Expression of cryIVA and cryIVB Genes, Independently or in Combination, in a Crystal-Negative Strain of Bacillus thuringiensis subsp. israelensis

Factors Related to Bacillus thuringiensis and Gut Physiology. Comment on Rajan, V. An Alkaline Foregut Protects Herbivores from Latex in Forage, but Increases Their Susceptibility to Bt Endotoxin. Life 2023, 13, 2195

A recent article has proposed that alkaline guts may lead to a general susceptibility to the biological control agent Bacillus thuringiensis and the pesticidal proteins derived from it. An analysis of the literature presented here clarifies our knowledge on the activity and safety of these agents, indicating that alkaline guts are not determinant of sensitivity and that the generalized conclusions proposed in the previous article cannot be substantiated.

New native Bacillus thuringiensis strains induce high insecticidal action against Culex pipiens pallens larvae and adults

Mosquitoes of many species are key disease vectors, killing millions of people each year. Bacillus thuringiensis-based insecticide formulations are largely recognized as among the most effective, ecologically safe, and long-lasting methods of managing insect pests. New B. thuringiensis strains with high mosquito control effectiveness were isolated, identified, genetically defined, and physiologically characterized. Eight B. thuringiensis strains were identified and shown to carry endotoxin-producing genes. Using a scanning electron microscope, results revealed typical crystal forms of various shapes in B. thuringiensis strains. Fourteen cry and cyt genes were found in the strains examined. Although the genome of the B. thuringiensis A4 strain had twelve cry and cyt genes, not all of them were expressed, and only a few protein profiles were observed. The larvicidal activity of the eight B. thuringiensis strains was found to be positive (LC50: 1.4-28.5 g/ml and LC95: 15.3-130.3 g/ml). Bioassays in a laboratory environment demonstrated that preparations containing B. thuringiensis spores and crystals were particularly active to mosquito larvae and adults. These new findings show that the novel preparation containing B. thuringiensis A4 spores and crystals mixture might be used to control larval and adult mosquitoes in a sustainable and ecologically friendly manner.

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.

Development of a cypovirus protein microcrystal-encapsulated Bacillus thuringiensis UV-tolerant and mosquitocidal δ-endotoxin

The δ-endotoxin Cry4Aa from Bacillus thuringiensis israelensis (Bti) has insecticidal characteristics specific to insects of the order Diptera. Although Cry4Aa has shown potential as an effective proteinaceous pesticide against mosquitoes, it has an ultraviolet (UV)-intolerant property that limits its outdoor use. Our previous research showed that protein microcrystal polyhedra from Bombyx mori cypovirus can encapsulate diverse foreign proteins and maintain long-term protein activity under hostile environmental conditions, including UV irradiation. In this study, we report the development of polyhedra encapsulating the Cry4Aa insecticidal activity domain by using a modified baculovirus expression system. We confirmed the oral intake of recombinant polyhedra introduced into the experimental environment by the larvae of a mosquito, Aedes albopictus, and delivery of encapsulated proteins into the digestive tract. The polyhedra encapsulating partial Cry4Aa showed mosquito larvicidal activity during incubation of larvae with 50% lethal-dose value of 23.717×10⁴ cubes for 10 Aedes albopictus larvae in 1 ml water. In addition, polyhedra showed a specific property to reduce the impact of UV-C irradiation on the activity of encapsulated partial Cry4Aa, thus demonstrating the effectiveness of encapsulating Bti δ-endotoxins inside polyhedra to increase the availability of proteinaceous pesticides for outdoor use for mosquito control.

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.

TOXiTAXi: a web resource for toxicity of Bacillus thuringiensis protein compositions towards species of various taxonomic groups

Bioinsecticides consisting of different sets of Bacillus thuringiensis (Bt) Cry, Cyt and Vip toxins are broadly used in pest control. Possible interactions (synergistic, additive or antagonistic) between these proteins can not only influence the overall efficacy of certain Bt-based bioinsecticide, but also raise questions regarding environmental safety. Here, we assemble, summarize and analyze the outcomes of experiments published over 30 years, investigating combinatorial effects among Bt Cry, Cyt and Vip toxins. We collected the results on 118 various two-to-five-component combinations that have been bioassayed against 38 invertebrate species. Synergism, additive effect and antagonism was indicated in 54%, 32% and 14% of experiments, respectively. Synergism was noted most frequently for Cry/Cyt combinations, followed by Cyt/Vip and Cry/Cry. In Cry/Vip combinations, antagonism is more frequent and higher in magnitude compared to other categories. Despite a significant number of tested Bt toxin combinations, most of them have been bioassayed only against one pest species. To aid the research on Bt pesticidal protein activity, we present TOXiTAXi ( http://www.combio.pl/toxitaxi/ ), a universal database and a dedicated web tool to conveniently gather and analyze the existing and future bioassay results on biocidal activity of toxins against various taxonomic groups.

Potential of Cry10Aa and Cyt2Ba, Two Minority δ-endotoxins Produced by Bacillus thuringiensis ser. israelensis, for the Control of Aedes aegypti Larvae

Bacillus thuringiensis ser. israelensis (Bti) has been widely used as microbial larvicide for the control of many species of mosquitoes and blackflies. The larvicidal activity of Bti resides in Cry and Cyt δ-endotoxins present in the parasporal crystal of this pathogen. The insecticidal activity of the crystal is higher than the activities of the individual toxins, which is likely due to synergistic interactions among the crystal component proteins, particularly those involving Cyt1Aa. In the present study, Cry10Aa and Cyt2Ba were cloned from the commercial larvicide VectoBac-12AS^® and expressed in the acrystalliferous Bt strain BMB171 under the cyt1Aa strong promoter of the pSTAB vector. The LC₅₀ values for Aedes aegypti second instar larvae estimated at 24 hpi for these two recombinant proteins (Cry10Aa and Cyt2Ba) were 299.62 and 279.37 ng/mL, respectively. Remarkable synergistic mosquitocidal activity was observed between Cry10Aa and Cyt2Ba (synergistic potentiation of 68.6-fold) when spore + crystal preparations, comprising a mixture of both recombinant strains in equal relative concentrations, were ingested by A. aegypti larvae. This synergistic activity is among the most powerful described so far with Bt toxins and is comparable to that reported for Cyt1A when interacting with Cry4Aa, Cry4Ba or Cry11Aa. Synergistic mosquitocidal activity was also observed between the recombinant proteins Cyt2Ba and Cry4Aa, but in this case, the synergistic potentiation was 4.6-fold. In conclusion, although Cry10Aa and Cyt2Ba are rarely detectable or appear as minor components in the crystals of Bti strains, they represent toxicity factors with a high potential for the control of mosquito populations.

