Monitoring resistance to Bacillus thuringiensis subsp. israelensis in the field by performing bioassays with each Cry toxin separately

Positive selection analysis of Cyt proteins from Bacillus thuringiensis: A conservative trend driven by negative (purifying) selection

Bacillus thuringiensis is a Gram-positive entomopathogenic bacterium that produces different pesticidal proteins: vegetative insecticidal proteins (Vpb1/Vpa2, Vip3, and Vpb4) during vegetative growth, which are secreted to the culture medium, and δ-endotoxins (Cry and Cyt) during sporulation, which accumulate into parasporal crystals. Cyt proteins are the smaller subset of δ-endotoxins targeting Diptera species. While Cry and Vip3 proteins undergo positive selection, our analysis suggests that Cyt proteins evolve following a conservative trend driven negative (purifying) selection.

Phenotypic adaptation of Chironomus riparius to chronic Bti exposure: effects on emergence time and nutrient content

Insects with aquatic larval and terrestrial adult life stages are a key component of coupled aquatic-terrestrial ecosystems. Thus, stressors applied to water bodies adversely affecting those larvae have the potential to influence the riparian zone through altered emergence, with differences in prey availability, timing, or nutrition. In this study, the common model organism Chironomus riparius, a species of Chironomidae (Diptera), was used. This selection was further motivated by its wide distribution in European freshwaters and its importance as prey for terrestrial predators. A stressor of high importance in this context is the globally used mosquito control agent Bacillus thuringiensis var. israelensis (Bti) which has been shown to affect Chironomidae. Here, we investigated the ability of chironomid populations to adapt to a regularly applied stressor, leading to a reduced impact of Bti. Therefore, the initial sensitivity of laboratory populations of C. riparius was investigated under the influence of field-relevant Bti treatments (three doses × two application days) and different food sources (high-quality TetraMin vs. low-quality Spirulina). Following a chronic exposure to Bti over six months, the sensitivity of pre-exposed and naïve populations was re-evaluated. Food quality had a strong impact on emergence timing and nutrient content. In addition, alterations in emergence time as well as protein and lipid contents of chronically exposed populations indicated a selection for individuals of advantageous energetics, potentially leading to a more efficient development while combating Bti. Signs of adaptation could be confirmed in five out of 36 tested scenarios suggesting adaptation to Bti at the population level. Adaptive responses of one or several species could theoretically (via eco-evolutionary dynamics) result in a community shift, favouring the prevalence of Bti-tolerant species. (In)direct effects of Bti and the adaptive responses at both population and community levels could affect higher trophic levels and may determine the fate of meta-ecosystems.

Chemical Probes to Interrogate the Extreme Environment of Mosquito Larval Guts

Mosquito control methods are vital to curtail the spread of life-threatening illnesses, such as dengue fever, malaria, and yellow fever. Vector control technologies must be selective to minimize deleterious effects on our ecosystem. Successful methods that control mosquito larva populations utilize the uniquely high alkaline nature of the midgut. Here, we present novel protected triazabutadienes (pTBD) that are deprotected under basic conditions of the larval midgut, releasing an aryl diazonium ion (ADI) that results in protein modification. The probes contain a bioorthogonal terminal alkyne handle, enabling a selective Cu-click reaction with an azidofluorophore for quantification by SDS PAGE and visualization using fluorescence microscopy. A control TBD, unable to release an ADI, did not label the midgut. We envision our chemical probes will aid in the development of new selective mosquito control methods, thus preventing the spread of mosquito-borne illnesses with minimal impact on other organisms in the ecosystem.

Development of a regional climate change model for Aedes vigilax and Aedes camptorhynchus (Diptera: Culicidae) in Perth, Western Australia

