Detection of an allele conferring resistance to Bacillus sphaericus binary toxin in Culex quinquefasciatus populations by molecular screening

Culex quinquefasciatus membrane-bound alkaline phosphatase is a putative receptor for Lysinibacillus sphaericus Tpp49Aa1 toxin

The binary toxin Cry48Aa1/Tpp49Aa1 produced by Lysinibacillus sphaericus exhibits potent toxicity against Culicidae larvae. Both Cry48Aa1 and Tpp49Aa1 toxins are crucial for binding to the toxin receptor in Culex quinquefasciatus larvae, albeit with different binding sites. Previous studies have identified Glu71, a membrane-bound α-glucosidase, as a putative binding protein for the Cry48Aa1 toxin, involved in the Cry48Aa1/Tpp49Aa1 toxicity. In this study, we employed pulldown assays to identify a group of Tpp49Aa1-binding proteins from C. quinquefasciatus solubilized midgut brush-border membrane proteins (BBMFs). RNA interference assays revealed that the silencing of an alkaline phosphatase gene (referred to as ALP1263) in C. quinquefasciatus resulted in a significant reduction in larval mortality upon exposure to Cry48Aa1/Tpp49Aa1 toxin in vivo. Furthermore, the ALP1263 protein exhibited specific and high-affinity binding to the Tpp49Aa1 toxin, with a dissociation constant (Kd) of approximately 57.3 nM. The dot blot analysis demonstrated that Tpp49Aa1 C-terminal region was essential for its interaction with the ALP1263 protein. In summary, our findings establish ALP1263 as a functional receptor for Tpp49Aa1 and emphasize its role in the toxicity of Cry48Aa1/Tpp49Aa1.

Mosquito-larvicidal Binary (BinA/B) proteins for mosquito control programs —advancements, challenges, and possibilities

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Binary (BinAB) toxin is primarily responsible for the larvicidal action of the WHO recognized mosquito-larvicidal bacterium Lysinibacillus sphaericus.

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BinAB is a single receptor-specific toxin, active against larvae of Culex and Anopheles, but not Aedes aegypti.

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The target receptor in Culex is Cqm1 protein, a GPI-anchored amylomaltase located apically in the lipid-rafts of the larval-midgut epithelium.

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Interaction of the toxin components with the receptor is critical for the larvicidal activity of the toxin.

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Evidences support the pore formation model for BinAB toxin internalization and the role of toxin-glycan interactions in the endoplasmic reticulum in mediating larval death.

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Targeted R&D efforts are required to maintain the sustainability and improve efficacy of the eco-friendly BinAB proteins for efficient mosquito control interventions.

The increasing global burden of mosquito-borne diseases require targeted, environmentally friendly, and sustainable approaches for effective vector control without endangering the non-target beneficial insect population. Biological interventions such as biopesticides, Wolbachia-mediated biological controls, or sterile insect techniques are used worldwide. Here we review Binary or BinAB toxin—the mosquito-larvicidal component of WHO-recognized Lysinibacillus sphaericus bacterium employed in mosquito control programs. Binary (BinAB) toxin is primarily responsible for the larvicidal effect of the bacterium. BinAB is a single-receptor-specific toxin and is effective against larvae of Culex and Anopheles, but not against Aedes aegypti. The receptor in Culex, the Cqm1 protein, has been extensively studied. It is a GPI-anchored amylomaltase and is located apically in the lipid rafts of the larval-midgut epithelium. The interaction of the toxin components with the receptor is crucial for the mosquito larvicidal activity of the BinAB toxin. Here we extend support for the pore formation model of BinAB toxin internalization and the role of toxin-glycan interactions in the endoplasmic reticulum in mediating larval death. BinAB is phylogenetically safe for humans, as Cqm1-like protein is not expected in the human proteome. This review aims to initiate targeted R&D efforts, such as applying fusion technologies (chimera of BinA, chemical modification of BinA), for efficient mosquito control interventions. In addition, the review also examines other areas such as bioremediation and cancer therapeutics, in which L. sphaericus is proving useful and showing potential for further development.

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.