Insecticide susceptibility of Aedes albopictus and Ae. aegypti from Brazil and the Swiss-Italian border region

BACKGROUND

Aedes aegypti and Ae. albopictus are two highly invasive mosquito species, both vectors of several viruses, including dengue, chikungunya and Zika. While Ae. aegypti is the primary vector in the tropics and sub-tropics, Ae. albopictus is increasingly under the public health watch as it has been implicated in arbovirus-transmission in more temperate regions, including continental Europe. Vector control using insecticides is the pillar of most control programmes; hence development of insecticide resistance is of great concern. As part of a Brazilian-Swiss Joint Research Programme we set out to assess whether there are any signs of existing or incipient insecticide resistance primarily against the larvicide Bacillus thuringiensis svar. israelensis (Bti), but also against currently applied and potentially alternative insecticides in our areas, Recife (Brazil) and the Swiss-Italian border region.

METHODS

Following World Health Organization guidelines, dose-response curves for a range of insecticides were established for both colonized and field caught Ae. aegypti and Ae. albopictus. The larvicides included Bti, two of its toxins, Cry11Aa and Cry4Ba, Lysinibacillus sphaericus, Vectomax CG®, a formulated combination of Bti and L. sphaericus, and diflubenzuron. In addition to the larvicides, the Swiss-Italian Ae. albopictus populations were also tested against five adulticides (bendiocarb, dichlorodiphenyltrichloroethane, malathion, permethrin and λ-cyhalothrin).

RESULTS

Showing a similar dose-response, all mosquito populations were fully susceptible to the larvicides tested and, in particular, to Bti which is currently used both in Brazil and Switzerland. In addition, there were no signs of incipient resistance against Bti as larvae were equally susceptible to the individual toxins, Cry11Aa and Cry4Ba. The field-caught Swiss-Italian populations were susceptible to the adulticides tested but DDT mortality rates showed signs of reduced susceptibility.

CONCLUSIONS

The insecticides currently used for mosquito control in Switzerland and Brazil are still effective against the target populations. The present study provides an important reference as relatively few insecticide susceptibility surveys have been carried out with Ae. albopictus.

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.

Toward mosquito control with a green alga: Expression of Cry toxins of Bacillus thuringiensis subsp. israelensis (Bti) in the chloroplast of Chlamydomonas

We are developing Chlamydomonas strains that can be used for safe and sustainable control of mosquitoes, because they produce proteins from Bacillus thuringiensis subsp. israelensis (Bti) in the chloroplast. Chlamydomonas has a number of advantages for this approach, including genetic controls that are not generally available with industrial algae. The Bti toxin has been used for mosquito control for > 30 years and does not engender resistance; it contains three Cry proteins, Cry4Aa (135 kDa), Cry4Ba (128 kDa) and Cry11Aa (72 kDa), and Cyt1Aa (25 kDa). To express the Cry proteins in the chloroplast, the three genes were resynthesized and cry4Aa was truncated to the first 700 amino acids (cry4Aa₇₀₀ ); also, since they can be toxic to host cells, the inducible Cyc6:Nac2-psbD expression system was used. Western blots of total protein from the chloroplast transformants showed accumulation of the intact polypeptides, and the relative expression level was Cry11Aa > Cry4Aa₇₀₀ > Cry4Ba. Quantitative western blots with purified Cry11Aa as a standard showed that Cry11Aa accumulated to 0.35% of total cell protein. Live cell bioassays in dH₂0 demonstrated toxicity of the cry4Aa₇₀₀ and cry11Aa transformants to larvae of Aedes aegypti and Culex quinquefasciatus. These results demonstrate that the Cry proteins that are most toxic to Aedes and Culex mosquitoes, Cry4Aa and Cry11Aa, can be successfully expressed in the chloroplast of Chlamydomonas.

Towards novel Cry toxins with enhanced toxicity/broader: a new chimeric Cry4Ba / Cry1Ac toxin

Attempts have been made to express or to merge different Cry proteins in order to enhance toxic effects against various insects. Cry1A proteins of Bacillus thuringiensis form a typical bipyramidal parasporal crystal and their protoxins contain a highly conserved C-terminal region. A chimerical gene, called cry(4Ba-1Ac), formed by a fusion of the N-terminus part of cry4Ba and the C-terminus part of cry1Ac, was constructed. Its transformation to an acrystalliferous B. thuringiensis strain showed that it was expressed as a chimerical protein of 116 kDa, assembled in spherical to amorphous parasporal crystals. The chimerical gene cry(4Ba-1Ac) was introduced in a B. thuringiensis kurstaki strain. In the generated crystals of the recombinant strain, the presence of Cry(4Ba-1Ac) was evidenced by MALDI-TOF. The recombinant strain showed an important increase of the toxicity against Culex pipiens larvae (LC₅₀ = 0.84 mg l^-1 ± 0.08) compared to the wild type strain through the synergistic activity of Cry2Aa with Cry(4Ba-1Ac). The enhancement of toxicity of B. thuringiensis kurstaki expressing Cry(4Ba-1Ac) compared to that expressing the native toxin Cry4Ba, might be related to its a typical crystallization properties. The developed fusion protein could serve as a potent toxin against different pests of mosquitoes and major crop plants.

Pre-selecting resistance against individual Bti Cry toxins facilitates the development of resistance to the Bti toxins cocktail

The bioinsecticide Bacillus thuringiensis subsp. israelensis is a larvicide used worldwide for mosquito control, which contains three Cry toxins and one Cyt toxin. We investigated for the first time in Aedes aegypti (1) the evolution of resistance and cross-resistance of strains selected with each Cry toxin, and (2) the effect of pre-selection with Cry toxin on the evolution of resistance to a mix of Bti toxins. Cross resistance was higher between Cry4Ba and Cry11Aa than between Cry4Aa and either Cry4Ba or Cry11Aa, suggesting both common and specific mechanisms of resistance. Pre-selecting resistance to each Cry toxins facilitated the development of resistance to the full Bti toxins cocktail.

