Recombinant bacteria for mosquito control

The Perpetual Vector Mosquito Threat and Its Eco-Friendly Nemeses

Mosquitoes are the most notorious arthropod vectors of viral and parasitic diseases for which approximately half the world's population, ~4,000,000,000, is at risk. Integrated pest management programs (IPMPs) have achieved some success in mitigating the regional transmission and persistence of these diseases. However, as many vector-borne diseases remain pervasive, it is obvious that IPMP successes have not been absolute in eradicating the threat imposed by mosquitoes. Moreover, the expanding mosquito geographic ranges caused by factors related to climate change and globalization (travel, trade, and migration), and the evolution of resistance to synthetic pesticides, present ongoing challenges to reducing or eliminating the local and global burden of these diseases, especially in economically and medically disadvantaged societies. Abatement strategies include the control of vector populations with synthetic pesticides and eco-friendly technologies. These "green" technologies include SIT, IIT, RIDL, CRISPR/Cas9 gene drive, and biological control that specifically targets the aquatic larval stages of mosquitoes. Regarding the latter, the most effective continues to be the widespread use of Lysinibacillus sphaericus (Ls) and Bacillus thuringiensis subsp. israelensis (Bti). Here, we present a review of the health issues elicited by vector mosquitoes, control strategies, and lastly, focus on the biology of Ls and Bti, with an emphasis on the latter, to which no resistance has been observed in the field.

Bacillus thuringiensis Cyt Proteins as Enablers of Activity of Cry and Tpp Toxins against Aedes albopictus

Aedes albopictus is a species of mosquito, originally from Southeast Asia, that belongs to the Culicidae family and the Dipteran insect order. The distribution of this vector has rapidly changed over the past decade, making most of the temperate territories in the world vulnerable to important human vector-borne diseases such as dengue, yellow fever, zika or chikungunya. Bacillus thuringiensis var. israeliensis (Bti)-based insecticides represent a realistic alternative to the most common synthetic insecticides for the control of mosquito larvae. However, several studies have revealed emerging resistances to the major Bti Crystal proteins such as Cry4Aa, Cry4Ba and Cry11Aa, making the finding of new toxins necessary to diminish the exposure to the same toxicity factors overtime. Here, we characterized the individual activity of Cyt1Aa, Cry4Aa, Cry4Ba and Cry11Aa against A. albopictus and found a new protein, Cyt1A-like, that increases the activity of Cry11Aa more than 20-fold. Additionally, we demonstrated that Cyt1A-like facilitates the activity three new Bti toxins: Cry53-like, Cry56A-like and Tpp36-like. All in all, these results provide alternatives to the currently available Bti products for the control of mosquito populations and position Cyt proteins as enablers of activity for otherwise non-active crystal proteins.

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.

Can (We Make) Bacillus thuringiensis Crystallize More Than Its Toxins?

The development of finely tuned and reliable crystallization processes to obtain crystalline formulations of proteins has received growing interest from different scientific fields, including toxinology and structural biology, as well as from industry, notably for biotechnological and medical applications. As a natural crystal-making bacterium, Bacillus thuringiensis (Bt) has evolved through millions of years to produce hundreds of highly structurally diverse pesticidal proteins as micrometer-sized crystals. The long-term stability of Bt protein crystals in aqueous environments and their specific and controlled dissolution are characteristics that are particularly sought after. In this article, we explore whether the crystallization machinery of Bt can be hijacked as a means to produce (micro)crystalline formulations of proteins for three different applications: (i) to develop new bioinsecticidal formulations based on rationally improved crystalline toxins, (ii) to functionalize crystals with specific characteristics for biotechnological and medical applications, and (iii) to produce microcrystals of custom proteins for structural biology. By developing the needs of these different fields to figure out if and how Bt could meet each specific requirement, we discuss the already published and/or patented attempts and provide guidelines for future investigations in some underexplored yet promising domains.

How Does Bacillus thuringiensis Crystallize Such a Large Diversity of Toxins?

Bacillus thuringiensis (Bt) is a natural crystal-making bacterium. Bt diversified into many subspecies that have evolved to produce crystals of hundreds of pesticidal proteins with radically different structures. Their crystalline form ensures stability and controlled release of these major virulence factors. They are responsible for the toxicity and host specificity of Bt, explaining its worldwide use as a biological insecticide. Most research has been devoted to understanding the mechanisms of toxicity of these toxins while the features driving their crystallization have long remained elusive, essentially due to technical limitations. The evolution of methods in structural biology, pushing back the limits in size of amenable protein crystals now allows access to be gained to structural information hidden within natural crystals of such toxins. In this review, we present the main parameters that have been identified as key drivers of toxin crystallization in Bt, notably in the light of recent discoveries driven by structural biology studies. Then, we develop how the future evolution of structural biology will hopefully unveil new mechanisms of Bt toxin crystallization, opening the door to their hijacking with the aim of developing a versatile in vivo crystallization platform of high academic and industrial interest.

Identification of a Novel Brevibacillus laterosporus Strain With Insecticidal Activity Against Aedes albopictus Larvae

Bacterial species able to produce proteins that are toxic against insects have been discovered at the beginning of the last century. However, up to date only two of them have been used as pesticides in mosquito control strategies targeting larval breeding sites: Bacillus thuringensis var. israelensis and Lysinibacillus sphaericus. Aiming to expand the arsenal of biopesticides, bacterial cultures from 44 soil samples were assayed for their ability to kill larvae of Aedes albopictus. A method to select, grow and test the larvicidal capability of spore-forming bacteria from each soil sample was developed. This allowed identifying 13 soil samples containing strains capable of killing Ae. albopictus larvae. Among the active isolates, one strain with high toxicity was identified as Brevibacillus laterosporus by 16S rRNA gene sequencing and by morphological characterization using transmission electron microscopy. The new isolate showed a larvicidal activity significantly higher than the B. laterosporus LMG 15441 reference strain. Its genome was phylogenomically characterized and compared to the available Brevibacillus genomes. Thus, the new isolate can be considered as a candidate adjuvant to biopesticides formulations that would help preventing the insurgence of resistance.

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.

