1 generations ranged from 75% to 100%. Our in silico analyses unveiled that Cry23Aa displays all the hallmark characteristics of β-pore-forming toxins (β-PFTs) that bind to sugar moieties in glycoproteins. Intriguingly, we also discovered a distinctive zinc-binding site within Cry23Aa, which appears to be involved in protein-protein interactions. Finally, we discuss the major structural features of Cry23Aa that likely play a role in the toxin's mechanism of action. In view of the low LC50 for CBW larvae and the significant accumulation of these toxins in the flower buds of both T0 and T1 plants, we anticipate that through successive generations of these transgenic lines, cotton plants engineered to overexpress cry23Aa and cry37Aa hold promise for effectively managing CBW infestations in cotton crops.

Mutational analysis of the transmembrane α4-helix of Bacillus thuringiensis mosquito-larvicidal Cry4Aa toxin

Cry4Aa, produced by Bacillus thuringiensis subsp. israelensis, exhibits specific toxicity to larvae of medically important mosquito genera. Cry4Aa functions as a pore-forming toxin, and a helical hairpin (α4-loop-α5) of domain I is believed to be the transmembrane domain that forms toxin pores. Pore formation is considered to be a central mode of Cry4Aa action, but the relationship between pore formation and toxicity is poorly understood. In the present study, we constructed Cry4Aa mutants in which each polar amino acid residues within the transmembrane α4 helix was replaced with glutamic acid. Bioassays using Culex pipiens mosquito larvae and subsequent ion permeability measurements using symmetric KCl solution revealed an apparent correlation between toxicity and toxin pore conductance for most of the Cry4Aa mutants. In contrast, the Cry4Aa mutant H178E was a clear exception, almost losing its toxicity but still exhibiting a moderately high conductivity of about 60% of the wild-type. Furthermore, the conductance of the pore formed by the N190E mutant (about 50% of the wild-type) was close to that of H178E, but the toxicity was significantly higher than that of H178E. Ion selectivity measurements using asymmetric KCl solution revealed a significant decrease in cation selectivity of toxin pores formed by H178E compared to N190E. Our data suggest that the toxicity of Cry4Aa is primarily pore related. The formation of toxin pores that are highly ion-permeable and also highly cation-selective may enhance the influx of cations and water into the target cell, thereby facilitating the eventual death of mosquito larvae.

Structure of the Lysinibacillus sphaericus Tpp49Aa1 pesticidal protein elucidated from natural crystals using MHz-SFX

The Lysinibacillus sphaericus proteins Tpp49Aa1 and Cry48Aa1 can together act as a toxin toward the mosquito Culex quinquefasciatus and have potential use in biocontrol. Given that proteins with sequence homology to the individual proteins can have activity alone against other insect species, the structure of Tpp49Aa1 was solved in order to understand this protein more fully and inform the design of improved biopesticides. Tpp49Aa1 is naturally expressed as a crystalline inclusion within the host bacterium, and MHz serial femtosecond crystallography using the novel nanofocus option at an X-ray free electron laser allowed rapid and high-quality data collection to determine the structure of Tpp49Aa1 at 1.62 Å resolution. This revealed the packing of Tpp49Aa1 within these natural nanocrystals as a homodimer with a large intermolecular interface. Complementary experiments conducted at varied pH also enabled investigation of the early structural events leading up to the dissolution of natural Tpp49Aa1 crystals-a crucial step in its mechanism of action. To better understand the cooperation between the two proteins, assays were performed on a range of different mosquito cell lines using both individual proteins and mixtures of the two. Finally, bioassays demonstrated Tpp49Aa1/Cry48Aa1 susceptibility of Anopheles stephensi, Aedes albopictus, and Culex tarsalis larvae-substantially increasing the potential use of this binary toxin in mosquito control.

The immunostimulatory role of an Enterococcus-dominated gut microbiota in host protection against bacterial and fungal pathogens in Galleria mellonella larvae

Understanding the intricate interplay between the gut microbiota and the immune response in insects is crucial, given its diverse impact on the pathogenesis of various microbial species. The microbiota's modulation of the host immune system is one such mechanism, although its complete impact on immune responses remains elusive. This study investigated the tripartite interaction between the gut microbiota, pathogens, and the host's response in Galleria mellonella larvae reared under axenic (sterile) and conventional (non-sterile) conditions. The influence of the microbiota on host fitness during infections was evaluated via two different routes: oral infection induced by Bacillus thuringiensis subsp. galleriae (Btg), and topical infection induced by Metarhizium robertsii (Mr). We observed that larvae without a microbiota can successfully fulfill their life cycle, albeit with more variation in their developmental time. We subsequently performed survival assays on final-instar larvae, using the median lethal dose (LD50) of Btg and Mr. Our findings indicated that axenic larvae were more vulnerable to an oral infection of Btg; specifically, a dose that was calculated to be half-lethal for the conventional group resulted in a 90%-100% mortality rate in the axenic group. Through a dual-analysis experimental design, we could identify the status of the gut microbiota using 16S rRNA sequencing and assess the level of immune-related gene expression in the same group of larvae at basal conditions and during infection. This analysis revealed that the microbiota of our conventionally reared population was dominated entirely by four Enterococcus species, and these species potentially stimulated the immune response in the gut, due to the increased basal expression of two antimicrobial peptides (AMPs)-gallerimycin and gloverin-in the conventional larvae compared with the axenic larvae. Furthermore, Enterococcus mundtii, isolated from the gut of conventional larvae, showed inhibition activity against Btg in vitro. Lastly, other immune effectors, namely, phenoloxidase activity in the hemolymph and total reactive oxygen/nitrogen species (ROS/RNS) in the gut, were tested to further investigate the extent of the stimulation of the microbiota on the immune response. These findings highlight the immune-modulatory role of the Enterococcus-dominated gut microbiota, an increasingly reported microbiota assemblage of laboratory populations of Lepidoptera, and its influence on the host's response to oral and topical infections.

Sporulation, Structure Assembly, and Germination in the Soil Bacterium Bacillus thuringiensis: Survival and Success in the Environment and the Insect Host

Bacillus thuringiensis (Bt) is a rod-shaped, Gram-positive soil bacterium that belongs to the phylum Firmicutes and the genus Bacillus. It is a spore-forming bacterium. During sporulation, it produces a wide range of crystalline proteins that are toxic to different orders of insects. Sporulation, structure assembly, and germination are essential stages in the cell cycle of B. thuringiensis. The majority of studies on these issues have focused on the model organism Bacillus subtilis, followed by Bacillus cereus and Bacillus anthracis. The machinery for sporulation and germination extrapolated to B. thuringiensis. However, in the light of recent findings concerning the role of the sporulation proteins (SPoVS), the germination receptors (Gr), and the cortical enzymes in Bt, the theory strengthened that conservation in sporulation, structure assembly, and germination programs drive the survival and success of B. thuringiensis in the environment and the insect host. In the present minireview, the latter pinpointed and reviewed.