Bacillus thuringiensis subsp. israelensis and its dipteran-specific toxins

Bacillus thuringiensis subsp. israelensis (Bti) is the first Bacillus thuringiensis to be found and used as an effective biological control agent against larvae of many mosquito and black fly species around the world. Its larvicidal activity resides in four major (of 134, 128, 72 and 27 kDa) and at least two minor (of 78 and 29 kDa) polypeptides encoded respectively by cry4Aa, cry4Ba, cry11Aa, cyt1Aa, cry10Aa and cyt2Ba, all mapped on the 128 kb plasmid known as pBtoxis. These six δ-endotoxins form a complex parasporal crystalline body with remarkably high, specific and different toxicities to Aedes, Culex and Anopheles larvae. Cry toxins are composed of three domains (perforating domain I and receptor binding II and III) and create cation-selective channels, whereas Cyts are composed of one domain that acts as well as a detergent-like membrane perforator. Despite the low toxicities of Cyt1Aa and Cyt2Ba alone against exposed larvae, they are highly synergistic with the Cry toxins and hence their combinations prevent emergence of resistance in the targets. The lack of significant levels of resistance in field mosquito populations treated for decades with Bti-bioinsecticide suggests that this bacterium will be an effective biocontrol agent for years to come.

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 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.

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Key Words

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

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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

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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

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Fate of Bacillus thuringiensis subsp. israelensis in the field: evidence for spore recycling and differential persistence of toxins in leaf litter

Bacillus thuringiensis subsp. israelensis is a bioinsecticide increasingly used worldwide for mosquito control. Despite its apparent low level of persistence in the field due to the rapid loss of its insecticidal activity, an increasing number of studies suggested that the recycling of B. thuringiensis subsp. israelensis can occur under specific, unknown conditions. Decaying leaf litters sampled in mosquito breeding sites in the French Rhône-Alpes region several months after a treatment were shown to exhibit a high level of larval toxicity and contained large amounts of spores. In the present article, we show that the high concentration of toxins found in these litters is consistent with spore recycling in the field, which gave rise to the production of new crystal toxins. Furthermore, in these toxic leaf litter samples, Cry4Aa and Cry4Ba toxins became the major toxins instead of Cyt1Aa in the commercial mixture. In a microcosm experiment performed in the laboratory, we also demonstrated that the toxins, when added in their crystal form to nontoxic leaf litter, exhibited patterns of differential persistence consistent with the proportions of toxins observed in the field-collected toxic leaf litter samples (Cry4 > Cry11 > Cyt). These results give strong evidence that B. thuringiensis subsp. israelensis recycled in specific breeding sites containing leaf litters, and one would be justified in asking whether mosquitoes can become resistant when exposed to field-persistent B. thuringiensis subsp. israelensis for several generations.

Aedes aegypti alkaline phosphatase ALP1 is a functional receptor of Bacillus thuringiensis Cry4Ba and Cry11Aa toxins

Bacillus thuringiensis subs. israelensis produces at least three Cry toxins (Cry4Aa, Cry4Ba, and Cry11Aa) that are active against Aedes aegypti larvae. Previous work characterized a GPI-anchored alkaline phosphatase (ALP1) as a Cry11Aa binding molecule from the gut of A. aegypti larvae. We show here that Cry4Ba binds ALP1, and that the binding and toxicity of Cry4Ba mutants located in loop 2 of domain II is correlated. Also, we analyzed the contribution of ALP1 toward the toxicity of Cry4Ba and Cry11Aa toxins by silencing the expression of this protein though RNAi. Efficient silencing of ALP1 was demonstrated by real-time quantitative PCR (qPCR) and Western blot. ALP1 silenced larvae showed tolerance to both Cry4Ba and Cry11Aa although the silenced larvae were more tolerant to Cry11Aa in comparison to Cry4Ba. Our results demonstrate that ALP1 is a functional receptor that plays an important role in the toxicity of the Cry4Ba and Cry11Aa proteins.

Investigation of the Cry4B-prohibitin interaction in Aedes aegypti cells

Bacillus thuringiensis (Bt) produces insecticidal toxins active against insects. Cry4B, one of the major insecticidal toxins produced by Bt subsp. israelensis, is highly toxic to mosquitoes in the genus Aedes: the major vectors of dengue, yellow fever, and chikungunya. Previous work has shown that Cry4B binds to several mid-gut membrane proteins in Aedes aegypti larvae including prohibitin, a protein recently identified as a receptor that also mediates entry of dengue virus into Aedes cells. This study confirms the interaction between Cry4B and prohibitin by co-immunoprecipitation analysis and demonstrates colocalization of prohibitin and Cry4B by confocal microscopy. While activated Cry4B toxin showed high larvicidal activity, it was not cytotoxic to two Aedes cell lines, allowing determination of its effect on dengue virus infectivity in the absence of Cry4B-induced cell lysis. Pre-exposure of Aedes cells to Cry4B resulted in a significant reduction in the number of infected cells compared to untreated cells.

Inheritance, stability, and dominance of cry resistance in Culex quinquefasciatus (Diptera: Culicidae) selected with the three cry toxins of Bacillus thuringiensis subsp. israelensis

Mendelian crosses were used to study the mode of inheritance of Cry toxin resistance in a Culex quinquefasciatus Say (Diptera: Culicidae) colony (CqAB11A) that evolved insecticide resistance under laboratory selection with a deletion mutant of Bacillus thuringiensis subsp. israelensis de Barjac lacking the Cyt1Aa toxin component but containing its three major Cry toxins, Cry4Aa, Cry4Ba, and Cry11Aa. High levels of resistance were observed to Cry toxins. F1 offspring of reciprocal crosses to a sensitive colony showed intermediate levels of resistance with no maternal effect, indicating autosomal inheritance. Dose-response data for backcross offspring deviated significantly from the monofactorial model when tested with Cry4Aa + Cry4Ba + Cry11Aa, Cry4Aa + Cry4Ba, or Cry11Aa. However, tests with Cry11Ba from B. thuringiensis subsp. jegathesan (Seleena, Lee, Lecadet) fit the monofactorial model. Dominance of F1 offspring was calculated at different concentrations of Cry-toxin suspensions and, as reported for other Cry-resistant Culex, generally decreased as concentration increased. A subset of colony CqAB11A was reared without selection pressure for 18 generations with little change in susceptibility, indicating a highly homozygous population. Consistent with reports for other Cry-resistant Culex, the data show these mosquitoes evolved resistance to B. thuringiensis Cry toxins at multiple loci in response to selection pressure and that cross-resistance to Cry11Ba was conferred by one of those loci.