Identification of Lysinibacillus sphaericus Binary toxin binding proteins in a malarial mosquito cell line by proteomics: A novel approach towards improving mosquito control

Bacterial insecticidal proteins, such as the Bin toxin from Lysinibacillus sphaericus, could be used more extensively to control insecticide resistant mosquitoes. This study was aimed at identification of mosquito cell proteins binding Bin toxin. Results showed that purified toxin was toxic to Anopheles gambiae larvae and Ag55 cultured cells. Clathrin heavy chain (an endocytosis protein) and glycolytic enzymes such as pyruvate kinase, enolase and dihydrolipoamide dehydrogenase were identified as binders of Bin toxin. The viability of Ag55 cells in the presence of endocytosis inhibitor, pitstop2, was significantly decreased upon Bin treatment, while the inhibitor chlorpromazine did not affect Bin toxicity. Bin toxin treatment decreased ATP production and mitochondrial respiration in Ag55 cells, whereas non-mitochondrial oxygen consumption significantly increased after Bin toxin treatment. These findings are steps towards understanding how Bin toxin kills mosquitoes. SIGNIFICANCE: Mosquitoes are vectors of pathogens causing human diseases such as dengue fever, yellow fever, zika virus and malaria. An insecticidal toxin from Lysinibacillus sphaericus called Binary, or Bin, toxin could be used more extensively to control insecticide resistant mosquitoes. Bin toxin enter cells in susceptible mosquitoes and induces apoptosis or autophagy. In the current research, we used the malaria mosquito Anopheles gambiae Ag55 cell line as a model. A proteomic-based approach identified proteins that interact with Bin toxin. Interacting proteins include clathrin heavy chain (endocytosis protein) and glycolysis enzymes such as pyruvate kinase, enolase and dihydrolipoamide dehydrogenase. In Ag55 cell toxicity assays, an endocytosis inhibitor, pitstop2, increased Bin toxicity. Real time assays with a Seahorse™ flux analyzer showed that Bin significantly affects mitochondrial respiration, a result consistent with cell death via apoptosis or autophagy. These research findings add insights into how an unusual binary protein exploits cellular machinery to kill mosquitoes.

Genetically modified entomopathogenic bacteria, recent developments, benefits and impacts: A review

Entomopathogenic bacteria (EPBs), insect pathogens that produce pest-specific toxins, are environmentally-friendly alternatives to chemical insecticides. However, the most important problem with EPBs application is their limited field stability. Moreover, environmental factors such as solar radiation, leaf temperature, and vapor pressure can affect the pathogenicity of these pathogens and their toxins. Scientists have conducted intensive research to overcome such problems. Genetic engineering has great potential for the development of new engineered entomopathogens with more resistance to adverse environmental factors. Genetically modified entomopathogenic bacteria (GM-EPBs) have many advantages over wild EPBs, such as higher pathogenicity, lower spraying requirements and longer-term persistence. Genetic manipulations have been mostly applied to members of the bacterial genera Bacillus, Lysinibacillus, Pseudomonas, Serratia, Photorhabdus and Xenorhabdus. Although many researchers have found that GM-EPBs can be used safely as plant protection bioproducts, limited attention has been paid to their potential ecological impacts. The main concerns about GM-EPBs and their products are their potential unintended effects on beneficial insects (predators, parasitoids, pollinators, etc.) and rhizospheric bacteria. This review address recent update on the significant role of GM-EPBs in biological control, examining them through different perspectives in an attempt to generate critical discussion and aid in the understanding of their potential ecological impacts.

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.

Functional Bacillus thuringiensis Cyt1Aa Is Necessary To Synergize Lysinibacillus sphaericus Binary Toxin (Bin) against Bin-Resistant and -Refractory Mosquito Species

The binary (Bin) toxin from Lysinibacillus sphaericus is effective to mosquito larvae, but its utilization is threatened by the development of insect resistance. Bin toxin is composed of the BinB subunit required for binding to midgut receptors and the BinA subunit that causes toxicity after cell internalization, mediated by BinB. Culex quinquefasciatus resistance to this toxin is caused by mutations that prevent expression of Bin toxin receptors in the midgut. Previously, it was shown that the Cyt1Aa toxin from Bacillus thuringiensis subsp. israelensis restores Bin toxicity to Bin-resistant C. quinquefasciatus and to Aedes aegypti larvae, which are naturally devoid of functional Bin receptors. Our goal was to elucidate the mechanism involved in Cyt1Aa synergism with Bin in such larvae. In vivo assays showed that the mixture of Bin toxin, or its BinA subunit, with Cyt1Aa was effective to kill resistant larvae. However, no specific binding interaction between Cyt1Aa and the Bin toxin, or its subunits, was observed. The synergy between Cyt1Aa and Bin toxins is dependent on functional Cyt1Aa, as demonstrated by using the nontoxic Cyt1AaV122E mutant toxin affected in oligomerization and membrane insertion, which was unable to synergize Bin toxicity in resistant larvae. The synergism correlated with the internalization of Bin or BinA into anterior and medium midgut epithelial cells, which occurred only in larvae treated with wild-type Cyt1Aa toxin. This toxin is able to overcome failures in the binding step involving BinB receptor by allowing the internalization of Bin toxin, or its BinA subunit, into the midgut cells.IMPORTANCE One promising management strategy for mosquito control is the utilization of a mixture of L. sphaericus and B. thuringiensis subsp. israelensis insecticidal toxins. From this set, Bin and Cyt1Aa toxins synergize and display toxicity to resistant C. quinquefasciatus and to A. aegypti larvae, whose midgut cells lack Bin toxin receptors. Our data set provides evidence that functional Cyt1Aa is essential for internalization of Bin or its BinA subunit into such cells, but binding interaction between Bin and Cyt1Aa is not observed. Thus, this mechanism contrasts with that for the synergy between Cyt1Aa and the B. thuringiensis subsp. israelensis Cry toxins, where active Cyt1Aa is not necessary but a specific binding between Cry and Cyt1Aa is required. Our study established the initial molecular basis of the synergy between Bin and Cyt1Aa, and these findings enlarge our knowledge of their mode of action, which could help to develop improved strategies to cope with insect resistance.

Serial femtosecond crystallography on in vivo-grown crystals drives elucidation of mosquitocidal Cyt1Aa bioactivation cascade

Cyt1Aa is the one of four crystalline protoxins produced by mosquitocidal bacterium Bacillus thuringiensis israelensis (Bti) that has been shown to delay the evolution of insect resistance in the field. Limiting our understanding of Bti efficacy and the path to improved toxicity and spectrum has been ignorance of how Cyt1Aa crystallizes in vivo and of its mechanism of toxicity. Here, we use serial femtosecond crystallography to determine the Cyt1Aa protoxin structure from sub-micron-sized crystals produced in Bti. Structures determined under various pH/redox conditions illuminate the role played by previously uncharacterized disulfide-bridge and domain-swapped interfaces from crystal formation in Bti to dissolution in the larval mosquito midgut. Biochemical, toxicological and biophysical methods enable the deconvolution of key steps in the Cyt1Aa bioactivation cascade. We additionally show that the size, shape, production yield, pH sensitivity and toxicity of Cyt1Aa crystals grown in Bti can be controlled by single atom substitution.