Mosquito-borne disease is a significant public health issue and within Australia Ross River virus (RRV) is the most reported. This study combines a mechanistic model of mosquito development for two mosquito vectors; Aedes vigilax and Aedes camptorhynchus, with climate projections from three climate models for two Representative Concentration Pathways (RCPs), to examine the possible effects of climate change and sea-level rise on a temperate tidal saltmarsh habitat in Perth, Western Australia. The projections were run under no accretion and accretion scenarios using a known mosquito habitat as a case study. This improves our understanding of the possible implications of sea-level rise, accretion and climate change for mosquito control programmes for similar habitats across temperate tidal areas found in Southwest Western Australia. The output of the model indicate that the proportion of the year mosquitoes are active increases. Population abundances of the two Aedes species increase markedly. The main drivers of changes in mosquito population abundances are increases in the frequency of inundation of the tidal wetland and size of the area inundated, increased minimum water temperature, and decreased daily temperature fluctuations as water depth increases due to sea level changes, particularly under the model with no accretion. The effects on mosquito populations are more marked for RCP 8.5 when compared to RCP 4.5 but were consistent among the three climate change models. The results indicate that Ae. vigilax is likely to be the most abundant species in 2030 and 2050, but that by 2070 Aedes camptorhynchus may become the more abundant species. This increase would put considerable pressure on existing mosquito control programmes and increase the risk of mosquito-borne disease and nuisance biting to the local community, and planning to mitigate these potential impacts should commence now.

Chemical probes to interrogate the extreme environment of mosquito larval guts

Mosquito control methods are vital for the spread of life-threatening illnesses such as dengue fever, malaria, and yellow fever. Vector control technologies must be selective to minimize deleterious effects to our ecosystem. Successful methods that control mosquito larva populations utilize the uniquely high alkaline nature of the midgut. Here, we present novel protected triazabutadienes (pTBD) which are deprotected under basic conditions of the larval midgut, releasing an aryl diazonium ion (ADI) that results in protein modification. The probes contain a bioorthogonal terminal alkyne handle, enabling a selective Cu-click reaction with an azido-fluorophore for quantification by SDS PAGE and visualization using fluorescence microscopy. A control TBD, unable to release an ADI, did not label the midgut. We envision our chemical probes will aid in the development of new selective mosquito control methods thus preventing the spread of mosquito-borne illnesses with minimal impact on other organisms in the ecosystem.

MosChito rafts as a promising biocontrol tool against larvae of the common house mosquito, Culex pipiens

Mosquito control is of paramount importance, in particular, in light of the major environmental alterations associated with human activities, from climate change to the altered distribution of pathogens, including those transmitted by Arthropods. Here, we used the common house mosquito, Culex pipiens to test the efficacy of MosChito raft, a novel tool for mosquito larval control. MosChito raft is a floating hydrogel matrix, composed of chitosan, genipin and yeast cells, as bio-attractants, developed for the delivery of a Bacillus thuringiensis israeliensis (Bti)-based bioinsecticide to mosquito larvae. To this aim, larvae of Cx. pipiens were collected in field in Northern Italy and a novel colony of mosquito species (hereafter: Trescore strain) was established. MosChito rafts, containing the Bti-based formulation, were tested on Cx. pipiens larvae from the Trescore strain to determine the doses to be used in successive experiments. Thus, bioassays with MosChito rafts were carried out under semi-field conditions, both on larvae from the Trescore strain and on pools of larvae collected from the field, at different developmental stages. Our results showed that MosChito raft is effective against Cx. pipiens. In particular, the observed mortality was over 50% after two days exposure of the larvae to MosChito rafts, and over 70-80% at days three to four, in both laboratory and wild larvae. In conclusion, our results point to the MosChito raft as a promising tool for the eco-friendly control of a mosquito species that is not only a nuisance insect but is also an important vector of diseases affecting humans and animals.

Digitally managed larviciding as a cost-effective intervention for urban malaria: operational lessons from a pilot in São Tomé and Príncipe guided by the Zzapp system

BACKGROUND

Once a mainstay of malaria elimination operations, larval source management (LSM)-namely, the treatment of mosquito breeding habitats-has been marginalized in Africa in favour of long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS). However, the development of new technologies, and mosquitoes' growing resistance to insecticides used in LLINs and IRS raise renewed interest in LSM.

METHODS

A digitally managed larviciding (DML) operation in three of the seven districts of São Tomé and Príncipe (STP) was launched by the Ministry of Health (MOH) and ZzappMalaria LTD. The operation was guided by the Zzapp system, consisting of a designated GPS-based mobile application and an online dashboard, which facilitates the detection, sampling and treatment of mosquito breeding sites. During the operation, quality assurance (QA) procedures and field management methods were developed and implemented.