Cadherin binding is not a limiting step for Bacillus thuringiensis subsp. israelensis Cry4Ba toxicity to Aedes aegypti larvae

Bacillus thuringiensis subsp. israelensis produces three Cry toxins (Cry4Aa, Cry4Ba and Cry11Aa) that are active against Aedes aegypti larvae. The identification of the rate-limiting binding steps of Cry toxins that are used for insect control in the field, such as those of B. thuringiensis subsp. israelensis, should provide targets for improving insecticides against important insect pests. Previous studies showed that Cry11Aa binds to cadherin receptor fragment CR7-11 (cadherin repeats 7-11) with high affinity. Binding to cadherin has been proposed to facilitate Cry toxin oligomer formation. In the present study, we show that Cry4Ba binds to CR7-11 with 9-fold lower binding affinity compared with Cry11Aa. Oligomerization assays showed that Cry4Ba is capable of forming oligomers when proteolytically activated in vitro in the absence of the CR7-11 fragment in contrast with Cry11Aa that formed oligomers only in the presence of CR7-11. Pore-formation assays in planar lipid bilayers showed that Cry4Ba oligomers were proficient in opening ion channels. Finally, silencing the cadherin gene by dsRNA (double-stranded RNA) showed that silenced larvae were more tolerant to Cry11Aa in contrast with Cry4Ba, which showed similar toxic levels to those of control larvae. These findings show that cadherin binding is not a limiting step for Cry4Ba toxicity to A. aegypti larvae.

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.

Activity of a Brazilian strain of Bacillus thuringiensis israelensis against the cotton Boll Weevil Anthonomus grandis Boheman (Coleoptera: Tenebrionidae)

A Brazilian Bacillus thuringiensis subspecies israelensis, toxic to Diptera, including mosquitoes, was found also to show toxicity to the coleopteran boll weevil Anthonomus grandis Boheman at an equivalent level to that of the standard coleopteran-active B. thuringiensis subspecies tenebrionis T08017. Recombinant B. thuringiensis strains expressing the individual Cyt1Aa, Cry4Aa, Cry4Ba and Cry11Aa toxins from this strain were assessed to evaluate their potential contribution to the activity against A. grandis, either alone or in combination. Whilst individual toxins produced mortality, none was sufficiently potent to allow calculation of LC50 values. Combinations of toxins were unable to attain the same potency as the parental B. thuringiensis subsp. israelensis, suggesting a major role for other factors produced by this strain.

Fitness costs of resistance to Bti toxins in the dengue vector Aedes aegypti

Sustainable insect vector disease control strategies involve delaying the evolution of resistance to insecticides in natural populations. The evolutionary dynamics of resistance in the field is highly dependent on the fitness cost of resistance alleles. To successfully manage resistance evolution in target species, it is not only important to find evidence of fitness cost in resistant insects, but also to determine at which stage of the insect's life it is expressed. Here, we show that resistance costs to the bacterio-insecticide Bacillus thuringiensis subsp. israelensis (Bti) are expressed at all the life-stages of the dengue vector Aedes aegypti, including egg survival, larval development time, and female fecundity. We show that the storage of eggs for 4 months is long enough to counter-select resistance alleles. This suggests that Bti resistance is not likely to evolve in temperate climates where most mosquito species overwinter as eggs. In tropical regions with a rapid turn-over of generations, resistance alleles are likely to be counter-selected in only few generations without treatment through fitness costs expressed in terms of larval development time and female fecundity. We discuss the implications of our findings in terms of sustainable management strategies in light of the challenge of preserving the long-term efficiency of this environmentally safe anti-mosquito bio-insecticide.

Dominant negative phenotype of Bacillus thuringiensis Cry1Ab, Cry11Aa and Cry4Ba mutants suggest hetero-oligomer formation among different Cry toxins

Background

Bacillus thuringiensis Cry toxins are used worldwide in the control of different insect pests important in agriculture or in human health. The Cry proteins are pore-forming toxins that affect the midgut cell of target insects. It was shown that non-toxic Cry1Ab helix α-4 mutants had a dominant negative (DN) phenotype inhibiting the toxicity of wildtype Cry1Ab when used in equimolar or sub-stoichiometric ratios (1∶1, 0.5∶1, mutant∶wt) indicating that oligomer formation is a key step in toxicity of Cry toxins.

Methodology/Principal Findings

The DN Cry1Ab-D136N/T143D mutant that is able to block toxicity of Cry1Ab toxin, was used to analyze its capacity to block the activity against Manduca sexta larvae of other Cry1 toxins, such as Cry1Aa, Cry1Ac, Cry1Ca, Cry1Da, Cry1Ea and Cry1Fa. Cry1Ab-DN mutant inhibited toxicity of Cry1Aa, Cry1Ac and Cry1Fa. In addition, we isolated mutants in helix α-4 of Cry4Ba and Cry11Aa, and demonstrate that Cry4Ba-E159K and Cry11Aa-V142D are inactive and completely block the toxicity against Aedes aegypti of both wildtype toxins, when used at sub-stoichiometric ratios, confirming a DN phenotype. As controls we analyzed Cry1Ab-R99A or Cry11Aa-E97A mutants that are located in helix α-3 and are affected in toxin oligomerization. These mutants do not show a DN phenotype but were able to block toxicity when used in 10∶1 or 100∶1 ratios (mutant∶wt) probably by competition of binding with toxin receptors.

Conclusions/Significance

We show that DN phenotype can be observed among different Cry toxins suggesting that may interact in vivo forming hetero-oligomers. The DN phenotype cannot be observed in mutants affected in oligomerization, suggesting that this step is important to inhibit toxicity of other toxins.