Bacillus thuringiensis israelensis (Bti) produces the naturally-crystalline proteinaceous toxin Cyt1Aa that is toxic to mosquito larvae. Here the authors grow recombinant nanocrystals of the Cyt1Aa protoxin in vivo and use serial femtosecond crystallography to determine its structure at different redox and pH conditions and by combining their structural data with further biochemical, toxicological and biophysical analyses provide mechanistic insights into the Cyt1Aa bioactivation cascade.

Interaction between Insects, Toxins, and Bacteria: Have We Been Wrong So Far?

Toxins are a major virulence factor produced by many pathogenic bacteria. In vertebrates, the response of hosts to the bacteria is inseparable from the response to the toxins, allowing a comprehensive understanding of this tripartite host-pathogen-toxin interaction. However, in invertebrates, this interaction has been investigated by two complementary but historically distinct fields of research: toxinology and immunology. In this article, I highlight how such dichotomy between these two fields led to a biased, or even erroneous view of the ecology and evolution of the interaction between insects, toxins, and bacteria. I focus on the reason behind such a dichotomy, on how to bridge the fields together, and on confounding effects that could bias the outcome of the experiments. Finally, I raise four questions at the border of the two fields on the cross-effects between toxins, bacteria, and spores that have been largely underexplored to promote a more comprehensive view of this interaction.

Transstadial transmission of larval hemocoelic infection negatively affects development and adult female longevity in the mosquito Anopheles gambiae

During all life stages, mosquitoes are exposed to pathogens, and employ an immune system to resist or limit infection. Although much attention has been paid to how adult mosquitoes fight infection, little is known about how an infection during the larval stage affects the biology of the resultant adult. In this study, we investigated whether a bacterial infection in the hemocoel of the African malaria mosquito, Anopheles gambiae, is transstadially transmitted from larvae to adults (both females and males), and whether immune stimulation in the hemocoel as a larva alters development or biological traits of the adult. Specifically, larvae were injected in the hemocoel with either fluorescent microspheres or Escherichia coli, and the following traits were examined: transstadial transmission, larval development to adulthood, adult survival, and adult body size. Our results show that transstadial transmission of hemocoel contents occurs from larvae to pupae and from pupae to adults, but that bacterial prevalence and intensity varies with age. Injury, immune stimulation or infection decreases the proportion of larvae that undergo pupation and eclosion, infection decreases the longevity of adult females, and treatment has complex effects on the body size of the resultant adults. The present study adds larval hemocoelic infection to the known non-genetic factors that reduce overall fitness by negatively affecting development and adult biological traits that influence mosquito vector competence.

Highly Effective Broad Spectrum Chimeric Larvicide That Targets Vector Mosquitoes Using a Lipophilic Protein

Two mosquitocidal bacteria, Bacillus thuringiensis subsp. israelensis (Bti) and Lysinibacillus sphaericus (Ls) are the active ingredients of commercial larvicides used widely to control vector mosquitoes. Bti’s efficacy is due to synergistic interactions among four proteins, Cry4Aa, Cry4Ba, Cry11Aa, and Cyt1Aa, whereas Ls’s activity is caused by Bin, a heterodimer consisting of BinA, the toxin, and BinB, a midgut-binding protein. Cyt1Aa is lipophilic and synergizes Bti Cry proteins by increasing midgut binding. We fused Bti’s Cyt1Aa to Ls’s BinA yielding a broad-spectrum chimeric protein highly mosquitocidal to important vector species including Anopheles gambiae, Culex quinquefasciatus, and Aedes aegypti, the latter an important Zika and Dengue virus vector insensitive to Ls Bin. Aside from its vector control potential, our bioassay data, in contrast to numerous other reports, provide strong evidence that BinA does not require conformational interactions with BinB or microvillar membrane lipids to bind to its intracellular target and kill mosquitoes.

Contributions of 5'-UTR and 3'-UTR cis elements to Cyt1Aa synthesis in Bacillus thuringiensis subsp. israelensis

The biopesticide used most effectively to control mosquito and blackfly vectors of human diseases worldwide is Bacillus thuringiensis subsp. israelensis. The high efficacy of this bacterium is due to synergistic interactions among four protein entomotoxins assembled individually into a single parasporal body (PB) during sporulation. Cyt1Aa, the primary synergist, is the most abundant toxin, comprising approximately 55% of the PB's mass. The other proteins are Cry11Aa at ∼35%, and Cry4Aa and Cry4Ba, which together account for the remaining ∼10%. The molecular genetic basis for the comparatively large amount of Cyt1Aa synthesized is unknown. Here, in addition to the known strong BtI (σ^E) and BtII (σ^K) promoters, we demonstrate a third promoter (BtIII) that has high identity to the σ^E promoter of Bacillus subtilis, contributes to the large amount of Cyt1Aa synthesized. We also show that a cyt1Aa-BtIII construct was not functional in a σ^E-deficient strain of B. subtilis. Comparison of transcription levels and protein profiles for recombinant strains containing different combinations of BtI, BtII and BtIII, or each promoter alone, showed that BtIII is active throughout sporulation. We further demonstrate that a stable stem-loop in the 3'-untranslated region (3'-UTR, predicted ΔG=-27.6) contributes to the high level of Cyt1Aa synthesized.

Anopheles gambiae larvae mount stronger immune responses against bacterial infection than adults: evidence of adaptive decoupling in mosquitoes

BACKGROUND

The immune system of adult mosquitoes has received significant attention because of the ability of females to vector disease-causing pathogens while ingesting blood meals. However, few studies have focused on the immune system of larvae, which, we hypothesize, is highly robust due to the high density and diversity of microorganisms that larvae encounter in their aquatic environments and the strong selection pressures at work in the larval stage to ensure survival to reproductive maturity. Here, we surveyed a broad range of cellular and humoral immune parameters in larvae of the malaria mosquito, Anopheles gambiae, and compared their potency to that of newly-emerged adults and older adults.