RESULTS

12,788 water bodies were located and treated a total of 128,864 times. The reduction impact on mosquito population and on malaria incidence was 74.90% and 52.5%, respectively. The overall cost per person protected (PPP) was US$ 0.86. The cost varied between areas: US$ 0.44 PPP in the urban area, and US$ 1.41 PPP in the rural area. The main cost drivers were labour, transportation and larvicide material.

CONCLUSION

DML can yield highly cost-effective results, especially in urban areas. Digital tools facilitate standardization of operations, implementation of QA procedures and monitoring of fieldworkers' performance. Digitally generated spatial data also have the potential to assist integrated vector management (IVM) operations. A randomized controlled trial (RCT) with a larger sample is needed to further substantiate findings.

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.

Aromatic Residues on the Side Surface of Cry4Ba-Domain II of Bacillus thuringiensis subsp. israelensis Function in Binding to Their Counterpart Residues on the Aedes aegypti Alkaline Phosphatase Receptor

Receptor binding is a prerequisite process to exert the mosquitocidal activity of the Cry4Ba toxin of Bacillus thuringiensis subsp. israelensis. The beta-sheet prism (domain II) and beta-sheet sandwich (domain III) of the Cry4Ba toxin have been implicated in receptor binding, albeit the precise binding mechanisms of these remain unclear. In this work, alanine scanning was used to determine the contribution to receptor binding of some aromatic and hydrophobic residues on the surface of domains II and III that are predicted to be responsible for binding to the Aedes aegypti membrane-bound alkaline phosphatase (Aa-mALP) receptor. Larvicidal activity assays against A. aegypti larvae revealed that aromatic residues (Trp₃₂₇ on the β2 strand, Tyr₃₄₇ on the β3-β4 loop, and Tyr₃₅₉ on the β4 strand) of domain II were important to the toxicity of the Cry4Ba toxin. Quantitative binding assays using enzyme-linked immunosorbent assay (ELISA) showed similar decreasing trends in binding to the Aa-mALP receptor and in toxicity of the Cry4Ba mutants Trp327Ala, Tyr347Ala, and Tyr359Ala, suggesting that a possible function of these surface-exposed aromatic residues is receptor binding. In addition, binding assays of the Cry4Ba toxin to the mutants of the binding residues Gly₅₁₃, Ser₄₉₀, and Phe₄₉₇ of the Aa-mALP receptor supported the binding function of Trp₃₂₇, Tyr₃₄₇, and Tyr₃₅₉ of the Cry4Ba toxin, respectively. Altogether, our results showed for the first time that aromatic residues on a side surface of the Cry4Ba domain II function in receptor binding. This finding provides greater insight into the possible molecular mechanisms of the Cry4Ba toxin.

Perspectives of vector management in the control and elimination of vector-borne zoonoses

The complex transmission profiles of vector-borne zoonoses (VZB) and vector-borne infections with animal reservoirs (VBIAR) complicate efforts to break the transmission circuit of these infections. To control and eliminate VZB and VBIAR, insecticide application may not be conducted easily in all circumstances, particularly for infections with sylvatic transmission cycle. As a result, alternative approaches have been considered in the vector management against these infections. In this review, we highlighted differences among the environmental, chemical, and biological control approaches in vector management, from the perspectives of VZB and VBIAR. Concerns and knowledge gaps pertaining to the available control approaches were discussed to better understand the prospects of integrating these vector control approaches to synergistically break the transmission of VZB and VBIAR in humans, in line with the integrated vector management (IVM) developed by the World Health Organization (WHO) since 2004.