Food and feed safety of the Bacillus thuringiensis derived protein Vpb4Da2, a novel protein for control of western corn rootworm

Western corn rootworm (WCR), Diabrotica virgifera virgifera, LeConte, is an insect pest that poses a significant threat to the productivity of modern agriculture, causing significant economic and crop losses. The development of genetically modified (GM) crops expressing one or more proteins that confer tolerance to specific insect pests, such as WCR, was a historic breakthrough in agricultural biotechnology and continues to serve as an invaluable tool in pest management. Despite this, evolving resistance to existing insect control proteins expressed in current generation GM crops requires continued identification of new proteins with distinct modes of action while retaining targeted insecticidal efficacy. GM crops expressing insecticidal proteins must undergo extensive safety assessments prior to commercialization to ensure that they pose no increased risk to the health of humans or other animals relative to their non-GM conventional counterparts. As part of these safety evaluations, a weight of evidence approach is utilized to assess the safety of the expressed insecticidal proteins to evaluate any potential risk in the context of dietary exposure. This study describes the food and feed safety assessment of Vpb4Da2, a new Bacillus thuringiensis insecticidal protein that confers in planta tolerance to WCR. Vpb4Da2 exhibits structural and functional similarities to other insect control proteins expressed in commercialized GM crops. In addition, the lack of homology to known toxins or allergens, a lack of acute toxicity in mice, inactivation by conditions commonly experienced in the human gut or during cooking/food processing, and the extremely low expected dietary exposure to Vpb4Da2 provide a substantial weight of evidence to demonstrate that the Vpb4Da2 protein poses no indication of a risk to the health of humans or other animals.

Molecular Characterization of Native Bacillus thuringiensis Strains from Root Nodules with Toxicity Against the Fall Armyworm (FAW, Spodoptera frugiperda) and Brinjal Ash Weevil (Myllocerus subfasciatus)

The fall armyworm is an exotic pest which destroys a wide variety of crops Querywhereas the brinjal ash weevil is a serious pest of eggplant and other solanaceous vegetables. The goal of this research is to find a sustainable and ecologically friendly bio-control agent for managing FAW and brinjal ash weevils. Twelve natural Bacillus thuringiensis strains were isolated from cowpea root nodules, and the Gram-positive cells with characteristic Bt crystal structures were discovered using phase contrast and scanning electron microscopy. There were bipyramidal, cuboidal, rhombus, and spherical crystals. The Bt cry gene content was characterized by PCR analysis, which revealed the presence of cry1, cry1I, cry3, cry7, cry7,8, cry14, cry26, and cry55 genes. The identity of Bt was confirmed by cloning and sequencing the cry genes. In the nucleotide sequences, no pseudo genes or indels were found in cry sequences. SDS-PAGE examination indicated the presence of bands ranging in size from 13 to 130 kDa, with 50-60 kDa being the most common. When compared to the control, the new native Bt strains were lethal, with pathogenicity ranging from 93 to 100% against S. frugiperda larvae and M. subfasciatus adults. The studies revealed that the native strains with conserved regions of 16S rRNA genes were compared to NCBI database sequences and classified as native Bt strains with 99-100% similarity to known Bt strains. In conclusion, native Bt strains from cowpea root nodules were shown to have bio-insecticidal activity against fall armyworm larvae and brinjal ash weevil adults.

Role of Lectin in the Response of Aedes aegypti Against Bt Toxin

Aedes aegypti is one of the world’s most dangerous mosquitoes, and a vector of diseases such as dengue fever, chikungunya virus, yellow fever, and Zika virus disease. Currently, a major global challenge is the scarcity of antiviral medicine and vaccine for arboviruses. Bacillus thuringiensis var israelensis (Bti) toxins are used as biological mosquito control agents. Endotoxins, including Cry4Aa, Cry4Ba, Cry10Aa, Cry11Aa, and Cyt1Aa, are toxic to mosquitoes. Insect eradication by Cry toxin relies primarily on the interaction of cry toxins with key toxin receptors, such as aminopeptidase (APN), alkaline phosphatase (ALP), cadherin (CAD), and ATP-binding cassette transporters. The carbohydrate recognition domains (CRDs) of lectins and domains II and III of Cry toxins share similar structural folds, suggesting that midgut proteins, such as C-type lectins (CTLs), may interfere with interactions among Cry toxins and receptors by binding to both and alter Cry toxicity. In the present review, we summarize the functional role of C-type lectins in Ae. aegypti mosquitoes and the mechanism underlying the alteration of Cry toxin activity by CTLs. Furthermore, we outline future research directions on elucidating the Bti resistance mechanism. This study provides a basis for understanding Bti resistance, which can be used to develop novel insecticides.

The RNA Chaperone Protein Hfq Regulates the Characteristic Sporulation and Insecticidal Activity of Bacillus thuringiensis

Bacillus thuringiensis (Bt) is one of the most widely used bio-insecticides at present. It can produce many virulence factors and insecticidal crystal proteins during growth and sporulation. Hfq, on the other hand, is a bacterial RNA chaperone that can regulate the function of different kinds of RNAs, thereby affecting various bacterial phenotypes. To further explore the physiological functions of Hfq in Bt, we took BMB171 as the starting strain, knocked out one, two, or three hfq genes in its genome in different combinations, and compared the phenotypic differences between the deletion mutant strains and the starting strain. We did observe significant changes in several phenotypes, including motility, biofilm formation, sporulation, and insecticidal activity against cotton bollworm, among others. Afterward, we found through transcriptome studies that when all hfq genes were deleted, 32.5% of the genes in Bt were differentially transcribed, with particular changes in the sporulation-related and virulence-related genes. The above data demonstrated that Hfq plays a pivotal role in Bt and can regulate its various physiological functions. Our study on the regulatory mechanism of Hfq in Bt, especially the mining of the regulatory network of its sporulation and insecticidal activity, could lay a theoretical foundation for the better utilization of Bt as an effective insecticide.

Differentiation of Bacillus cereus and Bacillus thuringiensis Using Genome-Guided MALDI-TOF MS Based on Variations in Ribosomal Proteins

Bacillus cereus and B. thuringiensis are closely related species that are relevant to foodborne diseases and biopesticides, respectively. Unambiguous differentiation of these two species is crucial for bacterial taxonomy. As genome analysis offers an objective but time-consuming classification of B. cereus and B. thuringiensis, in the present study, matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) was used to accelerate this process. By combining in silico genome analysis and MALDI-TOF MS measurements, four species-specific peaks of B. cereus and B. thuringiensis were screened and identified. The species-specific peaks of B. cereus were m/z 3211, 6427, 9188, and 9214, and the species-specific peaks of B. thuringiensis were m/z 3218, 6441, 9160, and 9229. All the above peaks represent ribosomal proteins, which are conserved and consistent with the phylogenetic relationship between B. cereus and B. thuringiensis. The specificity of the peaks was robustly verified using common foodborne pathogens. Thus, we concluded that genome-guided MALDI-TOF MS allows high-throughput differentiation of B. cereus and B. thuringiensis and provides a framework for differentiating other closely related species.

Herbivorous Juvenile Grass Carp (Ctenopharyngodon idella) Fed with Genetically Modified MON 810 and DAS-59122 Maize Varieties Containing Cry Toxins: Intestinal Histological, Developmental, and Immunological Investigations

Feeding experiments with juvenile grass carp (Ctenopharyngodon idella) fed with genetically modified maize MON 810 or DAS-59122 dried leaf biomass were carried out with 1-, 3- and 6-month exposures. Dosages of 3–7 μg/fish/day Cry1Ab or 18-55 μg/fish/day Cry34Ab1 toxin did not cause mortality. No difference occurred in body or abdominal sac weights. No differences appeared in levels of inorganic phosphate, calcium, fructosamine, bile acids, triglycerides, cholesterol, and alanine and aspartame aminotransferases. DAS-59122 did not alter blood parameters tested after 3 months of feeding. MON 810 slightly decreased serum albumin levels compared to the control, only in one group. Tapeworm (Bothriocephalus acheilognathi) infection changed the levels of inorganic phosphate and calcium. Cry34Ab1 toxin appeared in blood (12.6 ± 1.9 ng/mL), but not in the muscle. It was detected in B. acheilognathi. Cry1Ab was hardly detectable in certain samples near the limit of detection. Degradation of Cry toxins was extremely quick in the fish gastrointestinal tract. After 6 months of feeding, only mild indications in certain serum parameters were observed: MON 810 slightly increased the level of apoptotic cells in the blood and reduced the number of thrombocytes in one group; DAS-59122 mildly increased the number of granulocytes compared to the near-isogenic line.