RESULTS

We found that larvae kill bacteria in their hemocoel with equal or greater efficiency compared to newly-emerged adults, and that antibacterial ability declines further with adult age, indicative of senescence. This phenotype correlates with more circulating hemocytes and a differing spatial arrangement of sessile hemocytes in larvae relative to adults, as well as with the individual hemocytes of adults carrying a greater phagocytic burden. The hemolymph of larvae also possesses markedly stronger antibacterial lytic and melanization activity than the hemolymph of adults. Finally, infection induces a stronger transcriptional upregulation of immunity genes in larvae than in adults, including differences in the immunity genes that are regulated.

CONCLUSIONS

These results demonstrate that immunity is strongest in larvae and declines after metamorphosis and with adult age, and suggest that adaptive decoupling, or the independent evolution of larval and adult traits made possible by metamorphosis, has occurred in the mosquito lineage.

Biological Control Strategies for Mosquito Vectors of Arboviruses

Historically, biological control utilizes predatory species and pathogenic microorganisms to reduce the population of mosquitoes as disease vectors. This is particularly important for the control of mosquito-borne arboviruses, which normally do not have specific antiviral therapies available. Although development of resistance is likely, the advantages of biological control are that the resources used are typically biodegradable and ecologically friendly. Over the past decade, the advancement of molecular biology has enabled optimization by the manipulation of genetic materials associated with biological control agents. Two significant advancements are the discovery of cytoplasmic incompatibility induced by Wolbachia bacteria, which has enhanced replacement programs, and the introduction of dominant lethal genes into local mosquito populations through the release of genetically modified mosquitoes. As various arboviruses continue to be significant public health threats, biological control strategies have evolved to be more diverse and become critical tools to reduce the disease burden of arboviruses.

Persistent Infection by Wolbachia wAlbB Has No Effect on Composition of the Gut Microbiota in Adult Female Anopheles stephensi

The bacteria in the midgut of Anopheles stephensi adult females from laboratory colonies were studied by sequencing the V4 region of 16S rRNA genes, with respect to three experimental factors: stable or cured Wolbachia infection; sugar or blood diet; and age. Proteobacteria and Bacteroidetes dominated the community [>90% of operational taxonomic units (OTUs)]; most taxa were in the classes Flavobacteriia, Gammaproteobacteria, and Alphaproteobacteria, and were assigned to Elizabethkingia (46.9%), Asaia (6.4%) and Pseudomonas (6.0%), or unclassified Enterobacteriaceae (37.2%). Bacterial communities were similar between Wolbachia-cured and Wolbachia-infected mosquito lines, indicating that the gut microbiota were not dysregulated in the presence of Wolbachia. The proportion of Enterobacteriaceae was higher in mosquitoes fed a blood meal compared to those provided a sugar meal. Collectively, the bacterial community had a similar structure in older Wolbachia-infected mosquitoes 8 days after the blood meal, as in younger Wolbachia-infected mosquitoes before a blood meal, except that older mosquitoes had a higher proportion of Enterobacteriaceae and lower proportion of Elizabethkingia. Consistent presence of certain predominant bacteria (Elizabethkingia, Asaia, Pseudomonas, and Enterobacteriaceae) suggests they would be useful for paratransgenesis to control malaria infection, particularly when coupled to a Wolbachia-based intervention strategy.

Functional integration of the circulatory, immune, and respiratory systems in mosquito larvae: pathogen killing in the hemocyte-rich tracheal tufts

BACKGROUND

As both larvae and adults, mosquitoes encounter a barrage of immune insults, ranging from microbe-rich communities in larval habitats to ingested blood-borne pathogens in adult blood meals. Given that mosquito adults have evolved an efficient means of eliminating infections in their hemocoel (body cavity) via the coordinated action of their immune and circulatory systems, the goal of the present study was to determine whether such functional integration is also present in larvae.

RESULTS

By fluorescently labeling hemocytes (immune cells), pericardial cells, and the heart, we discovered that fourth instar larvae, unlike adults, contain segmental hemocytes but lack the periostial hemocytes that surround the ostia (heart valves) in abdominal segments 2-7. Instead, larvae contain an abundance of sessile hemocytes at the tracheal tufts, which are respiratory structures that are unique to larvae, are located in the posterior-most abdominal segment, and surround what in larvae are the sole incurrent openings for hemolymph entry into the heart. Injection of fluorescent immune elicitors and bacteria into the larval hemocoel then showed that tracheal tuft hemocytes mount rapid and robust immune responses against foreign insults. Indeed, green fluorescent protein-labeled Escherichia coli flowing with the hemolymph rapidly aggregate exclusively at the tracheal tufts, where they are killed within 24 h post-infection via both phagocytosis and melanization.

CONCLUSION

Together, these findings show that the functional integration of the circulatory, respiratory, and immune systems of mosquitoes varies drastically across life stages.

A Mixture of Bacillus thuringiensis subsp. israelensis With Xenorhabdus nematophila -Cultured Broth Enhances Toxicity Against Mosquitoes Aedes albopictus and Culex pipiens pallens (Diptera: Culicidae)

Xenorhabdus and Photorhabdus spp. (Enterobacteriaceae) can synthesize and release secondary metabolites that play crucial roles in their pathogenicity by suppressing the immunity of target insects. The insect immunity contributes to defense against the pathogenicity of Bacillus thuringiensis (Bt). This study tested a hypothesis that bacterial immunosuppresants could enhance the susceptibility of mosquitoes ( Aedes albopictus and Culex pipiens pallens ) to Bt. Three symbiotic bacteria [ X. nematophila (Xn), X. hominickii (Xh), and P. temperata temperata (Ptt)] were cultured in nutrient broth to allow them to produce secondary metabolites. Bacillus thuringiensis israelensis (BtI) was highly toxic to both culicid mosquitoes with median lethal concentration (LC 50 , spores/ml) of 2.9 × 10 5 and 2.2 × 10 5 at 16 h after treatment, respectively. Addition of each bacteria-cultured broth enhanced BtI toxicity to these mosquito larvae. The LC 50 values of BtI to Ae. albopictus larvae were reduced to 1.5 × 10 5 in Xn mixture, 1.7 × 10 5 in Xh mixture, and 1.9 × 10 5 in Ptt mixture. The LC 50 values of BtI to Cx. pipiens pallens larvae were also reduced to 1.2 × 10 5 in Xn mixture, 1.3 × 10 5 in Xh mixture, and 1.5 × 10 5 in Ptt mixture. Adding benzylideneacetone or oxindole produced from Xn and Ptt also enhanced BtI toxicities to these mosquito larvae. Based on these results, we developed a new mosquitocidal Bt formulation called "Dip-Kill" consisting of 80% Xn-cultured broth, 10% BtI (10 10 spores/ml), and 10% preservative. Dip-Kill at 1,000 ppm was superior to a commercial BtI product at its recommended dose.