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.

Insecticide resistance in Culex quinquefasciatus Say, 1823 in Brazil: a review

Culex quinquefasciatus is a successful invasive species broadly distributed in subtropical regions, including Brazil. It is an extremely annoying mosquito due to its nocturnal biting behavior, in high-density populations and it is a potential bridge between sylvatic arbovirus from birds to man in urban territories. Herein, we present a review concerning the methods of chemical control employed against Cx. quinquefasciatus in Brazil since the 1950's and insecticide resistance data registered in the literature. As there is no specific national programme for Cx. quinquefasciatus control in Brazil, the selection of insecticide resistance is likely due in part to the well-designed chemical campaigns against Aedes aegypti and the elevated employment of insecticides by households and private companies. There are very few publications about insecticide resistance in Cx. quinquefasciatus from Brazil when compared to Ae. aegypti. Nevertheless, resistance to organophosphates, carbamate, DDT, pyrethroids and biolarvicides has been registered in Cx. quinquefasciatus populations from distinct localities of the country. Concerning physiological mechanisms selected for resistance, distinct patterns of esterases, as well as mutations in the acetylcholinesterase (ace-1) and voltage-gated sodium channel (NaV) genes, have been identified in natural populations. Given environmental changes and socioeconomical issues in the cities, in recent years we have been experiencing an increase in the number of disease cases caused by arboviruses, which may involve Cx. quinquefasciatus participation as a key vector. It is urgent to better understand the efficiency and susceptibility status to insecticides, as well as the genetic background of known resistant mechanisms already present in Cx. quinquefasciatus populations for an effective and rapid chemical control when eventually required.

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.

Zika virus replication in the mosquito Culex quinquefasciatus in Brazil

Zika virus (ZIKV) is a flavivirus that has recently been associated with an increased incidence of neonatal microcephaly and other neurological disorders. The virus is primarily transmitted by mosquito bite, although other routes of infection have been implicated in some cases. The Aedes aegypti mosquito is considered to be the main vector to humans worldwide; however, there is evidence that other mosquito species, including Culex quinquefasciatus, transmit the virus. To test the potential of Cx. quinquefasciatus to transmit ZIKV, we experimentally compared the vector competence of laboratory-reared Ae. aegypti and Cx. quinquefasciatus. Interestingly, we were able to detect the presence of ZIKV in the midgut, salivary glands and saliva of artificially fed Cx. quinquefasciatus. In addition, we collected ZIKV-infected Cx. quinquefasciatus from urban areas with high microcephaly incidence in Recife, Brazil. Corroborating our experimental data from artificially fed mosquitoes, ZIKV was isolated from field-caught Cx. quinquefasciatus, and its genome was partially sequenced. Collectively, these findings indicate that there may be a wider range of ZIKV vectors than anticipated.

The LspC3-41I restriction-modification system is the major determinant for genetic manipulations of Lysinibacillus sphaericus C3-41

Background

Lysinibacillus sphaericus has been widely used in integrated mosquito control program and it is one of the minority bacterial species unable to metabolize carbohydrates. In consideration of the high genetic conservation at genomic level and difficulty of genetic horizontal transfer, it is hypothesized that effective restriction-modification (R-M) systems existed in mosquitocidal L. sphaericus.

Results

In this study, six type II R-M systems including LspC3–41I were predicted in L. sphaericus C3–41 genome. It was found that the cell free extracts (CFE) from this strain shown similar restriction and methylation activity on exogenous Bacillus/Escherichia coli shuttle vector pBU4 as the HaeIII, which is an isoschizomer of BspRI. The Bsph_0498 (encoding the predicted LspC3–41IR) knockout mutant Δ0498 and the complement strain RC0498 were constructed. It was found that the unmethylated pBU4 can be digested by the CFE of C3–41 and RC0498, but not by that of Δ0498. Furthermore, the exogenous plasmid pBU4 can be transformed at very high efficacy into Δ0498, low efficacy into RC0498, but no transformation into C3–41, indicating that LspC3–41I might be a major determinant for the genetic restriction barrier of strain C3–41. Besides, lspC3–41IR and lspC3–41IM genes are detected in other two strains besides C3–41 of the tested 16 L. sphaericus strains, which all belonging to serotype H5 and MLST sequence type (ST) 1. Furthermore, the three strains are not horizontal transferred, and this restriction could be overcome by in vitro methylation either by the host CFE or by commercial methytransferase M. HaeIII. The results provide an insight to further study the genetic restriction, modification and evolution of mosquitocidal L. sphaericus, also a theoretical basis and a method for the genetic manipulations of L. sphaericus.