A deep learning model to detect novel pore-forming proteins

Many pore-forming proteins originating from pathogenic bacteria are toxic against agricultural pests. They are the key ingredients in several pesticidal products for agricultural use, including transgenic crops. There is an urgent need to identify novel pore-forming proteins to combat development of resistance in pests to existing products, and to develop products that are effective against a broader range of pests. Existing computational methodologies to search for these proteins rely on sequence homology-based approaches. These approaches are based on similarities between protein sequences, and thus are limited in their usefulness for discovering novel proteins. In this paper, we outline a novel deep learning model trained on pore-forming proteins from the public domain. We compare different ways of encoding protein information during training, and contrast it with traditional approaches. We show that our model is capable of identifying known pore formers with no sequence similarity to the proteins used to train the model, and therefore holds promise for identifying novel pore formers.

Genetic Modification Approaches for Parasporins Bacillus thuringiensis Proteins with Anticancer Activity

Bacillus thuringiensis (Bt) is a bacterium capable of producing Cry toxins, which are recognized for their bio-controlling actions against insects. However, a few Bt strains encode proteins lacking insecticidal activity but showing cytotoxic activity against different cancer cell lines and low or no cytotoxicity toward normal human cells. A subset of Cry anticancer proteins, termed parasporins (PSs), has recently arisen as a potential alternative for cancer treatment. However, the molecular receptors that allow the binding of PSs to cells and their cytotoxic mechanisms of action have not been well established. Nonetheless, their selective cytotoxic activity against different types of cancer cell lines places PSs as a promising alternative treatment modality. In this review, we provide an overview of the classification, structures, mechanisms of action, and insights obtained from genetic modification approaches for PS proteins.

Enhancing the resilience of transgenic cotton for insect resistance

BACKGROUND

The efficacy of Bt crystal proteins has been compromised due to their extensive utilization in the field. The second-generation Bt vegetative insecticidal proteins could be the best-suited alternative to combat resistance build-up due to their broad range affinity with midgut receptors of insects.

MATERIAL AND RESULTS

The codon-optimized synthetic vegetative insecticidal proteins (Vip3Aa) gene under the control of CaMV35S promoter was transformed into a locally developed transgenic cotton variety (CKC-01) expressing cry1Ac and cry2A genes. Transformation efficiency of 1.63% was recorded. The highest Vip3Aa expression (51.98-fold) was found in MS3 transgenic cotton plant. Maximum Vip3Aa protein concentration (4.23 µg/mL) was calculated in transgenic cotton plant MS3 through ELISA. The transgenic cotton plant (MS3) showed one copy number on both chromatids in the homozygous form at chromosome 8 at the telophase stage. Almost 99% mortality of H. armigera was recorded in transgenic cotton plants expressing double crystal proteins pyramided with Vip3Aa gene as contrasted to transgenic cotton plant expressing only double crystal protein with 70% mortality.

CONCLUSIONS

The results obtained during this study suggest that the combination of Bt cry1Ac, cry2A, and Vip3Aa toxins is the best possible alternative approach to combat chewing insects.

Microbiota and transcriptome changes of Culex pipiens pallens larvae exposed to Bacillus thuringiensis israelensis

Culex pipiens pallens is an important vector of lymphatic filariasis and epidemic encephalitis. Mosquito control is the main strategy used for the prevention of mosquito-borne diseases. Bacillus thuringiensis israelensis (Bti) is an entomopathogenic bacterium widely used in mosquito control. In this study, we profiled the microbiota and transcriptional response of the larvae of Cx. pipiens pallens exposed to different concentrations of Bti. The results demonstrated that Bti induced a significant effect on both the microbiota and gene expression of Cx. pipiens pallens. Compared to the control group, the predominant bacteria changed from Actinobacteria to Firmicutes, and with increase in the concentration of Bti, the abundance of Actinobacteria was gradually reduced. Similar changes were also detected at the genus level, where Bacillus replaced Microbacterium, becoming the predominant genus in Bti-exposed groups. Furthermore, alpha diversity analysis indicated that Bti exposure changed the diversity of the microbota, possibly because the dysbiosis caused by the Bti infection inhibits some bacteria and provides opportunities to other opportunistic taxa. Pathway analysis revealed significant enhancement for processes associated with sphingolipid metabolism, glutathione metabolism and glycerophospholipid metabolism between all Bti-exposed groups and control group. Additionally, genes associated with the Toll and Imd signaling pathway were found to be notably upregulated. Bti infection significantly changed the bacterial community of larvae of Cx. pipiens pallens.

Structural and functional characterization of Mpp75Aa1.1, a putative beta-pore forming protein from Brevibacillus laterosporus active against the western corn rootworm

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is a major corn pest of significant economic importance in the United States. The continuous need to control this corn maize pest and the development of field-evolved resistance toward all existing transgenic maize (Zea mays L.) expressing Bacillus thuringiensis (Bt) insecticidal proteins against WCR has prompted the development of new insect-protected crops expressing distinct structural classes of insecticidal proteins. In this current study, we describe the crystal structure and functional characterization of Mpp75Aa1.1, which represents the first corn rootworm (CRW) active insecticidal protein member of the ETX_MTX2 sub-family of beta-pore forming proteins (β-PFPs), and provides new and effective protection against WCR feeding. The Mpp75Aa1.1 crystal structure was solved at 1.94 Å resolution. The Mpp75Aa1.1 is processed at its carboxyl-terminus by WCR midgut proteases, forms an oligomer, and specifically interacts with putative membrane-associated binding partners on the midgut apical microvilli to cause cellular tissue damage resulting in insect death. Alanine substitution of the surface-exposed amino acids W206, Y212, and G217 within the Mpp75Aa1.1 putative receptor binding domain I demonstrates that at least these three amino acids are required for WCR activity. The distinctive spatial arrangement of these amino acids suggests that they are part of a receptor binding epitope, which may be unique to Mpp75Aa1.1 and not present in other ETX_MTX2 proteins that do not have WCR activity. Overall, this work establishes that Mpp75Aa1.1 shares a mode of action consistent with traditional WCR-active Bt proteins despite significant structural differences.