Discovery of novel natural products for mosquito control

Vector control plays a key role in reducing the public health burden of mosquito-borne diseases. Today's vector control strategies largely rely on synthetic insecticides that can have a negative environmental impact when applied outdoors and often become inefficient because of the mosquitoes' ability to develop resistance. An alternative and promising approach to circumvent these challenges involves the implementation of insecticides derived from nature (biopesticides) for vector control. Biopesticides can constitute naturally occurring organisms or substances derived from them that have lifespan-shortening effects on disease vectors such as mosquitoes. Here we present the discovery and evaluation of natural product-based biological control agents that can potentially be developed into biopesticides for mosquito control. We screened a natural product collection comprising 390 compounds and initially identified 26 molecules with potential ability to kill the larval stages of the yellow fever mosquito Aedes aegypti, which is responsible for transmitting viruses such as dengue, Zika, chikungunya and yellow fever. Natural products identified as hits in the screen were further evaluated for their suitability for biopesticide development. We show that a selection of the natural product top hits, bactobolin, maytansine and ossamycin, also killed the larval stages of the malaria-transmitting mosquito Anopheles gambiae as well as the adult form of both species. We have further explored the usefulness of crude extracts and preparations from two of the best candidates' sources (organisms of origin) for mosquitocidal activity, that is extracts from the two bacteria Burkholderia thailandensis and Streptomyces hygroscopicus var. ossamyceticus.

Proteomic Response of Aedes aegypti Larvae to Silver/Silver Chloride Nanoparticles Synthesized Using Bacillus thuringiensis subsp. israelensis Metabolites

Simple Summary

Aedes aegypti is a vector of important mosquito-borne diseases. Green synthesized nanoparticles (NPs) have been used to control the larvae of mosquitoes including A. aegypti. However, the molecular responses of A. aegypti larvae to green synthesized NPs remain unexplored. This work analyzed protein expression in A. aegypti larvae in response to treatment using green synthesized silver/silver chloride nanoparticles (Ag/AgCl NPs) based on two-dimensional polyacrylamide gel electrophoresis. Fifteen differentially expressed protein spots were selected for identification using mass spectrometry. The results showed that the six upregulated proteins in A. aegypti larvae responsible for green synthesized Ag/AgCl NP treatment were involved in mitochondrial dysfunction, DNA and protein damage, inhibition of cell proliferation, and cell apoptosis, implying the modes of actions of Ag/AgCl NPs. This finding has provided greater insight into the possible mechanisms of green synthesized Ag/AgCl NPs on the control of A. aegypti larvae.

Abstract

Silver/silver chloride nanoparticles (Ag/AgCl NPs) are an alternative approach to control the larvae of Aedes aegypti, a vector of mosquito-borne diseases. However, the molecular mechanisms of Ag/AgCl NPs to A. aegypti have not been reported. In this work, Ag/AgCl NPs were synthesized using supernatant, mixed toxins from Bacillus thuringiensis subsp. israelensis (Bti), and heterologously expressed Cry4Aa and Cry4Ba toxins. The images from scanning electron microscopy revealed that the Ag/AgCl NPs were spherical in shape with a size range of 25–100 nm. The larvicidal activity against A. aegypti larvae revealed that the Ag/AgCl NPs synthesized using the supernatant of Bti exhibited higher toxicity (LC₅₀ = 0.133 μg/mL) than the Ag/AgCl NPs synthesized using insecticidal proteins (LC₅₀ = 0.148–0.217 μg/mL). The proteomic response to Ag/AgCl NPs synthesized using the supernatant of Bti in A. aegypti larvae was compared to the ddH₂O-treated control. Two-dimensional gel electrophoresis analysis revealed 110 differentially expressed proteins, of which 15 were selected for identification using mass spectrometry. Six upregulated proteins (myosin I heavy chain, heat shock protein 70, the F₀F₁-type ATP synthase beta subunit, methyltransferase, protein kinase, and condensin complex subunit 3) that responded to Ag/AgCl NP treatment in A. aegypti were reported for NP treatments in different organisms. These results suggested that possible mechanisms of action of Ag/AgCl NPs on A. aegypti larvae are: mitochondrial dysfunction, DNA and protein damage, inhibition of cell proliferation, and cell apoptosis. The findings from this work provide greater insight into the action of green synthesized Ag/AgCl NPs on the control of A. aegypti larvae.