Multiple receptors as targets of Cry toxins in mosquitoes

Bacillus thuringiensis (Bt) produces inclusions that are composed of proteins known as crystal proteins or Cry toxins. Due to their high specificity and their safety to humans and the environment, these Cry toxins are considered to be valuable alternatives to chemical pesticides in insect control programs. It is believed that Cry toxin-induced membrane pore formation is responsible for insect toxicity. The molecular mechanism of pore formation involves recognition and subsequent binding of the toxin to membrane receptors. This binding is accompanied by toxin oligomerization and transfer of domain I helices of the toxin to the lipid-water interface. This toxin insertion creates pores that lyse the cells. Several receptors from lepidopteran, coleopteran, and dipteran insects have been well characterized. This paper provides an overview of the understanding of the interactions between Cry toxin and multiple receptors in mosquitoes, in particular Aedes aegypti and reviews the manner by which the receptors were identified and characterized, with a focus on three proteins, cadherin, alkaline phosphatase, and aminopeptidase-N.

Persistence of Bacillus thuringiensis israelensis (Bti) in the environment induces resistance to multiple Bti toxins in mosquitoes

BACKGROUND

The simultaneous production of six different toxins by Bacillus thuringiensis israelensis (Bti) is thought to delay the evolution of resistance in treated mosquito populations. Recent studies have shown that Bti can persist and proliferate in the environment, thereby imposing continuous selective pressure on mosquito populations, raising concerns about the long-term effectiveness of this bioinsecticide. In order to evaluate the effect of Bti persistence on the evolution of resistance, the authors selected a laboratory Aedes aegypti L. strain with field-collected leaf litter containing Bti toxins.

RESULTS

It is shown that resistance to each individual Bti toxin (up to 30-fold) can be obtained after only a few generations of selection. However, the resistance to commercial Bti and to environmental Bti remains low (twofold and 3.4-fold respectively) in the selected strain. Furthermore, some selected individuals exhibited resistance to Cry4B but not to Cry4A, suggesting that two distinct resistance mechanisms are involved in the resistance to these two toxins.

CONCLUSION

Considering that resistance to Cry toxins might act as a first step to resistance to a complete Bti toxin mixture, the present results highlight the importance of testing each toxin individually in order accurately to monitor Bti toxin resistance evolution in field populations.

Influence of Exposure to Single versus Multiple Toxins of Bacillus thuringiensis subsp. israelensis on Development of Resistance in the Mosquito Culex quinquefasciatus (Diptera: Culicidae)

The impending widespread use of transgenic crop plants encoding a single insecticidal toxin protein of Bacillus thuringiensis has focused attention on the perceived risk of rapid selection of resistance in target insects. We have used Bacillus thuringiensis subsp. israelensis toxins as a model system and determined the speed and magnitude of evolution of resistance in colonies of the mosquito Culex quinquefasciatus during selection for 28 consecutive generations with single or multiple toxins. The parental strain was synthesized by combining approximately 500 larvae from each of 19 field collections obtained from the states of California, Oregon, Louisiana, and Tennessee. At least 10,000 larvae were selected in each generation of each line at an average mortality level of 84%. The susceptibilities of the parental and selected lines were compared in parallel tests in every third generation by using fresh suspensions of toxin powders. The normal toxin complement of B. thuringiensis subsp. israelensis consists of four toxins, CryIVA, CryIVB, CryIVD, and CytA. Resistance became evident first in the line that was selected with a single toxin (CryIVD), attaining the highest level (resistance ratio [RR], >913 at 95% lethal concentration) by generation F(inf28) when the study was completed. Resistance evolved more slowly and to a lower level (RR, >122 by F(inf25)) in the line selected with two toxins (CryIVA+CryIVB) and lower still (RR, 91 by F(inf28)) in the line selected with three toxins (CryIVA+CryIVB+ CryIVD). Resistance was remarkably low (RR, 3.2) in the line selected with all four toxins. The results reveal the importance of the full complement of toxins found in natural populations of B. thuringiensis subsp. israelensis as an effective approach to resistance management.

Synergistic activity between Bacillus thuringiensis Cry1Ab and Cry1Ac toxins against maize stem borer (Chilo partellus Swinhoe)

AIM

To select a toxin combination for the management of maize stem borer (Chilo partellus) and to understand possible mechanism of synergism among Bacillus thuringiensis Cry1A toxins tested.

METHODS AND RESULTS

Three Cry1A toxins were over expressed in Escherichia coli strain JM105 and used for diet overlay insect bioassay against C. partellus neonate larvae, both alone and in combinations. Probit analysis revealed that the three Cry1A toxins tested have synergistic effect against C. partellus larvae. In vitro binding analysis of fluorescein isothiocyanate (FITC)-labelled Cry1A toxins to midgut brush border membrane vesicle (BBMV) shows that increase in toxicity is directly correlated to an increase in binding of toxin mix.

CONCLUSIONS

A high Cry1Ac to Cry1Ab ratio leads to an increase in efficacy of these toxins towards C. partellus larvae and this increase in toxicity comes from an increase in toxin binding.

SIGNIFICANCE AND IMPACT OF THE STUDY

Use of Cry1Ab and Cry1Ac combination could be an effective approach to control C. partellus. Furthermore, we show it first time that possible reason behind increase in toxicity of synergistic Cry1A proteins is an increase in toxin binding.

Potential effect of a putative sigma(H)-driven promoter on the over expression of the Cry1Ac toxin of Bacillus thuringiensis

Sequence analysis of the upstream region of the cry1Ac gene in the HD-73 strain of B. thuringiensis showed a putative sigma(H)-like promoter. The potential regulating role of this sequence was tested by transforming an acrystaliferous mutant of the HD-73 strain with three different constructs: (1) a construct consisting of the sigma(H)-, sigma(E)- and sigma(K)-like promoters, the 0A box, and the cry1Ac coding sequence (EK0AH); (2) a derivative construct that lacked the sigma(H)-promoter (EK0A); and (3) a second derivative construct that lacked the sigma(H)-promoter and the 0A box (EK). Crystals from the recombinant and the wild-type (Bt HD-73) strains were measured by transmission electron microscopy. Statistically significant differences in crystal size were detected between all the transformed and the wild-type strains, with averages of 1.54, 1.31, 1.05, and 0.95microm for the EK0AH, EK0A, HD-73, and EK constructs, respectively. SDS-PAGE analyses of the EK0AH construct corroborated a higher expression level of the cry1Ac gene than that of the EK0A construct, as well as the lower expression of the EK construct. Interestingly, RT-PCR analyses indicated that the recombinant strain carrying the construct EK0AH started the transcription of the cry gene earlier than the Bt HD-73 strain, as observed when a kinetics study was carried out, which may explain the larger crystals and the higher expression of the construct with the putative sigma(H)-like promoter, along with the vector's high copy number.