Conclusions

LspC3–41I is identified as the major determinant for the restriction barrier of L. sphaericus C3–41. Only three strains of the tested 16 L. sphaericus strains, which all belonging to serotype H5 and ST1 by MLST scheme, contain LspC3–41I system. Two different methods can be used to overcome the restriction barrier of the three isolates to get transformants efficiently: 1) to methylate plasmid DNA prior to the electroporation; and 2) to delete the major restriction endonuclease encoding gene lspC3–41IR.

A new allele conferring resistance to Lysinibacillus sphaericus is detected in low frequency in Culex quinquefasciatus field populations

Background

The Cqm1 α-glucosidase of Culex quinquefasciatus larvae acts as the midgut receptor for the binary toxin of the biolarvicide Lysinibacillus sphaericus. Mutations within the cqm1 gene can code for aberrant polypeptides that can no longer be properly expressed or bind to the toxin, leading to insect resistance. The cqm1_REC and cqm1_REC-2 alleles were identified in a laboratory selected colony and both displayed mutations that lead to equivalent phenotypes of refractoriness to L. sphaericus. cqm1_REC was first identified as the major resistance allele in this colony but it was subsequently replaced by cqm1_REC-2, suggesting the better adaptive features of the second allele. The major aim of this study was to evaluate the occurrence of cqm1_REC-2 and track its origin in field populations where cqm1_REC was previously identified.

Methods

The screening of the cqm1_REC-2 allele was based on more than 2000 C. quinquefasciatus larvae from five localities in the city of Recife, Brazil and used a multiplex PCR assay that is also able to identify cqm1_REC. Full-length sequencing of the cqm1_REC-2 and selected cqm1 samples was performed to identify further polymorphisms between these alleles.

Results

The cqm1_REC-2 allele was found in field samples, specifically in two heterozygous individuals from a single locality with an overall frequency and distribution much lower than that observed for cqm1_REC. The full-length sequences from these two cqm1_REC-2 copies were almost identical to the cqm1_REC-2 derived from the resistant colony but displayed more than 30 SNPs when compared with cqm1 and cqm1_REC. The cqm1_REC and cqm1_REC-2 resistant alleles were found to be associated with two distinct sets of wild-type cqm1 variants found in field populations.

Conclusions

The cqm1_REC-2 allele occurs in populations in Recife and was probably already present in the samples used to establish the laboratory resistant colony. The data generated indicates that cqm1_REC-2 can be selected in field populations, although its low frequency and distribution in Recife suggest that cqm1_REC-2 presents a lower risk of selection compared to cqm1_REC.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1347-2) contains supplementary material, which is available to authorized users.

Susceptibility status of Culex quinquefasciatus (Diptera: Culicidae) populations to the chemical insecticide temephos in Pernambuco, Brazil

BACKGROUND

Culex quinquefasciatus is the vector of many agents of human diseases, including Wuchereria bancrofti, the parasite that causes bancroftian filariasis, an endemic disease in Pernambuco State, Brazil. Although temephos is not currently used to control C. quinquefasciatus, the species might be under a selection process from incidental exposure to this compound. This study aimed to evaluate the susceptibility status of C. quinquefasciatus to temephos, using bioassays, and to investigate its putative resistance mechanisms through biochemical assays and screening of the G119S mutation in the acetylcholinesterase gene, which is associated with organophosphate resistance, carried out by PCR and sequencing.