Bacillus thuringiensis: From biopesticides to anticancer agents

Bacillus thuringiensis (Bt) is a ubiquitous bacterium that produces several proteins that are toxic to different invertebrates such as insects, nematodes, mites, and also some protozoans. Among these, Cry and Cyt proteins are most explored as biopesticides for their action against agricultural pests and vectors of human diseases. In 2000, a group of researchers from Japan isolated parasporal inclusion proteins from B. thuringiensis, and reported their cytotoxic action against human leukemia. Later, other proteins with similar antitumor properties were also isolated from this bacterium and these cytotoxic proteins with specific activity against human cancer cells were named parasporins. At present, nineteen different parasporins are registered and classified in six families. These parasporins have been described to have specific in vitro antitumor activity against several cancer cell lines. The antitumor activity makes parasporins possible candidates as anticancer agents. Various research groups around the world are involved in isolating and characterizing in vitro antitumor activity of these proteins and many articles reporting such activities in detail have been published. However, there are virtually no data regarding the antitumor activity of parasporins in vivo. This review summarizes the properties of these potentially useful antitumor agents of natural origin, focusing on their in vivo activity thus also highlighting the importance of testing these proteins in animal models for a possible application in clinical oncology.

Bacillus thuringiensis Cry4Ba Insecticidal ToxinExploits Leu615 in Its C-Terminal Domain to Interact with a Target Receptor-Aedes aegypti Membrane-Bound Alkaline Phosphatase

In addition to the receptor-binding domain (DII), the C-terminal domain (DIII) of three-domain Cry insecticidal δ-endotoxins from Bacillus thuringiensis has been implicated in target insect specificity, yet its precise mechanistic role remains unclear. Here, the 21 kDa high-purity isolated DIII fragment derived from the Cry4Ba mosquito-specific toxin was achieved via optimized preparative FPLC, allowing direct rendering analyses for binding characteristics toward its target receptor—Aedes aegypti membrane-bound alkaline phosphatase (Aa-mALP). Binding analysis via dotblotting revealed that the Cry4Ba-DIII truncate was capable of specific binding to nitrocellulose-bound Aa-mALP, with a binding signal comparable to its 65 kDa Cry4Ba-R203Q full-length toxin. Further determination of binding affinity via sandwich ELISA revealed that Cry4Ba-DIII exhibited a rather weak binding to Aa-mALP with a dissociation constant (K_d) of ≈1.1 × 10⁻⁷ M as compared with the full-length toxin. Intermolecular docking between the Cry4Ba-R203Q active toxin and Aa-mALP suggested that four Cry4Ba-DIII residues, i.e., Glu⁵²², Asn⁵⁵², Asn⁵⁷⁶, and Leu⁶¹⁵, are potentially involved in such toxin–receptor interactions. Ala substitutions of each residue (E522A, N552A, N576A and L615A) revealed that only the L615A mutant displayed a drastic decrease in biotoxicity against A. aegypti larvae. Additional binding analysis revealed that the L615A-impaired toxin also exhibited a reduction in binding capability to the surface-immobilized Aa-mALP receptor, while two bio-inactive DII-mutant toxins, Y332A and F364A, which almost entirely lost their biotoxicity, apparently retained a higher degree of binding activity. Altogether, our data disclose a functional importance of the C-terminal domain of Cry4Ba for serving as a potential receptor-binding moiety in which DIII-Leu⁶¹⁵ could conceivably be exploited for the binding to Aa-mALP, highlighting its contribution to toxin interactions with such a target receptor in mediating larval toxicity.

Aedes aegypti continuously exposed to Bacillus thuringiensis svar. israelensis does not exhibit changes in life traits but displays increased susceptibility for Zika virus

Background

Aedes aegypti can transmit arboviruses worldwide, and Bacillus thuringiensis svar. israelensis (Bti)-based larvicides represent an effective tool for controlling this species. The safety of Bti and lack of resistance have been widely reported; however, little is known regarding the impact of the extensive use of these larvicides on the life traits of mosquitoes. Therefore, this study investigated biological parameters, including susceptibility to arbovirus, of an Ae. aegypti strain (RecBti) subjected to 29 generations of exposure to Bti compared with the RecL reference strain.

Methods

The biological parameters of individuals reared under controlled conditions were compared. Also, the viral susceptibility of females not exposed to Bti during their larval stage was analysed by oral infection and followed until 14 or 21 days post-infection (dpi).

Results

RecBti individuals did not display alterations in the traits that were assessed (fecundity, fertility, pupal weight, developmental time, emergence rate, sex ratio and haematophagic capacity) compared to RecL individuals. Females from both strains were susceptible to dengue serotype 2 (DENV-2) and Zika virus (ZIKV). However, RecBti females showed significantly higher rates of ZIKV infection compared with RecL females at 7 (90% versus 68%, Chi-square: χ² = 7.27, df = 1, P = 0.006) and 14 dpi (100% versus 87%, Chi-square: χ² = 7.69, df = 1, P = 0.005) and for dissemination at 7 dpi (83.3% versus 36%, Fisher’s exact test: P < 0.0001, OR = 0.11, 95% CI 0.03–0.32). Quantification of DENV-2 and ZIKV viral particles produced statistically similar results for females from both strains.

Conclusions

Prolonged exposure of Ae. aegypti larvae to Bti did not alter most of the evaluated biological parameters, except that RecBti females exhibited a higher vector susceptibility for ZIKV. This finding is related to a background of Bti exposure for several generations but not to a previous exposure of the tested females during the larval stage. This study highlights mosquito responses that could be associated with the chronic exposure to Bti in addition to the primary larvicidal effect elicited by this control agent.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04880-6.

Bacterial Toxins Active against Mosquitoes: Mode of Action and Resistance

Larvicides based on the bacteria Bacillus thuringiensis svar. israelensis (Bti) and Lysinibacillus sphaericus are effective and environmentally safe compounds for the control of dipteran insects of medical importance. They produce crystals that display specific and potent insecticidal activity against larvae. Bti crystals are composed of multiple protoxins: three from the three-domain Cry type family, which bind to different cell receptors in the midgut, and one cytolytic (Cyt1Aa) protoxin that can insert itself into the cell membrane and act as surrogate receptor of the Cry toxins. Together, those toxins display a complex mode of action that shows a low risk of resistance selection. L. sphaericus crystals contain one major binary toxin that display an outstanding persistence in field conditions, which is superior to Bti. However, the action of the Bin toxin based on its interaction with a single receptor is vulnerable for resistance selection in insects. In this review we present the most recent data on the mode of action and synergism of these toxins, resistance issues, and examples of their use worldwide. Data reported in recent years improved our understanding of the mechanism of action of these toxins, showed that their combined use can enhance their activity and counteract resistance, and reinforced their relevance for mosquito control programs in the future years.

Adjuvants: friends in vaccine formulations against infectious diseases

Infectious diseases represent a major cause of deaths worldwide. No vaccine or effective treatment exists nowadays, especially against intracellular pathogens. The increase in multiple drug and superbug antibiotic resistance strains, excessive medication, or misuse of drugs has prompted the search for other safe and effective alternatives. Consistent with this, adjuvants (Latin word "adjuvare": "help or aid") co-administered (Exo) in vaccines have emerged as a promising alternative to initiate and boost an innate, downstream signal that led to adaptative immune response. Nowadays, a promising model of strong immunogens and adjuvants at mucosal sites are the microbial bacterial toxins. Other adjuvants that are also used and might successfully replace aluminum salts in combination with nanotechnology are CpG-ODN, poly IC, type I IFNs, mRNA platforms. Therefore, in the present review, we focused to revisit the old to the new adjuvants compounds, the properties that make them friends in vaccine formulations against infectious diseases.