Three reasons why expanded use of natural enemy solutions may offer sustainable control of human infections

1. Many infectious pathogens spend a significant portion of their life cycles in the environment or in animal hosts, where ecological interactions with natural enemies may influence pathogen transmission to people. Yet, our understanding of natural enemy opportunities for human disease control is lacking, despite widespread uptake and success of natural enemy solutions for pest and parasite management in agriculture. 2. Here we explore three reasons why conserving, restoring, or augmenting specific natural enemies in the environment could offer a promising complement to conventional clinical strategies to fight environmentally mediated pathogens and parasites. (1) Natural enemies of human infections abound in nature, largely understudied and undiscovered. (2) Natural enemy solutions could provide ecological options for infectious disease control where conventional interventions are lacking. And, (3) Many natural enemy solutions could provide important co-benefits for conservation and human well-being. 3. We illustrate these three arguments with a broad set of examples whereby natural enemies of human infections have been used or proposed to curb human disease burden, with some clear successes. However, the evidence base for most proposed solutions is sparse, and many opportunities likely remain undiscovered, highlighting opportunities for future research.

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

Background

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

Methods

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

Results

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

Conclusions

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

Graphical abstract

Supplementary Information

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

Prospects and Pitfalls: Next-Generation Tools to Control Mosquito-Transmitted Disease

Mosquito-transmitted diseases, including malaria and dengue, are a major threat to human health around the globe, affecting millions each year. A diverse array of next-generation tools has been designed to eliminate mosquito populations or to replace them with mosquitoes that are less capable of transmitting key pathogens. Many of these new approaches have been built on recent advances in CRISPR/Cas9-based genome editing. These initiatives have driven the development of pathogen-resistant lines, new genetics-based sexing methods, and new methods of driving desirable genetic traits into mosquito populations. Many other emerging tools involve microorganisms, including two strategies involving Wolbachia that are achieving great success in the field. At the same time, other mosquito-associated bacteria, fungi, and even viruses represent untapped sources of new mosquitocidal or antipathogen compounds. Although there are still hurdles to be overcome, the prospect that such approaches will reduce the impact of these diseases is highly encouraging.

A novel anti-dipteran Bacillus thuringiensis strain: Unusual Cry toxin genes in a highly dynamic plasmid environment

Bacillus thuringiensis emerged as a major bioinsecticide on the global market. It offers a valuable alternative to chemical products classically utilized to control pest insects. Despite the efficiency of several strains and products available on the market, the scientific community is always on the lookout for novel toxins that can replace or supplement the existing products. In this study, H3, a novel B. thuringiensis strain showing mosquitocidal activity, was isolated from Lebanese soil and characterized at an in vivo, genomic and proteomic levels. H3 parasporal crystal is toxic on its own but displays an unusual killing profile with a higher LC50 than the reference B. thuringiensis serovar israelensis crystal proteins. In addition, H3 has a different toxicity order: it is more toxic to Aedes albopictus and Anopheles gambiae than to Culex pipiens Whole genome sequencing and crystal analysis revealed that H3 can produce eleven novel Cry proteins, eight of which are assembled in genes with an orf1-gap-orf2 organization, where orf2 is a potential Cry4-type crystallization domain. Moreover, pH3-180, the toxin-carrying plasmid, holds a wide repertoire of mobile genetic elements that amount to ca 22% of its size., including novel insertion sequences and class II transposable elements Two other large plasmids present in H3 carry genetic determinants for the production of many interesting molecules - such as chitinase, cellulase and bacitracin - that may add up to H3 bioactive properties. This study therefore reports a novel mosquitocidal Bacillus thuringiensis strain with unusual Cry toxin genes in a rich mobile DNA environment.IMPORTANCE Bacillus thuringiensis, a soil entomopathogenic bacteria, is at the base of many sustainable eco-friendly bio-insecticides. Hence stems the need to continually characterize insecticidal toxins. H3 is an anti-dipteran B. thuringiensis strain, isolated from Lebanese soil, whose parasporal crystal contains eleven novel Cry toxins and no Cyt toxins. In addition to its individual activity, H3 showed potential as a co-formulant with classic commercialized B. thuringiensis products, to delay the emergence of resistance and to shorten the time required for killing. On a genomic level, H3 holds three large plasmids, one of which carries the toxin-coding genes, with four occurrences of the distinct orf1-gap-orf2 organization. Moreover, this plasmid is extremely rich in mobile genetic elements, unlike its two co-residents. This highlights the important underlying evolutionary traits between toxin-carrying plasmids and the adaptation of a B. thuringiensis strain to its environment and insect host spectrum.