Highly toxic and broad-spectrum insecticidal local Bacillus strains engineered using protoplast fusion

Protoplast fusion was performed between a local Bacillus thuringiensis UV-resistant mutant 66/1a (Bt) and Bacillus sphaericus GHAI (Bs) to produce new Bacillus strains with a wider spectrum of action against different insects. Bt is characterized as sensitive to polymyxin and streptomycin and resistant to rifampicin and has shown 87% mortality against Spodoptera littoralis larvae at concentration of 1.5 × 10(7) cells/mL after 7 days of feeding; Bs is characterized as resistant to polymyxin and streptomycin and sensitive to rifampicin and has been shown to have 100% mortality against Culex pipiens after 1 day of feeding at the same concentration as that of Bt. Among a total of 64 Bt::Bs fusants produced on the selective medium containing polymyxin, streptomycin, and rifampicin, 17 fusants were selected because of their high mortality percentages against S. littoralis (Lepidoptera) and C. pipiens (Diptera). While Bt harboured 3 plasmids (600, 350, and 173 bp) and Bs had 2 plasmids (544 and 291 bp), all the selected fusants acquired plasmids from both parental strains. SDS-PAGE protein analysis of the 17 selected fusants and their parental strains confirmed that all fusant strains acquired and expressed many specific protein bands from the 2 parental strains, especially the larvicidal proteins to both lepidopteran and dipteran species with molecular masses of 65, 70, 80, 88, 100, and 135 kDa. Four protein bands with high molecular masses of 281, 263, 220, and 190 kDa, which existed in the Bt parental strain and did not exist in the Bs parental strain, and 2 other protein bands with high molecular masses of 185 and 180 kDa, which existed in the Bs parental strain and did not exist in the Bt parental strain, were expressed in most fusants. The results indicated the expression of some cry genes encoded for insecticidal crystal proteins from Bt and the binary toxin genes from Bs in all fusant strains. The recombinant fusants have more efficient and potential values for agricultural application compared with both the insecticidal Bt and the mosquitocidal Bs strains alone against S. littoralis and C. pipiens larvae, respectively.

Comparison of metabolic capacities and inference of gene content evolution in mosquito-associated Spiroplasma diminutum and S. taiwanense

Mosquitoes are hosts of several Spiroplasma species that belong to different serogroups. To investigate the genetic mechanisms that may be involved in the utilization of similar hosts in these phylogenetically distinct bacteria, we determined the complete genome sequences of Spiroplasma diminutum and S. taiwanense for comparative analysis. The genome alignment indicates that their chromosomal organization is highly conserved, which is in sharp contrast to the elevated genome instabilities observed in other Spiroplasma lineages. Examination of the substrate utilization strategies revealed that S. diminutum can use a wide range of carbohydrates, suggesting that it is well suited to living in the gut (and possibly the circulatory system) of its mosquito hosts. In comparison, S. taiwanense has lost several carbohydrate utilization genes and acquired additional sets of oligopeptide transporter genes through tandem duplications, suggesting that proteins from digested blood meal or lysed host cells may be an important nutrient source. Moreover, one glycerol-3-phosphate oxidase gene (glpO) was found in S. taiwanense but not S. diminutum. This gene is linked to the production of reactive oxygen species and has been shown to be a major virulence factor in Mycoplasma mycoides. This finding may explain the pathogenicity of S. taiwanense observed in previous artificial infection experiments, while no apparent effect was found for S. diminutum. To infer the gene content evolution at deeper divergence levels, we incorporated other Mollicutes genomes for comparative analyses. The results suggest that the losses of biosynthetic pathways are a recurrent theme in these host-associated bacteria.

Ingestibility, digestibility, and engineered biological control potential of Flavobacterium hibernum, isolated from larval mosquito habitats

Flavobacterium hibernum, isolated from larval habitats of the eastern tree hole mosquito, A. triseriatus, remained suspended in the larval feeding zone much longer (8 days) than other bacteria. Autofluorescent protein markers were developed for the labeling of F. hibernum with a strong flavobacterial expression system. Green fluorescent protein (GFP)-tagged F. hibernum cells were quickly consumed by larval mosquitoes at an ingestion rate of 9.5 × 10(4)/larva/h. The ingested F. hibernum cells were observed mostly in the foregut and midgut and rarely in the hindgut, suggesting that cells were digested and did not pass the gut viably. The NanoLuc luciferase reporter system was validated for quantitative larval ingestion rate and bacterial fate analyses. Larvae digested 1.87 × 10(5) cells/larva/h, and few F. hibernum cells were excreted intact. Expression of the GFP::Cry11A fusion protein with the P20 chaperone protein from Bacillus thuringiensis H-14 was successfully achieved in F. hibernum. Whole-cell bioassays of recombinant F. hibernum exhibited high larvicidal activity against A. triseriatus in microplates and in microcosms simulating tree holes. F. hibernum cells persisted in microcosms at 100, 59, 30, and 10% of the initial densities at days 1, 2, 3, and 6, respectively, when larvae were absent, while larvae consumed nearly all of the F. hibernum cells within 3 days of their addition to microcosms.

Interaction between mosquito-larvicidal Lysinibacillus sphaericus binary toxin components: analysis of complex formation

The two components (BinA and BinB) of Lysinibacillus sphaericus binary toxin together are highly toxic to Culex and Anopheles mosquito larvae, and have been employed world-wide to control mosquito borne diseases. Upon binding to the membrane receptor an oligomeric form (BinA2.BinB2) of the binary toxin is expected to play role in pore formation. It is not clear if these two proteins interact in solution as well, in the absence of receptor. The interactions between active forms of BinA and BinB polypeptides were probed in solution using size-exclusion chromatography, pull-down assay, surface plasmon resonance, circular dichroism, and by chemically crosslinking BinA and BinB components. We demonstrate that the two proteins interact weakly with first association and dissociation rate constants of 4.5×10(3) M(-1) s(-1) and 0.8 s(-1), resulting in conformational change, most likely, in toxic BinA protein that could kinetically favor membrane translocation of the active oligomer. The weak interactions between the two toxin components could be stabilized by glutaraldehyde crosslinking. The cross-linked complex, interestingly, showed maximal Culex larvicidal activity (LC50 value of 1.59 ng mL(-1)) reported so far for combination of BinA/BinB components, and thus is an attractive option for development of new bio-pesticides for control of mosquito borne vector diseases.