IDOPS, a Profile HMM-Based Tool to Detect Pesticidal Sequences and Compare Their Genetic Context

Biopesticide-based crop protection is constantly challenged by insect resistance. Thus, expansion of available biopesticides is crucial for sustainable agriculture. Although Bacillus thuringiensis is the major agent for pesticide bioprotection, the number of bacteria species synthesizing proteins with biopesticidal potential is much higher. The Bacterial Pesticidal Protein Resource Center (BPPRC) offers a database of sequences for the control of insect pests, grouped in structural classes. Here we present IDOPS, a tool that detects novel biopesticidal sequences and analyzes them within their genetic environment. The backbone of the IDOPS detection unit is a curated collection of high-quality hidden Markov models that is in accordance with the BPPRC nomenclature. IDOPS was positively benchmarked with BtToxin_Digger and Cry_Processor. In addition, a scan of the UniProtKB database using the IDOPS models returned an abundance of new pesticidal protein candidates distributed across all of the structural groups. Gene expression depends on the genomic environment, therefore, IDOPS provides a comparative genomics module to investigate the genetic regions surrounding pesticidal genes. This feature enables the investigation of accessory elements and evolutionary traits relevant for optimal toxin expression and functional diversification. IDOPS contributes and expands our current arsenal of pesticidal proteins used for crop protection.

Production of Lysinibacillus sphaericus Mosquitocidal Protein Mtx2 from Bacillus subtilis as a Secretory Protein

BACKGROUND

Mtx2 is a mosquitocidal toxin produced during the vegetative growth of Lysinibacillus sphaericus. The protein shows synergism with other toxins against mosquito larvae; hence it could be used in mosquito control formulations. The protein expression system is needed for Mtx2 development as a biocontrol agent.

OBJECTIVE

The objective of the study was to set up a Bacillus subtilis system to produce Mtx2 as a secreted protein since the protein contains a putative signal peptide.

METHODS

Initially, four different promoters (P43, Pspac, PxylA, and PyxiE) were compared for their strength using GFP as a reporter in B. subtilis. Subsequently, six different signal peptides (SacB, Epr, AmyE, AprE, LipA, and Vip3A)were tested in conjunction with the selected promoter and mtx2 to evaluate levels of Mtx2 secreted by B. subtilis WB800, an extracellular protease-deficient strain.

RESULTS

The promoter PyxiE showed the highest GFP intensity and was selected for further study. Mtx2 was successfully produced as a secreted protein from signal peptides LipA and AmyE, and exhibited larvicidal activity against Aedesaegypti.

CONCLUSION

B. subtilis was successfully developed as a host for the production of secreted Mtx2 and the protein retained its larvicidal activity. Although the Mtx2 production level still needs improvement, the constructed plasmids could be used to produce other soluble proteins.

Vegetative Insecticidal Protein (Vip): A Potential Contender From Bacillus thuringiensis for Efficient Management of Various Detrimental Agricultural Pests

Bacillus thuringiensis (Bt) bacterium is found in various ecological habitats, and has natural entomo-pesticidal properties, due to the production of crystalline and soluble proteins during different growth phases. In addition to Cry and Cyt proteins, this bacterium also produces Vegetative insecticidal protein (Vip) during its vegetative growth phase, which is considered an excellent toxic candidate because of the difference in sequence homology and receptor sites from Cry proteins. Vip proteins are referred as second-generation insecticidal proteins, which can be used either alone or in complementarity with Cry proteins for the management of various detrimental pests. Among these Vip proteins, Vip1 and Vip2 act as binary toxins and have toxicity toward pests belonging to Hemiptera and Coleoptera orders, whereas the most important Vip3 proteins have insecticidal activity against Lepidopteran pests. These Vip3 proteins are similar to Cry proteins in terms of toxicity potential against susceptible insects. They are reported to be toxic toward pests, which can’t be controlled with Cry proteins. The Vip3 proteins have been successfully pyramided along with Cry proteins in transgenic rice, corn, and cotton to combat resistant pest populations. This review provides detailed information about the history and importance of Vip proteins, their types, structure, newly identified specific receptors, and action mechanism of this specific class of proteins. Various studies conducted on Vip proteins all over the world and the current status have been discussed. This review will give insights into the significance of Vip proteins as alternative promising candidate toxic proteins from Bt for the management of pests in most sustainable manner.

Antagonistic Effect of Truncated Fragments of Bacillus thuringiensis Vip3Aa on the Larvicidal Activity of its Full-length Protein

BACKGROUND

Vip3Aa is a vegetative insecticidal protein produced by Bacillus thuringiensis. The protein is produced as an 88-kDa protoxin that could be processed by insect gut proteases into a 22-kDa N-terminal and a 66-kDa C-terminal fragments. The C-terminal part could bind to a specific receptor while the N-terminal part is required for toxicity and structural stability.

OBJECTIVE

To demonstrate the antagonistic effect of truncated fragments on the insecticidal activity of the full-length Vip3Aa.

METHODS

The full-length protein (Vip3Aa), a 66-kDa C-terminal fragment (Vip3Aa-D199) and a predicted carbohydrate binding module (CBM) were produced in Escherichia coli. Purified proteins were mixed at different ratios and fed to Spodoptera litura and Spodoptera exigua larvae. Mortality was recorded and compared between larvae fed with individual toxin and mixtures of the full-length and truncated toxins.

RESULTS

Production level of the Vip3Aa-D199 was significantly decreased comparing to that of the full-length protein. Vip3Aa-D199 and CBM fragment were not toxic to insect larvae whereas Vip3Aa showed high toxicity with LC₅₀ about 200 ng/cm². Feeding the larvae with mixtures of the Vip3Aa and Vip3Aa-D199 at different ratios revealed antagonistic effect of the Vip3Aa-D199 on the toxicity of Vip3Aa. Results showed that the lethal time (LT ₅₀ and LT ₉₅) of larvae fed the mixture toxins was longer than those fed the Vip3Aa alone. In addition, a CBM fragment could inhibit toxicity of the full-length Vip3Aa.

CONCLUSION

Our results demonstrated that the Vip3Aa-D199 and a CBM fragment could complete for the membrane binding thus rendering activity of the full-length Vip3Aa.

Potential for Bacillus thuringiensis and Other Bacterial Toxins as Biological Control Agents to Combat Dipteran Pests of Medical and Agronomic Importance

The control of dipteran pests is highly relevant to humans due to their involvement in the transmission of serious diseases including malaria, dengue fever, Chikungunya, yellow fever, zika, and filariasis; as well as their agronomic impact on numerous crops. Many bacteria are able to produce proteins that are active against insect species. These bacteria include Bacillus thuringiensis, the most widely-studied pesticidal bacterium, which synthesizes proteins that accumulate in crystals with insecticidal properties and which has been widely used in the biological control of insects from different orders, including Lepidoptera, Coleoptera, and Diptera. In this review, we summarize all the bacterial proteins, from B. thuringiensis and other entomopathogenic bacteria, which have described insecticidal activity against dipteran pests, including species of medical and agronomic importance.