RESULTS

The results showed that only mosquitoes from Santa Cruz do Capibaribe (SC) had an alteration in their susceptibility status (RR = 7.2-fold), while the other populations were all susceptible to the insecticide. Biochemical assays showed increased activity for all esterases in SC, as well as evidence of acetylcholinesterase insensitivity. The G119S mutation was detected in this population with a frequency of 0.11, but it was not found in the remaining populations.

CONCLUSION

These data show that mechanisms of temephos resistance have been selected in natural C. quinquefasciatus populations from Pernambuco, which could undermine future control actions.

Novel mutations associated with resistance to Bacillus sphaericus in a polymorphic region of the Culex quinquefasciatus cqm1 gene

Bin toxin from Bacillus sphaericus acts on Culex quinquefasciatus larvae by binding to Cqm1 midgut-bound receptors, and disruption of the cqm1 gene is the major cause of resistance. The goal of this work was to screen for a laboratory-selected resistance cqm1(REC) allele in field populations in the city of Recife, Brazil, and to describe other resistance-associated polymorphisms in the cqm1 gene. The cqm1(REC) allele was detected in the four nontreated populations surveyed at frequencies from 0.001 to 0.017, and sequence analysis from these samples revealed a novel resistant allele (cqm1(REC-D16)) displaying a 16-nucletotide (nt) deletion which is distinct from the 19-nt deletion associated with cqm1(REC). Yet a third resistant allele (cqm1(REC-D25)), displaying a 25-nt deletion, was identified in samples from a treated area exposed to B. sphaericus. A comparison of the three deletion events revealed that all are located within the same 208-nt region amplified during the screening procedure. They also introduce equivalent frameshifts in the sequence and generate the same premature stop codon, leading to putative transcripts encoding truncated proteins which are unable to locate to the midgut epithelium. The populations analyzed in this study contained a variety of alleles with mutations disrupting the function of the corresponding Bin toxin receptor. Their locations reveal a hot spot that can be exploited to assess the resistance risk through DNA screening.

Molecular diagnosis of infections and resistance in veterinary and human parasites

Since 1977, >2000 research papers described attempts to detect, identify and/or quantify parasites, or disease organisms carried by ecto-parasites, using DNA-based tests and 148 reviews of the topic were published. Despite this, only a few DNA-based tests for parasitic diseases are routinely available, and most of these are optional tests used occasionally in disease diagnosis. Malaria, trypanosomiasis, toxoplasmosis, leishmaniasis and cryptosporidiosis diagnosis may be assisted by DNA-based testing in some countries, but there are very few cases where the detection of veterinary parasites is assisted by DNA-based tests. The diagnoses of some bacterial (e.g. lyme disease) and viral diseases (e.g. tick borne encephalitis) which are transmitted by ecto-parasites more commonly use DNA-based tests, and research developing tests for these species makes up almost 20% of the literature. Other important uses of DNA-based tests are for epidemiological and risk assessment, quality control for food and water, forensic diagnosis and in parasite biology research. Some DNA-based tests for water-borne parasites, including Cryptosporidium and Giardia, are used in routine checks of water treatment, but forensic and food-testing applications have not been adopted in routine practice. Biological research, including epidemiological research, makes the widest use of DNA-based diagnostics, delivering enhanced understanding of parasites and guidelines for managing parasitic diseases. Despite the limited uptake of DNA-based tests to date, there is little doubt that they offer great potential to not only detect, identify and quantify parasites, but also to provide further information important for the implementation of parasite control strategies. For example, variant sequences within species of parasites and other organisms can be differentiated by tests in a manner similar to genetic testing in medicine or livestock breeding. If an association between DNA sequence and phenotype has been demonstrated, then qualities such as drug resistance, strain divergence, virulence, and origin of isolates could be inferred by DNA-based tests. No such tests are in clinical or commercial use in parasitology and few tests are available for other organisms. Why have DNA-based tests not had a bigger impact in veterinary and human medicine? To explore this question, technological, biological, economic and sociological factors must be considered. Additionally, a realistic expectation of research progress is needed. DNA-based tests could enhance parasite management in many ways, but patience, persistence and dedication will be needed to achieve this goal.