Mtx toxins from Lysinibacillus sphaericus enhance mosquitocidal cry-toxin activity and suppress cry-resistance in Culex quinquefasciatus

The interaction of Mtx toxins from Lysinibacillus sphaericus (formerly Bacillus sphaericus) with Bacillus thuringiensis subsp. israelensis Cry toxins and the influence of such interactions on Cry-resistance were evaluated in susceptible and Cry-resistant Culex quinquefasciatus larvae. Mtx-1 and Mtx-2 were observed to be active against both susceptible and resistant mosquitoes; however varying levels of cross-resistance toward Mtx toxins were observed in the resistant mosquitoes. A 1:1 mixture of either Mtx-1 or Mtx-2 with different Cry toxins generally showed moderate synergism, but some combinations were highly toxic to resistant larvae and suppressed resistance. Toxin synergy has been demonstrated to be a powerful tool for enhancing activity and managing Cry-resistance in mosquitoes, thus Mtx toxins may be useful as components of engineered bacterial larvicides.

The Cry4B toxin of Bacillus thuringiensis subsp. israelensis kills Permethrin-resistant Anopheles gambiae, the principal vector of malaria

Resurgence of malaria has been attributed, in part, to the development of resistance by Anopheles gambiae, a principal vector of the disease, to various insecticidal compounds such as Permethrin. Permethrin, a neurotoxicant, is widely used to impregnate mosquito nets. An alternative strategy to control mosquitoes is the use of Bacillus thuringiensis subsp. israelensis (Bti) because there is no observable resistance in the field to the bacterium. Bti kills mosquitoes by targeting cadherin molecules residing in the midgut epithelium of larvae of the insect. Cry proteins (Cry4A, Cry4B, Cry10A and Cry11A) produced by the bacterium during the sporulation phase of its life cycle bind to the cadherin molecules, which serve as receptors for the proteins. These Cry proteins have variable specificity to a variety of mosquitoes, including Culex and Aedes as well as Anopheles. Importantly, selective mosquitocidal action is occasioned by binding of the respective Cry toxins to cadherins distinctive to individual mosquito species. Differential fractionation of the four Cry proteins from a novel Bti isolate (M1) and cloning and expression of their genes in Escherichia coli revealed that Cry4B is the only Cry protein that exerts insecticidal action against An. gambiae. Indeed, it does so against a Permethrin-resistant strain of the mosquito. The other three Cry proteins are ineffective. Multiple sequence alignments of the four Cry proteins revealed a divergent sequence motif in the Cry4B toxin, which most likely determines binding of the toxin to its cognate receptor, BT-R3, in An. gambiae and to its specific toxicity. A model showing Cry4B toxin binding to BT-R3 is presented.

Gut content identification of larvae of the Anopheles gambiae complex in western Kenya using a barcoding approach

Although larvae feeding and food source are vital to the development, survival and population regulation of African malaria vectors, the prey organisms of Anopheles gambiae larvae in the natural environment have not been well studied. This study used a molecular barcoding approach to investigate the natural diets of Anopheles gambiae s.l. larvae in western Kenya. Gut contents from third- and fourth-instar larvae from natural habitats were dissected and DNA was extracted. The 18S ribosomal DNA gene was amplified, the resulting clones were screened using a restriction fragment length polymorphism method and nonmosquito clones were sequenced. Homology search and phylogenetic analyses were then conducted using the sequences of non-mosquito clones to identify the putative microorganisms ingested. The phylogenetic analyses clustered ingested microorganisms in four clades, including two clades of green algae (Chlorophyta, Chlorophyceae Class, Chlamydomonadales and Chlorococcales families), one fungal clade, and one unknown eukaryote clade. In parallel, using the same approach, an analysis of the biodiversity present in the larval habitats was carried out. This present study demonstrated the feasibility of the barcoding approach to infer the natural diets of Anopheles gambiae larvae. Our analysis suggests that despite the wide range of microorganisms available in natural habitats, mosquito larvae fed on specific groups of algae. The novel tools developed from this study can be used to improve our understanding of the larval ecology of African malaria vectors and to facilitate the development of new mosquito control tools.

Bacillus thuringiensis Cry4Ba toxin employs two receptor-binding loops for synergistic interactions with Cyt2Aa2

We previously demonstrated that co-expression in Escherichia coli of Bacillus thuringiensis (Bt) subsp. israelensis Cry4Ba and Bt subsp. darmstadiensis Cyt2Aa2 shows high synergistic toxicity against target mosquito larvae. Here, further insights into synergistic interactions between these two toxins were revealed through bioactivity restoration of particular inactive Cry4Ba-mutant toxins altered within the receptor-binding domain. Specific mutations at β2-β3 (Y332A) or β4-β5 (F364A) loops, but neither at three other β-hairpin loops (β6-β7, β8-β9 and β10-β11) of Cry4Ba, adversely affect toxicity restoration by Cyt2Aa2. Binding analysis using quartz crystal microbalance verified a decrease in binding of these two bioinactive-mutant toxins (Y332A and F364A) to the immobilized Cyt2Aa2. This suggests that Cry4Ba utilizes these two critical aromatic loop-residues, Tyr(332) and Phe(364), for synergistic toxicity with its alternative receptor-Cyt2Aa2.

North American wetlands and mosquito control

Wetlands are valuable habitats that provide important social, economic, and ecological services such as flood control, water quality improvement, carbon sequestration, pollutant removal, and primary/secondary production export to terrestrial and aquatic food chains. There is disagreement about the need for mosquito control in wetlands and about the techniques utilized for mosquito abatement and their impacts upon wetlands ecosystems. Mosquito control in wetlands is a complex issue influenced by numerous factors, including many hard to quantify elements such as human perceptions, cultural predispositions, and political climate. In spite of considerable progress during the last decades, habitat protection and environmentally sound habitat management still remain inextricably tied to politics and economics. Furthermore, the connections are often complex, and occur at several levels, ranging from local businesses and politicians, to national governments and multinational institutions. Education is the key to lasting wetlands conservation. Integrated mosquito abatement strategies incorporate many approaches and practicable options, as described herein, and need to be well-defined, effective, and ecologically and economically sound for the wetland type and for the mosquito species of concern. The approach will certainly differ in response to disease outbreaks caused by mosquito-vectored pathogens versus quality of life issues caused by nuisance-biting mosquitoes. In this contribution, we provide an overview of the ecological setting and context for mosquito control in wetlands, present pertinent information on wetlands mosquitoes, review the mosquito abatement options available for current wetlands managers and mosquito control professionals, and outline some necessary considerations when devising mosquito control strategies. Although the emphasis is on North American wetlands, most of the material is applicable to wetlands everywhere.