A new Bacillus thuringiensis protein for Western corn rootworm control

The Western corn rootworm (WCR) Diabrotica virgifera virgifera LeConte is one of the most economically important insect pests in North America. Since 2003, transgenic maize expressing WCR-active proteins from Bacillus thuringiensis (Bt) have been widely adopted as the main approach to controlling WCR in the U.S. However, the emergence of field resistance to the Bt proteins in current commercial products has been documented in recent years, highlighting the need to develop additional tools for controlling this devasting pest. Here we report the discovery of Vpb4Da2 (initially assigned as Vip4Da2), a new insecticidal protein highly selective against WCR, through high-throughput genome sequencing of a Bt strain sourced from grain dust samples collected in the eastern and central regions of the US. Vpb4Da2 contains a sequence and domain signature distinct from families of other WCR-active proteins. Under field conditions, transgenic maize expressing Vpb4Da2 demonstrates commercial-level (at or below NIS 0.25) root protection against WCR, and reduces WCR beetle emergence by ≥ 97%. Our studies also conclude that Vpb4Da2 controls WCR populations that are resistant to WCR-active transgenic maize expressing Cry3Bb1, Cry34Ab1/Cry35Ab1 (reassigned as Gpp34Ab1/Tpp35Ab1), or DvSnf7 RNA. Based on these findings, Vpb4Da2 represents a valuable new tool for protecting maize against WCR.

Current Insights on Vegetative Insecticidal Proteins (Vip) as Next Generation Pest Killers

Bacillus thuringiensis (Bt) is a Gram negative soil bacterium. This bacterium secretes various proteins during different growth phases with an insecticidal potential against many economically important crop pests. One of the important families of Bt proteins is vegetative insecticidal proteins (Vip), which are secreted into the growth medium during vegetative growth. There are three subfamilies of Vip proteins. Vip1 and Vip2 heterodimer toxins have an insecticidal activity against many Coleopteran and Hemipteran pests. Vip3, the most extensively studied family of Vip toxins, is effective against Lepidopteron. Vip proteins do not share homology in sequence and binding sites with Cry proteins, but share similarities at some points in their mechanism of action. Vip3 proteins are expressed as pyramids alongside Cry proteins in crops like maize and cotton, so as to control resistant pests and delay the evolution of resistance. Biotechnological- and in silico-based analyses are promising for the generation of mutant Vip proteins with an enhanced insecticidal activity and broader spectrum of target insects.

The Tripartite Interaction of Host Immunity-Bacillus thuringiensis Infection-Gut Microbiota

Bacillus thuringiensis (Bt) is an important cosmopolitan bacterial entomopathogen, which produces various protein toxins that have been expressed in transgenic crops. The evolved molecular interaction between the insect immune system and gut microbiota is changed during the Bt infection process. The host immune response, such as the expression of induced antimicrobial peptides (AMPs), the melanization response, and the production of reactive oxygen species (ROS), varies with different doses of Bt infection. Moreover, B. thuringiensis infection changes the abundance and structural composition of the intestinal bacteria community. The activated immune response, together with dysbiosis of the gut microbiota, also has an important effect on Bt pathogenicity and insect resistance to Bt. In this review, we attempt to clarify this tripartite interaction of host immunity, Bt infection, and gut microbiota, especially the important role of key immune regulators and symbiotic bacteria in the Bt killing activity. Increasing the effectiveness of biocontrol agents by interfering with insect resistance and controlling symbiotic bacteria can be important steps for the successful application of microbial biopesticides.

The Independent Biological Activity of Bacillus thuringiensis Cry23Aa Protein Against Cylas puncticollis

The Cry23Aa/Cry37Aa proteins from Bacillus thuringiensis (Bt) have been described toxic to Cylas puncticollis larvae. In general, it is believed that Cry23Aa and Cry37Aa act jointly to exert the insecticidal activity, while there is no evidence of their toxicity individually. Therefore, in the present study, the contribution of each protein in the insecticidal activity toward C. puncticollis larvae has been assessed. The results showed that both proteins were toxic for C. puncticollis larvae when tested individually. Contrary to what was claimed previously, our results suggest that the presence of both proteins is not necessary to exert toxicity against C. puncticollis larvae. Also, the binding behavior of Cry23Aa protein to midgut receptors of C. puncticollis larvae has been determined. According to our results, Cry23Aa binds to C. puncticollis brush border membrane vesicles (BBMV) specifically and independently of Cry37Aa. Due to the lack of common binding sites, Cry23Aa can be pyramided with Cry3Aa protein for better management of C. puncticollis.

Insecticidal Activity of Bacillus thuringiensis Proteins Against Coleopteran Pests

Bacillus thuringiensis is the most successful microbial insecticide agent and its proteins have been studied for many years due to its toxicity against insects mainly belonging to the orders Lepidoptera, Diptera and Coleoptera, which are pests of agro-forestry and medical-veterinary interest. However, studies on the interactions between this bacterium and the insect species classified in the order Coleoptera are more limited when compared to other insect orders. To date, 45 Cry proteins, 2 Cyt proteins, 11 Vip proteins, and 2 Sip proteins have been reported with activity against coleopteran species. A number of these proteins have been successfully used in some insecticidal formulations and in the construction of transgenic crops to provide protection against main beetle pests. In this review, we provide an update on the activity of Bt toxins against coleopteran insects, as well as specific information about the structure and mode of action of coleopteran Bt proteins.

Unraveling the Composition of Insecticidal Crystal Proteins in Bacillus thuringiensis: a Proteomics Approach

Bacillus thuringiensis (Bt) is the most widely used active ingredient for biological insecticides. The composition of δ-endotoxins (Cry and Cyt proteins) in the parasporal crystal determines the toxicity profile of each Bt strain. However, a reliable method for their identification and quantification has not been available, due to the high sequence identity of the genes that encode the δ-endotoxins and the toxins themselves. Here, we have developed an accurate and reproducible mass spectrometry-based method (liquid chromatography-tandem mass spectrometry-multiple reaction monitoring [LC-MS/MS-MRM]) using isotopically labeled proteotypic peptides for each protein in a particular mixture to determine the relative proportion of each δ-endotoxin within the crystal. To validate the method, artificial mixtures containing Cry1Aa, Cry2Aa, and Cry6Aa were analyzed. Determination of the relative abundance of proteins (in molarity) with our method was in good agreement with the expected values. This method was then applied to the most common commercial Bt-based products, DiPel DF, XenTari GD, VectoBac 12S, and Novodor, in which between three and six δ-endotoxins were identified and quantified in each product. This novel approach is of great value for the characterization of Bt-based products, not only providing information on host range, but also for monitoring industrial crystal production and quality control and product registration for Bt-based insecticides.IMPORTANCE Bacillus thuringiensis (Bt)-based biological insecticides are used extensively to control insect pests and vectors of human diseases. Bt-based products provide greater specificity and biosafety than broad-spectrum synthetic insecticides. The biological activity of this bacterium resides in spores and crystals comprising complex mixtures of toxic proteins. We developed and validated a fast, accurate, and reproducible method for quantitative determination of the crystal components of Bt-based products. This method will find clear applications in the improvement of various aspects of the industrial production process of Bt. An important aspect of the production of Bt-based insecticides is its quality control. By specifically quantifying the relative proportion of each of the toxins that make up the crystal, our method represents the most consistent and repeatable evaluation procedure in the quality control of different batches produced in successive fermentations. This method can also contribute to the design of specific culture media and fermentation conditions that optimize Bt crystal composition across a range of Bt strains that target different pestiferous insects. Quantitative information on crystal composition should also prove valuable to phytosanitary product registration authorities that oversee the safety and efficacy of crop protection products.