Properties and applied use of the mosquitocidal bacterium, Bacillus sphaericus

Strains of Bacillus sphaericus exhibit varying levels of virulence against mosquito larvae. The most potent strain, B. sphaericus 2362, which is the active ingredient in the commercial product VectoLex^®, together with another well-known larvicide Bacillus thuringiensis subsp. israelensis, are used to control vector and nuisance mosquito larvae in many regions of the world. Although not all strains of B. sphaericus are mosquitocidal, lethal strains produce one or two combinations of three different types of toxins. These are (1) the binary toxin (Bin) composed of two proteins of 42 kDa (BinA) and 51 kDa (BinB), which are synthesized during sporulation and co-crystallize, (2) the soluble mosquitocidal toxins (Mtx1, Mtx2 and Mtx3) produced during vegetative growth, and (3) the two-component crystal toxin (Cry48Aa1/Cry49Aa1). Non-mosquitocidal toxins are also produced by certain strains of B. sphaericus, for examples sphaericolysin, a novel insecticidal protein toxic to cockroaches. Larvicides based on B. sphaericus-based have the advantage of longer persistence in treated habitats compared to B. thuringiensis subsp. israelensis. However, resistance is a much greater threat, and has already emerged at significant levels in field populations in China and Thailand treated with B. sphaericus. This likely occurred because toxicity depends principally on Bin rather than various combinations of crystal (Cry) and cytolytic (Cyt) toxins present in B. thuringiensis subsp. israelensis. Here we review both the general characteristics of B. sphaericus, particularly as they relate to larvicidal isolates, and strategies or considerations for engineering more potent strains of this bacterium that contain built-in mechanisms that delay or overcome resistance to Bin in natural mosquito populations.

Insecticide control of vector-borne diseases: when is insecticide resistance a problem?

Many of the most dangerous human diseases are transmitted by insect vectors. After decades of repeated insecticide use, all of these vector species have demonstrated the capacity to evolve resistance to insecticides. Insecticide resistance is generally considered to undermine control of vector-transmitted diseases because it increases the number of vectors that survive the insecticide treatment. Disease control failure, however, need not follow from vector control failure. Here, we review evidence that insecticide resistance may have an impact on the quality of vectors and, specifically, on three key determinants of parasite transmission: vector longevity, competence, and behaviour. We argue that, in some instances, insecticide resistance is likely to result in a decrease in vector longevity, a decrease in infectiousness, or in a change in behaviour, all of which will reduce the vectorial capacity of the insect. If this effect is sufficiently large, the impact of insecticide resistance on disease management may not be as detrimental as previously thought. In other instances, however, insecticide resistance may have the opposite effect, increasing the insect's vectorial capacity, which may lead to a dramatic increase in the transmission of the disease and even to a higher prevalence than in the absence of insecticides. Either way-and there may be no simple generality-the consequence of the evolution of insecticide resistance for disease ecology deserves additional attention.

Stability of Culex quinquefasciatus resistance to Bacillus sphaericus evaluated by molecular tools

Bacillus sphaericus binary toxin action on Culex quinquefasciatus larvae relies on the binding to Cqm1alpha-glucosidases, which act as midgut receptors. Resistance of two laboratory-selected colonies is associated with the allele cqm1(REC) that prevents Cqm1 expression as membrane-bound molecules. This study evaluated stability of resistance after the interruption of selection pressure and introduction of susceptible individuals in these colonies. Bioassays showed that frequency of resistant larvae did not decrease throughout 11 generations, under these conditions, and it was associated to a similar frequency of larvae lacking the Cqm1alpha-glucosidase receptor, detected by in gel enzymatic assays. Direct screening of the cqm1(REC) allele, by specific PCR, showed that its frequency remained stable throughout 11 generations. Parental resistant colony did not display biological costs regarding fecundity, fertility and pupal weight and data from susceptibility assays, enzymatic assays and PCR screening showed that cqm1(REC) was not disfavored in competition with the susceptible allele and persisted in the progenies, in the lack of selection pressure. Characterization of molecular basis of resistance is essential for developing diagnostic tools and data have relevant implication for the establishment of strategies for resistance management.