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.

An alpha-amylase is a novel receptor for Bacillus thuringiensis ssp. israelensis Cry4Ba and Cry11Aa toxins in the malaria vector mosquito Anopheles albimanus (Diptera: Culicidae)

Bacillus thuringiensis ssp. israelensis (Bti) produces four Cry toxins (Cry4Aa, Cry4Ba, Cry10Aa and Cry11Aa), and two Cyt proteins (Cyt1Aa and Cyt2Ba), toxic to mosquito-larvae of the genus Aedes, Anopheles and Culex, important human disease vectors that transmit dengue virus, malaria and filarial parasites respectively. Previous work showed that Bti is highly toxic to Anopheles albimanus, the main vector for transmission of malaria in Mexico. In this work, we analysed the toxicity of isolated Cry proteins of Bti and identified an An. albimanus midgut protein as a putative Cry4Ba and Cry11Aa receptor molecule. Biossays showed that Cry4Ba and Cry11Aa of Bti are toxic to An. albimanus larvae. Ligand blot assays indicated that a 70 kDa glycosylphosphatidylinositol-anchored protein present in midgut brush border membrane vesicles of An. albimanus interacts with Cry4Ba and Cry11Aa toxins. This protein was identified as an alpha-amylase by mass spectrometry and enzymatic activity assays. The cDNA that codes for the alpha-amylase was cloned by means of 5'- and 3'-RACE experiments. Recombinant alpha-amylase expressed in Escherichia coli specifically binds Cry4Ba and Cry11Aa toxins.

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.

The theoretical three-dimensional structure of Bacillus thuringiensis Cry5Aa and its biological implications

Cry5Aa is a crystal protein produced by Bacillus thuringiensis serovar. damstadiensis during its stationary phase, this delta-endotoxin is active against nematodes and has great potential for nematodes control. The theoretical model of the three-dimensional structure of Cry5Aa was predicted by homology modeling on the structures of the Cry1Aa which is specific to Lepidopteran insects. The structure of the Cry5Aa resembles previously reported Cry toxin structures but shows the following distinctions. Cry5Aa has a long insertion in alpha2 of domain I. Some loops in the domain II and III of Cry5Aa are exposed to the solvent. In this work we give a brief description of our model and hypothesize the residues of the Cry5Aa that could be important in receptor recognition and pore formation. This model will be helpful for the design of mutagenesis experiments aimed to the improvement of toxicity, and lead to a deep understanding of the mechanism of action of nematicidal toxins.

Variations in the mosquito larvicidal activities of toxins from Bacillus thuringiensis ssp. israelensis

Comparing activities of purified toxins from Bacillus thuringiensis ssp. israelensis against larvae of seven mosquito species (vectors of tropical diseases) that belong to three genera, gleaned from the literature, disclosed highly significant variations in the levels of LC(50) as well as in the hierarchy of susceptibilities. Similar toxicity comparisons were performed between nine transgenic Gram-negative species, four of which are cyanobacterial, expressing various combinations of cry genes, cyt1Aa and p20, against larvae of four mosquito species as potential agents for biological control. Reasons for inconsistencies are listed and discussed. Standard conditions for toxin isolation and presentation to larvae are sought. A set of lyophilized powders prepared identically from six Escherichia coli clones expressing combinations of four genes displayed toxicities against larvae of three mosquito species, with levels that differed between them but with identical hierarchy.

The Cry48Aa-Cry49Aa binary toxin from Bacillus sphaericus exhibits highly restricted target specificity

The Cry48Aa/Cry49Aa binary toxin of Bacillus sphaericus was recently discovered by its ability to kill Culex quinquefasciatus mosquito larvae through a novel interaction between its two components. We have investigated the target specificity of this toxin and show it to be non-toxic to coleopteran, lepidopteran and other dipteran insects, including closely related Aedes and Anopheles mosquitoes. This represents an unusually restricted target range for crystal toxins from either B. sphaericus or Bacillus thuringiensis. Gut extracts from Culex and Aedes larvae show differential processing of the Cry48Aa protein, with the location of cleavage sites in Culex reflecting those previously shown for the activation of Cry4 toxins in mosquitoes. Pre-activation of Cry48Aa/Cry49Aa with Culex extracts, however, fails to induce toxicity to Aedes larvae. Co-administration of Cry49Aa with Cry4Aa gives higher than predicted toxicity, perhaps suggesting weak synergism against Culex larvae between Cry49Aa and other three-domain Cry toxins.

Anopheles gambiae cadherin AgCad1 binds the Cry4Ba toxin of Bacillus thuringiensis israelensis and a fragment of AgCad1 synergizes toxicity

A midgut cadherin AgCad1 cDNA was cloned from Anopheles gambiae larvae and analyzed for its possible role as a receptor for the Cry4Ba toxin of Bacillus thuringiensis strain israelensis. The AgCad1 cadherin encodes a putative 1735-residue protein organized into an extracellular region of 11 cadherin repeats (CR) and a membrane-proximal extracellular domain (MPED). AgCad1 mRNA was detected in midgut of larvae by polymerase chain reaction (PCR). The AgCad1 protein was localized, by immunochemistry of sectioned larvae, predominately to the microvilli in posterior midgut. The localization of Cry4Ba binding was determined by the same technique, and toxin bound microvilli in posterior midgut. The AgCad1 protein was present in brush border membrane fractions prepared from larvae, and Cry4Ba toxin bound the same-sized protein on blots of those fractions. The AgCad1 protein was expressed transiently in Drosophila melanogaster Schneider 2 (S2) cells. 125I-Cry4Ba toxin bound AgCad1 from S2 cells in a competitive manner. Cry4Ba bound to beads extracted 200 kDa AgCad1 and a 29 kDa fragment of AgCad1 from S2 cells. A peptide containing the AgCad1 region proximal to the cell (CR11-MPED) was expressed in Escherichia coli. Although Cry4Ba showed limited binding to CR11-MPED, the peptide synergized the toxicity of Cry4Ba to larvae. AgCad1 in the larval brush border is a binding protein for Cry4Ba toxin. On the basis of binding results and CR11-MPED synergism of Cry4Ba toxicity, AgCad1 is probably a Cry4Ba receptor.