The Cytocidal Spectrum of Bacillus thuringiensis Toxins: From Insects to Human Cancer Cells

Bacillus thuringiensis (Bt) is a ubiquitous bacterium in soils, insect cadavers, phylloplane, water, and stored grain, that produces several proteins, each one toxic to different biological targets such as insects, nematodes, mites, protozoa, and mammalian cells. Most Bt toxins identify their particular target through the recognition of specific cell membrane receptors. Cry proteins are the best-known toxins from Bt and a great amount of research has been published. Cry are cytotoxic to insect larvae that affect important crops recognizing specific cell membrane receptors such as cadherin, aminopeptidase-N, and alkaline phosphatase. Furthermore, some Cry toxins such as Cry4A, Cry4B, and Cry11A act synergistically with Cyt toxins against dipteran larvae vectors of human disease. Research developed with Cry proteins revealed that these toxins also could kill human cancer cells through the interaction with specific receptors. Parasporins are a small group of patented toxins that may or may not have insecticidal activity. These proteins could kill a wide variety of mammalian cancer cells by recognizing specific membrane receptors, just like Cry toxins do. Surface layer proteins (SLP), unlike the other proteins produced by Bt, are also produced by most bacteria and archaebacteria. It was recently demonstrated that SLP produced by Bt could interact with membrane receptors of insect and human cancer cells to kill them. Cyt toxins have a structure that is mostly unrelated to Cry toxins; thereby, other mechanisms of action have been reported to them. These toxins affect mainly mosquitoes that are vectors of human diseases like Anopheles spp (malaria), Aedes spp (dengue, zika, and chikungunya), and Culex spp (Nile fever and Rift Valley fever), respectively. In addition to the Cry, Cyt, and parasporins toxins produced during spore formation as inclusion bodies, Bt strains also produce Vip (Vegetative insecticidal toxins) and Sip (Secreted insecticidal proteins) toxins with insecticidal activity during their vegetative growth phase.

Whole genome sequence analysis of the mosquitocidal Bacillus thuringiensis LLP29

Bacillus thuringiensis (Bt) is efficient, strongly specific, and avirulent to humans, making it one of the most popular biopesticides in the world. Bt LLP29 is a mosquitocidal strain that was first isolated from Magnolia denudata. To understand its molecular mechanism against mosquitoes, the genome of Bt LLP29 was sequenced and annotated in this study. The LLP29 genome was found to have a total length of 5.99 Mb, with an average G + C content of 35.21%. A total of 6107 coding sequences were also detected, together with 42 rRNAs and 124 tRNAs and 135 other RNAs. With the help of annotation databases, including GO, COG, KEGG, Nr and Swiss-Prot, most unigene functions were identified. At the same time, a collinear analysis was performed on the genome of LLP29. There were also some virulence genes detected, including cry, chitinase, zwittermicin and vip.

Development of a sensitive monoclonal antibody-based sandwich ELISA to detect Vip3Aa in genetically modified crops

Objectives

To develop a sensitive monoclonal antibody-based sandwich enzyme-linked immunosorbent assay (ELISA) to detect Vip3Aa in genetically modified (GM) crops and their products.

Results

Vegetative insecticidal proteins (Vips) are secreted by Bacillus thuringiensis (Bt) and are known to be toxic to Lepidoptera species. Vip3Aa family proteins, Vip3Aa19 and Vip3Aa20, were successfully applied in GM crops to confer an effective and persistent insecticidal resistance. A sensitive monoclonal antibody-based sandwich ELISA was developed to detect Vip3Aa in GM crops and their products. Two monoclonal antibodies were raised against the overexpressed and purified His-Vip3Aa20, were purified from mouse ascites and characterized. A sandwich ELISA method was developed using the 2G3-1D7 monoclonal antibody for capture and the biotin-labeled 1F9-1F5 monoclonal antibody for detection of Vip3Aa20. The linear detection range of the method was found to be approximately 31.25–500 pg/ml, with a sensitivity of 10.24 pg/ml.

Conclusions

The established ELISA was effective for detecting Vip3Aa family proteins other than Vip3Aa8, and was successfully applied in the detection of Vip3Aa20 and Vip3Aa19 expressed in transgenic maize and cotton.

Cry78Ba1, One Novel Crystal Protein from Bacillus thuringiensis with High Insecticidal Activity against Rice Planthopper

The rice planthopper is a very important hemipteran pest that preys on rice and substantially affects the safety of rice production. Moreover, the long-term prevention and control of these pests with chemical pesticides has led to an increase in the resistance of the rice planthopper as well as serious environmental pollution and food safety problems. Bacillus thuringiensis (Bt) has been used for the efficient and green control of a variety of rice pests. Therefore, based on the high-throughput screening of Bt strains that are active against the rice planthopper, we found that Bt strain B4F11 showed certain insecticidal activity against Laodelphax striatellus Fallén, and we have identified a novel insecticidal protein Cry78Ba1 from the Bt strain B4F11, which is expected to provide the specific and safe control of the rice planthopper. The Cry78Ba1 protein is composed of 380 amino acid residues with a molecular weight of 42.55 kDa and contains conserved Ricin_B_Lectin and Toxin_10 superfamily domains. It displays high insecticidal activity against L. striatellus with a lethal concentration (LC₅₀) of 9.723 μg/mL. More importantly, this Toxin_10-like protein does not display sequence homology to any known allergen and can be degraded and inactivated rapidly when heated at 90 °C and in simulated gastrointestinal fluid. In summary, Cry78Ba1 has great potential for applications in the efficient and safe prevention and control of the rice planthopper.

Role of the peritrophic matrix in insect-pathogen interactions

The peritrophic matrix (PM) is an acellular chitin and glycoprotein layer that lines the invertebrate midgut. The PM has long been considered a physical as well as a biochemical barrier, protecting the midgut epithelium from abrasive food particles, digestive enzymes and pathogens infectious per os. This short review will focus on the latter function, as a barrier to pathogens infectious per os. We focus on the evidence confirming the role of the PM as protective barrier against pathogenic microorganisms of insects, mainly bacteria and viruses, as well as the evolution of a variety of mechanisms used by pathogens to overcome the PM barrier.

Specific Cytotoxic Effects of Parasporal Crystal Proteins Isolated from Native Saudi Arabian Bacillus thuringiensis Strains against Cervical Cancer Cells

Currently, global efforts are being intensified towards the discovery of local Bacillus thuringiensis (Bt) isolates with unique anticancer properties. Parasporins (PS) are a group of Bt non-insecticidal crystal proteins with potential and specific in vitro anticancer activity. However, despite the significant therapeutic potential of PS-producing Bt strains, our current knowledge on the effects of these proteins is limited. Hence, the main objective of this study was to screen Bt-derived parasporal toxins for cytotoxic activities against colon (HT-29) and cervical (HeLa) cancerous cell lines. Nine non-larvicidal and non-hemolytic Bt strains, native to Saudi Arabia, were employed for the isolation of their parasporal toxins. 16S rDNA sequencing revealed a 99.5% similarity with a reference Bt strain. While PCR screening results indicated the absence of selected Cry (Cry4A, Cry4B, Cry10 and Cry11), Cyt (Cyt1 and Cyt2) and PS (PS2, PS3 and PS4) genes, it concluded presence of the PS1 gene. SDS-PAGE analysis revealed that proteolytically-cleavaged PS protein profiles exhibit patterns resembling those observed with PS1Aa1, with major bands at 56 kDa and 17 kDa (Bt7), and 41 kDa and 16 kDa (Bt5). Solubilized and trypsinized PS proteins from all Bt strains exhibited a marked and dose-dependent cytotoxicity against HeLa cancerous cells but not against HT-29 cells. IC₅₀ values ranged from 3.2 (Bt1) to 14.2 (Bt6) with an average of 6.8 µg/mL. The observed cytotoxicity of PS proteins against HeLa cells was specific as it was not evident against normal uterus smooth muscle cells. RT-qPCR analysis revealed the overexpression of caspase 3 and caspase 9 by 3.7, and 4.2 folds, respectively, indicative of the engagement of intrinsic pathway of apoptosis. To the best of our knowledge, this is the first report exploring and exploiting the versatile repertoire of Saudi Arabian environmental niches for the isolation of native and possibly novel Saudi Bt strains with unique and specific anticancer activity. In conclusion, native Saudi Bt-derived PS proteins might have a potential to join the arsenal of natural anticancer drugs.