The susceptibility of five African Anopheles species to Anabaena PCC 7120 expressing Bacillus thuringiensis subsp. israelensis mosquitocidal cry genes

BACKGROUND

Malaria, one of the leading causes of death in Africa, is transmitted by the bite of an infected female Anopheles mosquito. Problems associated with the development of resistance to chemical insecticides and concerns about the non-target effects and persistence of chemical insecticides have prompted the development of environmentally friendly mosquito control agents. The aim of this study was to evaluate the larvicidal activity of a genetically engineered cyanobacterium, Anabaena PCC 7120#11, against five African Anopheles species in laboratory bioassays.

FINDINGS

There were significant differences in the susceptibility of the anopheline species to PCC 7120#11. The ranking of the larvicidal activity of PCC 7120#11 against species in the An. gambiae complex was: An. merus

CONCLUSIONS

PCC 7120#11 exhibited good larvicidal activity against larvae of the An. gambiae complex, but relatively weak larvicidal activity against An. funestus. The study has highlighted the importance of evaluating a novel mosquitocidal agent against a range of malaria vectors so as to obtain a clear understanding of the agent's spectrum of activity and potential as a vector control agent.

Collapse

Key Words

• malaria vectors
• anopheles
• bacillus thuringiensis subsp. israelensis
• cry proteins
• cyanobacteria
• anabaena sp. pcc 7120
• genetic engineering
• bioassays
• larvicidal activity

Collapse

MESH Headings

• Africa
• Anabaena/genetics
• Anabaena/growth & development
• Anabaena/metabolism
• Animals
• Anopheles/drug effects
• Anopheles/microbiology
• Anopheles/physiology
• Bacillus thuringiensis Toxins
• Bacterial Proteins/biosynthesis
• Bacterial Proteins/genetics
• Endotoxins/biosynthesis
• Endotoxins/genetics
• Female
• Hemolysin Proteins/biosynthesis
• Hemolysin Proteins/genetics
• Larva/drug effects
• Larva/microbiology
• Larva/physiology
• Mosquito Control/methods
• Pest Control, Biological/methods
• Survival Analysis

Collapse

Grants Collapse

Affiliation(s)

Irene Ketseoglou School of Molecular and Cell Biology, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa
Gustav Bouwer School of Molecular and Cell Biology, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa

Collapse

The biological control of the malaria vector

The call for malaria control, over the last century, marked a new epoch in the history of this disease. Many control strategies targeting either the Plasmodium parasite or the Anopheles vector were shown to be effective. Yet, the emergence of drug resistant parasites and insecticide resistant mosquito strains, along with numerous health, environmental, and ecological side effects of many chemical agents, highlighted the need to develop alternative tools that either complement or substitute conventional malaria control approaches. The use of biological means is considered a fundamental part of the recently launched malaria eradication program and has so far shown promising results, although this approach is still in its infancy. This review presents an overview of the most promising biological control tools for malaria eradication, namely fungi, bacteria, larvivorous fish, parasites, viruses and nematodes.

Decreased toxicity of Bacillus thuringiensis subsp. israelensis to mosquito larvae after contact with leaf litter

Bacillus thuringiensis subsp. israelensis is a bacterium producing crystals containing Cry and Cyt proteins, which are toxic for mosquito larvae. Nothing is known about the interaction between crystal toxins and decaying leaf litter, which is a major component of several mosquito breeding sites and represents an important food source. In the present work, we investigated the behavior of B. thuringiensis subsp. israelensis toxic crystals sprayed on leaf litter. In the presence of leaf litter, a 60% decrease in the amount of Cyt toxin detectable by immunology (enzyme-linked immunosorbent assays [ELISAs]) was observed, while the respective proportions of Cry toxins were not affected. The toxicity of Cry toxins toward Aedes aegypti larvae was not affected by leaf litter, while the synergistic effect of Cyt toxins on all B. thuringiensis subsp. israelensis Cry toxins was decreased by about 20% when mixed with leaf litter. The toxicity of two commercial B. thuringiensis subsp. israelensis strains (VectoBac WG and VectoBac 12AS) and a laboratory-produced B. thuringiensis subsp. israelensis strain decreased by about 70% when mixed with leaf litter. Taken together, these results suggest that Cyt toxins interact with leaf litter, resulting in a decreased toxicity of B. thuringiensis subsp. israelensis in litter-rich environments and thereby dramatically reducing the efficiency of mosquitocidal treatments.

Development of an efficient expression system for Flavobacterium strains

Strong promoters were isolated from Flavobacterium johnsoniae in a promoter-trap vector incorporating a gfp reporter system, and were used to express fluorescent protein markers (including GFP, YFP, mOrange and mStrawberry) and insecticidal protein genes in Flavobacterium strains. Sequence analysis of trapped DNA fragments showed conserved Bacteroidetes promoter motifs (TTG-N(19)-TAnnTTTG) located upstream of putative open reading frames. Plasmids harboring these genomic DNA fragments from F. johnsoniae promoted strong production of fluorescent proteins in Flavobacterium hibernum but not in Escherichiacoli. The most potent promoter (PompA) identified in this work was cloned upstream of genes encoding fluorescent proteins, and these were co-expressed in Flavobacterium strains. The p42 and p51 genes (binary toxins from Bacillus sphaericus) when translationally fused to the 3'-end of gfp showed strong expression. Flavobacteria expressing these genes exhibited toxicity against larvae of the mosquitoes Culex quinquefasciatus, Anopheles gambiae, and Ochlerotatus triseriatus. However, transformants with the transcriptional fusion construct between cry11A with p20 from Bacillus thuringiensis did not express Cry11A protein indicating that constitutive expression of cry11A may be problematic in Flavobacterium.