Long-term exposure of Aedes aegypti to Bacillus thuringiensis svar. israelensis did not involve altered susceptibility to this microbial larvicide or to other control agents

Background

Bacillus thuringiensis svar. israelensis (Bti) is an effective and safe biolarvicide to control Aedes aegypti. Its mode of action based on four protoxins disfavors resistance; however, control in endemic areas that display high mosquito infestation throughout the year requires continuous larvicide applications, which imposes a strong selection pressure. Therefore, this study aimed to investigate the effects of an intensive Bti exposure on an Ae. aegypti strain (RecBti), regarding its susceptibility to Bti and two of its protoxins tested individually, to other control agents temephos and diflubenzuron, and its profile of detoxifying enzymes.

Methods

The RecBti strain was established using a large egg sample (10,000) from Recife city (Brazil) and more than 290,000 larvae were subjected to Bti throughout 30 generations. Larvae susceptibility to larvicides and the activity of detoxifying enzymes were determined by bioassays and catalytic assays, respectively. The Rockefeller strain was the reference used for these evaluations.

Results

Bti exposure yielded an average of 74% mortality at each generation. Larvae assessed in seven time points throughout the 30 generations were susceptible to Bti crystal (resistance ratio RR ≤ 2.8) and to its individual toxins Cry11Aa and Cry4Ba (RR ≤ 4.1). Early signs of altered susceptibility to Cry11Aa were detected in the last evaluations, suggesting that this toxin was a marker of the selection pressure imposed. RecBti larvae were also susceptible (RR ≤ 1.6) to the other control agents, temephos and diflubenzuron. The activity of the detoxifying enzymes α- and β-esterases, glutathione-S-transferases and mixed-function oxidases was classified as unaltered in larvae from two generations (F₁₉ and F₂₅), except for a β-esterases increase in F₂₅.

Conclusions

Prolonged exposure of Ae. aegypti larvae to Bti did not evolve into resistance to the crystal, and no cross-resistance with temephos and diflubenzuron were recorded, which supports their sustainable use with Bti for integrated control practices. The unaltered activity of most detoxifying enzymes suggests that they might not play a major role in the metabolism of Bti toxins, therefore resistance by this mechanism is unlikely to occur. This study also highlights the need to establish suitable criteria to classify the status of larval susceptibility/resistance.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3246-1) contains supplementary material, which is available to authorized users.

Larval Exposure to the Bacterial Insecticide Bti Enhances Dengue Virus Susceptibility of Adult Aedes aegypti Mosquitoes

Understanding the interactions between pathogens sharing the same host can be complicated for holometabolous animals when larval and adult stages are exposed to distinct pathogens. In medically important insect vectors, the effect of pathogen exposure at the larval stage may influence susceptibility to human pathogens at the adult stage. We addressed this hypothesis in the mosquito Aedes aegypti, a major vector of arthropod-borne viruses (arboviruses), such as the dengue virus (DENV) and the chikungunya virus (CHIKV). We experimentally assessed the consequences of sub-lethal exposure to the bacterial pathogen Bacillus thuringiensis subsp. israelensis (Bti), during larval development, on arbovirus susceptibility at the adult stage in three Ae. aegypti strains that differ in their genetic resistance to Bti. We found that larval exposure to Bti significantly increased DENV susceptibility, but not CHIKV susceptibility, in the Bti-resistant strains. However, there was no major difference in the baseline arbovirus susceptibility between the Bti-resistant strains and their Bti-susceptible parental strain. Although the generality of our results remains to be tested with additional arbovirus strains, this study supports the idea that the outcome of an infection by a pathogen depends on other pathogens sharing the same host even when they do not affect the same life stage of the host. Our findings may also have implications for Bti as a mosquito biocontrol agent, indicating that the sub-optimal Bti efficacy may have counter-productive effects by increasing vector competence, at least for some combinations of arbovirus and mosquito strains.

