Blood meal-induced inhibition of vector-borne disease by transgenic microbiota

Host factors influencing sexual differentiation and transmission of Plasmodium: A comprehensive review

Malaria, a severe parasitic disease caused by Plasmodium infections, remains a major global health challenge. Efforts to eradicate malaria are complicated by the parasite's intricate life cycle, which alternates between vertebrate hosts and mosquito vectors. Host-derived factors and parasite-sourced components exert crucial roles in regulating this biological process. This review explores the critical role of host-derived factors in shaping Plasmodium sexual differentiation and transmission. We examine how vertebrate and mosquito host-specific factors either promote or restrict parasite development, influencing the transition from vertebrates to mosquitoes. Understanding these host-mediated mechanisms is crucial for developing novel transmission-blocking strategies to reduce malaria prevalence. By highlighting key interactions between hosts and parasites, this review provides insights into potential interventions that could disrupt Plasmodium transmission and contribute to malaria control efforts.

Harnessing engineered symbionts to combat concurrent malaria and arboviruses transmission

Concurrent malaria and arbovirus infections pose significant public health challenges in tropical and subtropical regions, demanding innovative control strategies. Here, we describe a strategy that employs multifunctional engineered symbiotic bacteria to suppress concurrent transmission of malaria parasites, dengue, and Zika viruses by various vector mosquitoes. The symbiotic bacterium Serratia AS1, which efficiently spreads through Anopheles and Aedes populations, is engineered to simultaneously produce anti-Plasmodium and anti-arbovirus effector proteins controlled by a selected blood-induced promoter. Laboratory and outdoor field-cage studies show that the multifunctional engineered symbiotic strains effectively inhibit Plasmodium infection in Anopheles mosquitoes and arbovirus infection in Aedes mosquitoes. Our findings provide the foundation for the use of engineered symbiotic bacteria as a powerful tool to combat the concurrent transmission of malaria and arbovirus diseases.

Exploiting venom toxins in paratransgenesis to prevent mosquito-borne disease

Mosquitoes are responsible for the transmission of numerous pathogens, including Plasmodium parasites, arboviruses and filarial worms. They pose a significant risk to public health with over 200 million cases of malaria per annum and approximately 4 billion people at risk of arthropod-borne viruses (arboviruses). Mosquito populations are geographically expanding into temperate regions and their distribution is predicted to continue increasing. Mosquito symbionts, including fungi, bacteria and viruses, have desirable traits for mosquito disease control including spreading horizontally and vertically through mosquito populations and potentially colonising multiple important vector species. Paratransgenesis, genetic modification of mosquito symbionts with effectors to target the pathogen rather than the vector, is a promising strategy to prevent the spread of mosquito-borne diseases. A variety of effectors can be expressed but venom toxins are excellent effector candidates because they are target specific, potent and stable. However, the only toxins to be explored in mosquito paratransgenesis to date are scorpine and mutated phospholipase A2. To enhance the scope, effectiveness and durability of paratransgenesis, an expanded arsenal of effectors is required. This review discusses other potential toxin effectors for future paratransgenesis studies based on prior in vitro and in vivo antiparasitic and antiviral studies and highlights the need for further research and investment in this area. In terms of mosquito-borne diseases, paratransgenesis strategies have been developed to target Plasmodium. We postulate the potential to apply this principle to target arboviruses using antiviral toxin effectors.

Gene Drive and Symbiont Technologies for Control of Mosquito-Borne Diseases

Mosquito-borne diseases, such as dengue and malaria, pose a significant burden to global health. Current control strategies with insecticides are only moderately effective. Scalable solutions are needed to reduce the transmission risk of these diseases. Symbionts and genome engineering-based mosquito control strategies have been proposed to address these problems. Bacterial, fungal, and viral symbionts affect mosquito reproduction, reduce mosquito lifespan, and block pathogen transmission. Field tests of endosymbiont Wolbachia-based methods have yielded promising results, but there are hurdles to overcome due to the large-scale rearing and accurate sex sorting required for Wolbachia-based suppression approaches and the ecological impediments to Wolbachia invasion in replacement approaches. Genome engineering-based methods, in which mosquitoes are genetically altered for the modification or suppression of wild populations, offer an additional approach for control of mosquito-borne diseases. In particular, the use of gene drive alleles that bias inheritance in their favor is a potentially powerful approach. Several drives are frequency dependent, potentially giving them broadly similar population dynamics to Wolbachia. However, public acceptance and the behavior of released drives in natural mosquito populations remain challenges. We summarize the latest developments and discuss the knowledge gaps in both symbiont- and gene drive-based methods.