Evolution of resistance to the Bacillus sphaericus Bin toxin is phenotypically masked by combination with the mosquitocidal proteins of Bacillus thuringiensis subspecies israelensis

Two insecticidal bacteria are used as larvicides to control larvae of nuisance and vector mosquitoes in many countries, Bacillus thuringiensis ssp. israelensis and B. sphaericus. Field studies show both are effective, but serious resistance, as high as 50 000-fold, has evolved where B. sphaericus is used against Culex mosquitoes. To improve efficacy and deal with even greater potential problems of resistance, we previously developed several recombinant larvicidal bacteria that combine the best mosquitocidal proteins of these bacteria. In the present study, we report laboratory selection studies using our best recombinant strain against larvae of Culex quinquefasciatus. This recombinant, Bti/BsBin, is a strain of B. thuringiensis ssp. israelensis engineered to produce a large amount of the B. sphaericus binary (Bin) toxin, which makes it more than 10-fold as mosquitocidal as the its parental strains. Here we show that larvae exposed to Bti/BsBin failed to develop significant resistance after 30 successive generations of heavy selection pressure. The highest level of resistance obtained at the LC(95) level was 5.2-fold, but declined to less than two-fold at the 35th generation. Testing the selected populations against B. sphaericus alone showed resistance to Bin evolved, but was masked by combination with B. thuringiensis ssp. israelensis. These results suggest that recombinant bacterial strains have improved mosquito and vector management properties compared with the wild-type strains used in current commercial formulations, and should prove useful in controlling important human diseases such as malaria and filariasis on a long-term basis, even when used intensively under field conditions.

Anopheles gambiae alkaline phosphatase is a functional receptor of Bacillus thuringiensis jegathesan Cry11Ba toxin

Alkaline phosphatases (ALPs, EC 3.1.3.1) isolated from lepidopteran and dipteran species are identified as receptors for Cry1Ac and Cry11Aa toxins, respectively [Jurat-Fuentes, J. L., and Adang, M. J. (2004) Eur. J. Biochem. 7, 3127-3135; Fernandez, L. E., et al. (2006) Biochem. J. 396, 77-84]. In our study, an alkaline phosphatase cDNA (AgALP1) was cloned from the midgut of Anopheles gambiae larvae. The encoded 63 kDa protein has a predicted glycosylphosphatidylinositol (GPI) anchor omega-site ((526)Asp), an N-glycosylation site ((239)Asn-Leu-Thr), and an O-glycosylation site ((312)Ser). AgALP1(t) was expressed in Escherichia coli and used to prepare antiserum and to analyze the interaction of AgALP with mosquitocidal Cry11Ba toxin. Anti-AgALP serum localized AgALP to the apical brush border in the anterior and posterior midgut of larvae and detected a 65 kDa species on a blot of brush border membrane vesicles (BBMVs) protein prepared from larvae. ALP activity was released from larval BBMVs prepared by phosphatidylinositol-specific phospholipase C (PIPLC) treatment, and after separation by two-dimensional gel electrophoresis and blotting, a chain of doublet spots at 65 kDa was detected by anti-AgALP. A subset of these doublet spots bound Cry11Ba on a reprobed blot. Heterologously expressed AgALP1(t) bound [(125)I]Cry11Ba on dot blots and reduced the level of binding of [(125)I]Cry11Ba to brush border membrane vesicles by 41%, a percentage comparable to that of unlabeled Cry11Ba and aminopeptidase AgAPN2(t1) peptide. AgALP1(t) binds Cry11Ba toxin with a high affinity (23.9 nM) and shares a binding site on Cry11Ba with AgAPN2(t1). In bioassays against An. gambiae larvae, the presence of AgALP1(t) reduced larval mortality from 78 to 8%. We conclude that AgALP1 is a binding protein and a functional receptor for Cry11Ba toxin.

Cadherin fragments from Anopheles gambiae synergize Bacillus thuringiensis Cry4Ba's toxicity against Aedes aegypti larvae

A peptide from cadherin AgCad1 of Anopheles gambiae larvae was reported as a synergist of Bacillus thuringiensis subsp. israelensis Cry4Ba's toxicity to the Anopheles mosquito (G. Hua, R. Zhang, M. A. Abdullah, and M. J. Adang, Biochemistry 47:5101-5110, 2008). We report that CR11 to the membrane proximal extracellular domain (MPED) (CR11-MPED) and a longer peptide, CR9 to CR11 (CR9-11), from AgCad1 act as synergists of Cry4Ba's toxicity to Aedes aegypti larvae, but a Diabrotica virgifera virgifera cadherin-based synergist of Cry3 (Y. Park, M. A. F. Abdullah, M. D. Taylor, K. Rahman, and M. J. Adang, Appl. Environ. Microbiol. 75:3086-3092, 2009) did not affect Cry4Ba's toxicity. Peptides CR9-11 and CR11-MPED bound Cry4Ba with high affinity (13 nM and 23 nM, respectively) and inhibited Cry4Ba binding to the larval A. aegypti brush border membrane. The longer CR9-11 fragment was more potent than CR11-MPED in enhancing Cry4Ba against A. aegypti.

Purification and characterization of mosquitocidal Bacillus sphaericus BinA protein

Certain strains of Bacillus sphaericus produce a highly toxic mosquito-larvicidal binary toxin during sporulation. The binary toxin is composed of toxic BinA (41.9kDa) and receptor binding BinB (51.4kDa) polypeptides and is active against vectors of filariasis, encephalitis and malaria. The toxin has been tested with limited use for the control of vector mosquitoes for more than two decades. The binA gene from a local ISPC-8 strain of B. sphaericus that is highly toxic to Culex and Anopheles mosquito species was cloned into pET16b and expressed in Escherichia coli. The purified BinA protein differs by one amino acid (R197M) from BinA of the highest toxicity strains 1593/2362/C3-41. Majority of the expressed protein was observed in inclusion bodies. BinA inclusions alone from E. coli did not show toxic activity, like reported previously. However, the active form of BinA could be purified to homogeneity from the soluble fraction of E. coli cell lysate, grown at reduced temperature after isopropyl beta-d-thiogalactopyranoside induction. The purified BinA protein with and without poly-histidine tag showed LC(50) dose of 82.3 and 66.9ngml(-1), respectively, at 48h against Culex quinquefasciatus larvae. The secondary structure of BinA is expected to be mainly beta strands as estimated using far-UV circular dichroism. The estimates matched well with the secondary structure predictions using amino acid sequence. This is the first report of large-scale purification and accurate toxicity estimation of soluble B. sphaericus BinA. This can help in design and synthesis of improved bacterial insecticide.