Lack of Resistance in Aedes vexans Field Populations After 36 Years of Bacillus thuringiensis subsp. Israelensis Applications in the Upper Rhine Valley, Germany

Bacillus thuringiensis subsp. israelensis (Bti) has been widely and solely used against floodwater mosquitoes, mostly Aedes vexans, for 36 years in the Upper Rhine Valley by the German Mosquito Control Association. During this period, almost 5,000 tons of Bti formulations were applied to an area of approximately 400,000 ha. To investigate a possible resistance development after such a long-term and widespread application of Bti, the susceptibility of Ae. vexans larvae to Bti in 3 untreated (Lake Constance) and 6 treated areas on both sides of the Rhine within the Upper Rhine Valley was assessed by bioassays following World Health Organization guidelines. Comparing log-probit analyses, it was shown that neither the median lethal concentration (LC₅₀ values) nor slopes of the probit lines of bioassays of the larvae deriving from treated and untreated areas showed significant differences. These results have been confirmed by resistance ratios, which varied from 0.80 to 1.12 in all tests. The results provided the evidence that no restistance in the target species Ae. vexans has developed in the areas of the Upper Rhine Valley, despite the large-scale use of Bti for 36 years.

Mosquitocidal potential of silver nanoparticles synthesized using local isolates of Bacillus thuringiensis subsp. israelensis and their synergistic effect with a commercial strain of B. thuringiensis subsp. israelensis

Control of larval stages of Aedes aegypti is considered an effective approach for preventing outbreaks of dengue fever. In this work, silver nanoparticles (Ag NPs) were synthesized using the supernatant and insecticidal proteins from local isolates of Bacillus thuringiensis subsp. israelensis (Bti). Mosquitocidal activity assays against A. aegypti larvae revealed that the highest toxicity was obtained from the Ag NPs synthesized using supernatant of Bti K55 and the inclusion proteins of Bti K46 with a lethal concentration 50 (LC₅₀) of 0.001 and 0.008μg/mL, respectively. The synthesized nanoparticles were characterized using UV-vis absorption spectrophotometry, scanning electron microscopy (SEM), SEM coupled with energy dispersive X-ray spectroscopy, X-ray diffraction and Fourier-transform infrared spectroscopy. The synergistic studies revealed that the Ag NPs synthesized using supernatant of Bti K55 were synergized with commercial Bti cells with a synergistic factor (SF) of 3.3 and 10.0 for LC₅₀ and LC₉₀, respectively. In addition, the Ag NPs synthesized using inclusion proteins of Bti K46 were synergized with commercial Bti cells with a SF of 1.6 and 4.2 for LC₅₀ and LC₉₀, respectively. This study provided the first report of the synergistic effect between Bti and Ag NPs. Such a combination could represent an effective approach for the control of the dengue vector and possibly reducing the likelihood of increased insect resistance to chemical control.

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.

Malaria in penguins - current perceptions

Avian malaria is a mosquito-borne disease caused by protozoans of the genus Plasmodium, and it is considered one of the most important causes of morbidity and mortality in captive penguins, both in zoological gardens and rehabilitation centres. Penguins are known to be highly susceptible to this disease, and outbreaks have been associated with mortality as high as 50-80% of affected captive populations within a few weeks. The disease has also been reported in wild penguin populations, however, its impacts on the health and fitness of penguins in the wild is not clear. This review provides an overview of the aetiology, life cycle and epidemiology of avian malaria, and provides details on the strategies that can be employed for the diagnostic, treatment and prevention of this disease in captive penguins, discussing possible directions for future research.

Isolation and characterization of native Bacillus thuringiensis strains from Saudi Arabia with enhanced larvicidal toxicity against the mosquito vector Anopheles gambiae (s.l.)

BACKGROUND

Worldwide, mosquito vectors are transmitting several etiological agents of important human diseases, including malaria, causing millions of deaths every year. In Saudi Arabia, as elsewhere, vector-control is based mostly on chemical insecticides which may be toxic and cause environmental deprivation. Here, to support the development of bio-pesticide alternatives, a study was conducted to identify native Bacillus thuringiensis (Bt) isolates with improved toxicity against the malaria vector, Anopheles gambiae (s.l.).

METHODS

Sixty-eight Bt isolates were obtained from 300 soil and other samples collected from 16 sites across Saudi Arabia. Bt identification was based on morphological characteristics of colonies, shape of parasporal crystals and biochemical profiles. After characterization of their mosquitocidal activity, larvicidal strains were described through 16S ribosomal DNA gene sequencing, cry, cyt and chi genes PCR-amplification profiles, and SDS-PAGE protein analyses.