Evolutionary Features in the Structure and Function of Bacterial Toxins

Toxins can function both as a harmful and therapeutic molecule, depending on their concentrations. The diversity in their function allows us to ask some very pertinent questions related to their origin and roles: (a) What makes them such effective molecules? (b) Are there evolutionary features encoded within the structures of the toxins for their function? (c) Is structural hierarchy in the toxins important for maintaining their structure and function? (d) Do protein dynamics play a role in the function of toxins? and (e) Do the evolutionary connections to these unique features and functions provide the fundamental points in driving evolution? In light of the growing evidence in structural biology, it would be appropriate to suggest that protein dynamics and flexibility play a much bigger role in the function of the toxin than the structure itself. Discovery of IDPs (intrinsically disorder proteins), multifunctionality, and the concept of native aggregation are shaking the paradigm of the requirement of a fixed three-dimensional structure for the protein’s function. Growing evidence supporting the above concepts allow us to redesign the structure-function aspects of the protein molecules. An evolutionary model is necessary and needs to be developed to study these important aspects. The criteria for a well-defined model would be: (a) diversity in structure and function, (b) unique functionality, and (c) must belong to a family to define the evolutionary relationships. All these characteristics are largely fulfilled by bacterial toxins. Bacterial toxins are diverse and widely distributed in all three forms of life (Bacteria, Archaea and Eukaryotes). Some of the unique characteristics include structural folding, sequence and functional combination of domains, targeting a cellular process to execute their function, and most importantly their flexibility and dynamics. In this work, we summarize certain unique aspects of bacterial toxins, including role of structure in defining toxin function, uniqueness in their enzymatic function, and interaction with their substrates and other proteins. Finally, we have discussed the evolutionary aspects of toxins in detail, which will help us rethink the current evolutionary theories. A careful study, and appropriate interpretations, will provide answers to several questions related to the structure-function relationship of proteins, in general. Additionally, this will also allow us to refine the current evolution theories.

The C-terminal protoxin region of Bacillus thuringiensis Cry1Ab toxin has a functional role in binding to GPI-anchored receptors in the insect midgut

Bacillus thuringiensis Cry toxins are used worldwide for controlling insects. Cry1Ab is produced as a 130-kDa protoxin that is activated by proteolytic removal of an inert 500 amino-acid-long C-terminal region, enabling the activated toxin to bind to insect midgut receptor proteins, leading to its membrane insertion and pore formation. It has been proposed that the C-terminal region is only involved in toxin crystallization, but its role in receptor binding is undefined. Here we show that the C-terminal region of Cry1Ab protoxin provides additional binding sites for alkaline phosphatase (ALP) and aminopeptidase N (APN) insect receptors. ELISA, ligand blot, surface plasmon resonance, and pulldown assays revealed that the Cry1Ab C-terminal region binds to both ALP and APN but not to cadherin. Thus, the C-terminal region provided a higher binding affinity of the protoxin to the gut membrane that correlated with higher toxicity of protoxin than activated toxin. Moreover, Cry1Ab domain II loop 2 or 3 mutations reduced binding of the protoxin to cadherin but not to ALP or APN, supporting the idea that protoxins have additional binding sites. These results imply that two different regions mediate the binding of Cry1Ab protoxin to membrane receptors, one located in domain II-III of the toxin and another in its C-terminal region, suggesting an active role of the C-terminal protoxin fragment in the mode of action of Cry toxins. These results suggest that future manipulations of the C-terminal protoxin region could alter the specificity and increase the toxicity of B. thuringiensis proteins.

Global challenges faced by engineered Bacillus thuringiensis Cry genes in soybean (Glycine max L.) in the twenty-first century

The most important insect pests causing severe economic damages to soybean (Glycine max L.) production worldwide are Chrysodeixis includens (Walker, Noctuidae), Anticarsia gemmatalis (Hübner, Erebidae), Helicoverpa gelotopoeon (Dyar, Noctuidae), Crocidosema aporema (Walsingham; Tortricidae), Spodoptera albula (Walker, Noctuidae), S. cosmiodes (Walker, Noctuidae), S. eridania (Stoll, Noctuidae), S. frugiperda (Smith; Noctuidae), Helicoverpa armigera (Hübner, Noctuidae), H. zea (Boddie; Noctuidae) and Telenomus podisi (Hymenoptera,Platygastidae). Despite the success of biotech Bacillus thuringiensis (Bt)/herbicide tolerance (HT)-soybean in the past decade in terms of output, unforeseen mitigated performances have been observed due to changes in climatic events that favors the emergence of insect resistance. Thus, there is a need to develop hybrids with elaborated gene stacking to avert the upsurge in insect field tolerance to crystal (Cry) toxins in Bt-soybean. This study covers the performance of important commercial transgenic soybean developed to outwit destructive insects. New gene stacking soybean events such as Cry1Ac-, Cry1AF- and PAT-soybean (DAS-81419-2^®, Conkesta™ technology), and MON-87751-7 × MON-87701-2 × MON 87708 × MON 89788 (bearing Cry1A.105 [Cry1Ab, Cry1F, Cry1Ac], Cry2Ab, Cry1Ac) are being approved and deployed in fields. Following this deployment trend, we recommend herein that plant-mediated RNA interference into Bt-soybean, and the application of RNA-based pesticides that is complemented by other best agricultural practices such as refuge compliance, and periodic application of low-level insecticides could maximize trait durability in Bt-soybean production in the twenty-first century.

Role of plasmid plasticity and mobile genetic elements in the entomopathogen Bacillus thuringiensis serovar israelensis

Bacillus thuringiensis is a well-known biopesticide that has been used for more than 80 years. This spore-forming bacterium belongs to the group of Bacillus cereus that also includes, among others, emetic and diarrheic pathotypes of B. cereus, the animal pathogen Bacillus anthracis and the psychrotolerant Bacillus weihenstephanensis. Bacillus thuringiensis is rather unique since it has adapted its lifestyle as an efficient pathogen of specific insect larvae. One of the peculiarities of B. thuringiensis strains is the extent of their extrachromosomal pool, with strains harbouring more than 10 distinct plasmid molecules. Among the numerous serovars of B. thuringiensis, 'israelensis' is certainly emblematic since its host spectrum is apparently restricted to dipteran insects like mosquitoes and black flies, vectors of human and animal diseases such as malaria, yellow fever, or river blindness. In this review, the putative role of the mobile gene pool of B. thuringiensis serovar israelensis in its pathogenicity and dedicated lifestyle is reviewed, with specific emphasis on the nature, diversity, and potential mobility of its constituents. Variations among the few related strains of B. thuringiensis serovar israelensis will also be reported and discussed in the scope of this specialised insect pathogen, whose lifestyle in the environment remains largely unknown.