Conjugal transfer of a toxin-coding megaplasmid from Bacillus thuringiensis subsp. israelensis to mosquitocidal strains of Bacillus sphaericus

Both Bacillus sphaericus and Bacillus thuringiensis subsp. israelensis produce mosquitocidal toxins during sporulation and are extensively used in the field for control of mosquito populations. All the known toxins of the latter organism are known to be encoded on a large plasmid, pBtoxis. In an attempt to combine the best properties of the two bacteria, an erythromycin resistance-marked pBtoxis plasmid was transferred to B. sphaericus by a mating technique. The resulting transconjugant bacteria were significantly more toxic to Aedes aegypti mosquitoes and were able to overcome resistance to B. sphaericus in a resistant colony of Culex quinquefasciatus, apparently due to the production of Cry11A but not Cry4A or Cry4B. The stability of the plasmid in the B. sphaericus host was moderate during vegetative growth, but segregational instability was observed, which led to substantial rates of plasmid loss during sporulation.

Transfer and expression of the mosquitocidal plasmid pBtoxis in Bacillus cereus group strains

The toxicity of Bacillus thuringiensis subsp. israelensis to dipteran larvae (mosquitoes and black flies) depends on the presence of the pBtoxis plasmid. In this paper, two antibiotic resistance tagged pBtoxis were transferred by conjugation to other Bacillus cereus group strains. Among 15 potential recipients, only a lepidopteran active B. thuringiensis subspecies kurstaki and a B. cereus strain received the plasmid pBtoxis with a low transfer rate of about 10(-8) transconjugants/recipient. The resulting B. thuringiensis subspecies kurstaki transconjugant was active to both lepidopteran and dipteran targets and the B. cereus transconjugant was active against dipteran insects. Phase contrast microscopy showed that the B. cereus transconjugants could produce only round crystalline inclusion bodies while B. thuringiensis subspecies kurstaki transconjugant could produce both round and bipyramidal crystals during sporulation. SDS-PAGE revealed that all the major mosquitocidal proteins from pBtoxis could express in the two transconjugants, including Cry4Aa, Cry4Ba, Cry10Aa, Cry11Aa and Cyt1Aa. However, none of the experiment showed any indications of mobilising abilities of pBtoxis. The limited number of strains, which could receive and maintain pBtoxis using a conjugational helper plasmid, indicates a very narrow host range of the B. thuringiensis subsp. israelensis pBtoxis plasmid.

Association analysis between serotype, cry gene content, and toxicity to Helicoverpa armigera larvae among Bacillus thuringiensis isolates native to Spain

Serotyping, cry gene content, and toxicity to Helicoverpa armigera were determined for 178 isolates of Bacillus thuringiensis native to Spain. A total of 13 different cry1 and cry2 genes were detected when isolates were screened by PCR analysis. Results showed that cry2 and cry1Ia were the most frequent cry genes in the collection (74 and 57%, respectively); whereas cry1D, cry1Aa, cry1Ab, and cry1C were only moderately abundant (49, 48, 47, and 36%, respectively). The most uncommon cry genes were cry1Ac, cry1E, cry1B, cry1Ib, cry1Ad, cry1F, and cry1G, with frequencies of 24, 14, 13, 8, 5, 5, and 1%, respectively. The distribution of some cry genes was somewhat associated with particular serovars. For example, genes cry1C and cry1D were especially frequent in the serovar aizawai, while cry1B was very frequent in the serovar thuringiensis. Bioassays against H. armigera larvae showed a wide variation in the insecticidal potency, even among strains sharing the same set of cry genes and within the same serotype.

Crystal structure of the mosquito-larvicidal toxin Cry4Ba and its biological implications

Cry4Ba, isolated from Bacillus thuringiensis subsp. israelensis, is specifically toxic to the larvae of Aedes and Anopheles mosquitoes. The structure of activated Cry4Ba toxin has been determined by multiple isomorphous replacement with anomalous scattering and refined to R(cryst) = 20.5% and R(free)= 21.8% at 1.75 Angstroms resolution. It resembles previously reported Cry toxin structures but shows the following distinctions. In domain I the helix bundle contains only the long and amphipathic helices alpha3-alpha7. The N-terminal helices alpha1-alpha2b, absent due to proteolysis during crystallisation, appear inessential to toxicity. In domain II the beta-sheet prism presents short apical loops without the beta-ribbon extension of inner strands, thus placing the receptor combining sites close to the sheets. In domain III the beta-sandwich contains a helical extension from the C-terminal strand beta23, which interacts with a beta-hairpin excursion from the edge of the outer sheet. The structure provides a rational explanation of recent mutagenesis and biophysical data on this toxin. Furthermore, added to earlier structures from the Cry toxin family, Cry4Ba completes a minimal structural database covering the Coleoptera, Lepidoptera, Diptera and Lepidoptera/Diptera specificity classes. A multiple structure alignment found that the Diptera-specific Cry4Ba is structurally more closely similar to the Lepidoptera-specific Cry1Aa than the Coleoptera-specific Cry3Aa, but most distantly related to Lepidoptera/Diptera-specific Cry2Aa. The structures are most divergent in domain II, supporting the suggestion that this domain has a major role in specificity determination. They are most similar in the alpha3-alpha7 major fragment of domain I, which contains the alpha4-alpha5 hairpin crucial to pore formation. The collective knowledge of Cry toxin structure and mutagenesis data will lead to a more critical understanding of the structural basis for receptor binding and pore formation, as well as allowing the scope of diversity to be better appreciated.

Synergy between toxins of Bacillus thuringiensis subsp. israelensis and Bacillus sphaericus

Synergistic interactions among the multiple endotoxins of Bacillus thuringiensis subsp. israelensis de Barjac play an important role in its high toxicity to mosquito larvae and the absence of insecticide resistance in populations treated with this bacterium. A lack of toxin complexity and synergism are the apparent causes of resistance to Bacillus sphaericus Neide in particular Culex field populations. To identify endotoxin combinations of the two Bacillus species that might improve insecticidal activity and manage mosquito resistance to B. sphaericus, we tested their toxins alone and in combination. Most combinations of B. sphaericus and B. t. subsp. israelensis toxins were synergistic and enhanced toxicity relative to B. sphaericus, particularly against Culex quinquefasciatus Say larvae resistant to B. sphaericus and Aedes aegypti (L.), a species poorly susceptible to B. sphaericus. Toxicity also improved against susceptible Cx. quinquefasciatus. For example, when the CytlAa toxin from B. t. subsp. israelensis was added to Bin and Cry toxins, or when native B. t. subsp. israelensis was combined with B. sphaericus, synergism values as high as 883-fold were observed and combinations were 4-59,000-fold more active than B. sphaericus. These data, and previous studies using cytolytic toxins, validate proposed strategies for improving bacterial larvicides by combining B. sphaericus with B. t. subsp. israelensis or by engineering recombinant bacteria that express endotoxins from both strains. These combinations increase both endotoxin complexity and synergistic interactions and thereby enhance activity and help avoid insecticide resistance.