RESULTS

Spherical Bt crystals were predominant amongst the 68 isolates (34%), while irregular, bi-pyramidal and spore-attached crystals were found in 32, 13 and 21% of strains, respectively. LC50 and LC90 bioassays showed that 23/68 isolates were larvicidal, with distinct biochemical activity profiles compared to non-larvicidal Bt strains. Eight larvicidal strains showed larvicidal activity up to 3.4-fold higher (LC50 range: 3.90-7.40 μg/ml) than the reference Bti-H14 strain (LC50 = 13.33 μg/ml). Of these, 6 strains had cry and cyt gene profiles similar to Bti-H14 (cry4Aa, cry4Ba, cry10, cry11, cyt1Aa, cyt1Ab, cyt2Aa). The seventh strain (Bt63) displaying the highest larvicidal activity (LC50 = 3.90 μg/ml) missed the cry4Aa and cyt1Ab genes and had SDS-PAGE protein profiles and spore/crystal sizes distinct from Bti-H14. The eight strain (Bt55) with LC50 of 4.11μg/ml had cry and cyt gene profiles similar to Bti-H14 but gave a chi gene PCR product size of 2027bp. No strains harbouring cry2, cry17 + 27, cry24 + 40, cry25, cry29, cry30, or cyt2Ba were detected.

CONCLUSION

This study represents the first report of several Saudi indigenous Bt strains with significantly higher larvicidal efficacy against An. gambiae than the reference Bti-H14 strain. The very high toxicity of the Bt63 strain, combined with distinct cry and cyt genes and SDS-PAGE-protein profiles makes it a promising candidate for future applications in mosquito bio-control.

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.

Bioefficacy of crude extract of Cyperus aromaticus (Family: Cyperaceae) cultured cells, against Aedes aegypti and Aedes albopictus mosquitoes

OBJECTIVE

To evaluate the growth inhibition activity of the crude extract of Cyperus aromaticus (C. aromaticus) cultured cells against the 3rd instar larvae of Aedes aegypti (Linn.) and Aedes albopictus Skuse (Ae. albopictus) under laboratory conditions, and determine the sublethal effects (EI50) of the crude extract of C. aromaticus cultured cells on some biological and morphological parameters of both Aedes mosquito species during two generations as well.

METHODS

The cell suspension cultures of C. aromaticus were activated from five callus lines (P4, Pa, Z1, Z6 and Ml) derived from the root explants of in vitro plantlets. The cultured cells were extracted in chloroform and used as plant material for the present study. For detection of juvenile hormone III, the crude extracts were analyzed by HPLC. Then the crude extracts of the three C. aromaticus cultured cell lines which contained varied amounts of juvenile hormone III [high level (P4 cell line), medium level (Z1 cell line) and low level (Ml cell line)] were tested against Aedes mosquito species. Laboratory evaluation was performed against late third instar larvae of the Vector Control Research Unit strains of Ae. aegypti and Ae. albopictus using the standard WHO method. The effects of EI50 of the C. aromaticus cultured P4 cells on fecundity, fertility, growth period, sex ratio, adult size and longevity of Aedes mosquitoes were assessed.

RESULTS

Bioassay tests presented the remarkable growth inhibition activity of the crude extracts of C. aromaticus cultured cells against the two Aedes mosquitoes. Between the two mosquito species, Ae. albopictus was more susceptible to the crude extracts with lower EI50 values. EI50 of the crude extract of C. aromaticus cultured cells (P4) increased the sterility indices in the parental generation females in both Aedes mosquito species. A significant delay in the pupal formation and adult emergence were observed in the parental generation of the both mosquito species. The sex ratio of the adult population either parental or F1 generation of the Aedes mosquito species was not significantly affected by the EI50 dosage of the crude extract of C. aromaticus cultured P4 cells. A significant decrease in the wing length of the treated adult (female and male) of Aedes aegypti as well as the treated female of Ae. albopictus were observed. Longevity of the adult female of the parental generation of both Aedes mosquitoes as well as females of F1 generation of Ae. albopictus were significantly decreased.

CONCLUSIONS

The present study revealed the potential of the crude extract of C. aromaticus cultured cells in controlling vector mosquito populations in the effort to reduce the transmission of vector borne diseases.