Laboratory selection for resistance to Bacillus thuringiensis subsp. jegathesan or a component toxin, Cry11B, in Culex quinquefasciatus (Diptera: Culicidae)

The bacteria Bacillus thuringiensis subsp. israelensis and Bacillus sphaericus produce insecticidal toxins used to control mosquito larvae throughout the world. Unfortunately, there are few alternative insecticides with similar activity and environmental safety, which may limit the long-term success of these insecticides. Bacillus thuringiensis subsp. jegathesan is another bacterium with toxins that are active against mosquitoes and has potential for development as a commercial product. B. t. subsp. jegathesan would be ineffective if cross-resistance was detected or if treated mosquito populations evolved resistance. B. t. subsp. jegathesan was evaluated for its potential for selecting insecticide resistance in Culex quinquefasciatus Say. Susceptibility changes in mosquitoes selected with the wild-type strain were compared with susceptibility changes in mosquitoes selected with Cry11B, a component toxin of B. t. subsp. jegathesan. Resistance was detected in generation 18 in the Cry11B-selected colony, reached a maximum of 38-fold, and was present through generation 40. The B. t. subsp. jegathesan-selected colony evolved 13-fold resistance in generation 22, but resistance declined to 2.3-fold in generation 26 and remained low throughout the study. Cry11B-selected mosquitoes showed no significant resistance to the wild-type bacterium, whereas B. t. subsp. jegathesan-selected mosquitoes expressed significant resistance to Cry11B. Both colonies displayed cross-resistance to component toxins of B. t. subsp. israelensis, but they lacked cross-resistance to that wild-type strain. The patterns of resistance and cross-resistance in this study are consistent with the patterns previously observed in mosquitoes selected with B. t. subsp. israelensis and suggest that B. t. subsp. jegathesan might also be at low risk for resistance.

Introduction of Culex toxicity into Bacillus thuringiensis Cry4Ba by protein engineering

Bacillus thuringiensis mosquitocidal toxin Cry4Ba has no significant natural activity against Culex quinquefasciatus or Culex pipiens (50% lethal concentrations [LC(50)], >80,000 and >20,000 ng/ml, respectively). We introduced amino acid substitutions in three putative loops of domain II of Cry4Ba. The mutant proteins were tested on four different species of mosquitoes, Aedes aegypti, Anopheles quadrimaculatus, C. quinquefasciatus, and C. pipiens. Putative loop 1 and 2 exchanges eliminated activity towards A. aegypti and A. quadrimaculatus. Mutations in a putative loop 3 resulted in a final increase in toxicity of >700-fold and >285-fold against C. quinquefasciatus (LC(50) congruent with 114 ng/ml) and C. pipiens (LC(50) 37 ng/ml), respectively. The enhanced protein (mutein) has very little negative effect on the activity against Anopheles or AEDES: These results suggest that the introduction of short variable sequences of the loop regions from one toxin into another might provide a general rational design approach to enhancing B. thuringiensis Cry toxins.

Binding properties of Bacillus thuringiensis Cry1C delta-endotoxin to the midgut epithelial membranes of Culex pipiens

The Cry1C delta-endotoxin from Bacillus thuringiensis is toxic to both lepidopteran and dipteran insect larvae. To analyze the dipteran-specific insecticidal mechanisms, we investigated the properties of Cry1C binding to the epithelial cell membrane of the larval midgut from the mosquito Culex pipiens in comparison with dipteran-specific Cry4A. Immunohistochemical staining of the larval midgut sections from Culex pipiens showed that Cry1C and Cry4A bound to the microvilli of the epithelial cells. The Cry1C binding to brush border membrane vesicles from the mosquito larvae was specific and irreversible, and did not compete with Cry4A. By ligand blotting analyses, we detected several Cry1C-binding proteins, the Cry1C binding to which did compete with excess unlabeled Cry4A. These results suggested that Cry1C and Cry4A recognized the same binding site(s) on the epithelial cell surface but that their interaction with the target membrane differed.

Loss of catabolite repression function of HPr, the phosphocarrier protein of the bacterial phosphotransferase system, affects expression of the cry4A toxin gene in Bacillus thuringiensis subsp. israelensis

HPr, the phosphocarrier protein of the bacterial phosphotransferase system, mediates catabolite repression of a number of operons in gram-positive bacteria. In order to participate in the regulatory process, HPr is activated by phosphorylation of a conserved serine-46 residue. To study the potential role of HPr in the regulation of Cry4A protoxin synthesis in Bacillus thuringiensis subsp. israelensis, we produced a catabolite repression-negative mutant by replacing the wild-type copy of the ptsH gene with a mutated copy in which the conserved serine residue of HPr was replaced with an alanine. HPr isolated from the mutant strain was not phosphorylated at Ser-45 by HPr kinase, but phosphorylation at His-14 was found to occur normally. The enzyme I and HPr kinase activities of the mutant were not affected. Analysis of the B. thuringiensis subsp. israelensis mutant harboring ptsH-S45A in the chromosome showed that cry4A expression was derepressed from the inhibitory effect of glucose. The mutant strain produced both cry4A and sigma(35) gene transcripts 4 h ahead of the parent strain, but there was no effect on sigma(28) synthesis. In wild-type B. thuringiensis subsp. israelensis cells, cry4A mRNA was observed from 12 h onwards, while in the mutant it appeared at 8 h and was produced for a longer period. The total amount of cry4A transcripts produced by the mutant was higher than by the parent strain. There was a 60 to 70% reduction in the sporulation efficiency of the mutant B. thuringiensis subsp. israelensis strain compared to the wild-type strain.