Symbiotic Bacteria: Wolbachia, Midgut Microbiota in Mosquitoes and Their Importance for Vector Prevention Strategies

Mosquito-borne illnesses pose a significant threat to eradication under existing vector management measures. Chemo-based vector control strategies (use of insecticides) raise a complication of resistance and environmental pollution. Biological control methods are an alternative approach to overcoming this complication arising from insecticides. The mosquito gut microbiome is essential to supporting the factors that involve metabolic regulation and metamorphic development (from juvenile to adult), as well as the induction of an immune response. The induced immune response includes the JAK-STAT, IMD, and Toll pathways due to the microbial interaction with the midgut cells (MG cells) that prevent disease transmission to humans. The aforementioned sequel to the review provides information about endosymbiont Wolbachia, which contaminates insect cells, including germline and somatic cytoplasm, and inhibits disease-causing pathogen development and transmission by competing for resources within the cell. Moreover, it reduces the host population via cytoplasmic incompatibility (CI), feminization, male killing, and parthenogenesis. Furthermore, the Cif factor in Wolbachia is responsible for CI induction that produces inviable cells with the translocating systems and the embryonic defect-causing protein factor, WalE1 (WD0830), which manipulates the host actin. This potential of Wolbachia can be used to design a paratransgenic system to control vectors in the field. An extracellular symbiotic bacterium such as Asaia, which is grown in the growth medium, is used to transfer lethal genes within itself. Besides, the genetically transferred symbiotic bacteria infect the wild mosquito population and are easily manifold. So, it might be suitable for vector control strategies in the future.

UK mosquitoes are competent to transmit Usutu virus at native temperatures

Usutu virus (USUV) is an emerging zoonotic virus transmitted primarily by Culex mosquitoes. Since its introduction into Europe from Africa during the late 20th century, it has caused mortality within populations of passerine birds and captive owls, and can on occasion lead to disease in humans. USUV was first detected in the UK in 2020 and has become endemic, having been detected in either birds and/or mosquitoes every subsequent year. Importantly, the vector competence of indigenous mosquitoes for the circulating UK (London) USUV strain at representative regional temperatures is still to be elucidated. This study assessed the vector competence of five field-collected mosquito species/biotypes, Culex pipiens biotype molestus, Culex pipiens biotype pipiens, Culex torrentium, Culiseta annulata and Aedes detritus for the London USUV strain, with infection rates (IR) and transmission rates (TR) evaluated between 7 and 28 days post-infection. Infection and transmission were observed in all species/biotypes aside from Ae. detritus and Cx. torrentium. For Cx. pipiens biotype molestus, transmission potential suggests these populations should be monitored further for their role in transmission to humans. Furthermore, both Cx. pipiens biotype pipiens and Cs. annulata were shown to be competent vectors at 19 °C indicating the potential for geographical spread of the virus to other UK regions.

Colonization of Anopheles coustani, a neglected malaria vector in Madagascar

Anopheles coustani has long been recognized as a secondary malaria vector in Africa. It has recently been involved in the transmission of both Plasmodium falciparum and P. vivax in Madagascar. As most secondary malaria vectors, An. coustani mainly bites outdoors, which renders the control of this mosquito species difficult using classical malaria control measures, such as the use of bed nets or indoor residual spraying of insecticides. For a better understanding of the biology and vector competence of a vector species, it is useful to rear the species in the laboratory. The absence of a colony hinders the assessment of the bionomics of a species and the development of adapted control strategies. Here, we report the first successful establishment of an An. coustani colony from mosquitoes collected in Madagascar. We used a forced copulation procedure as this mosquito species will not mate in cages. We describe our mosquito colonization procedure with detailed biological features concerning larval to adult development and survival, recorded over the first six critical generations. The procedure should be easily applicable to An. coustani from different African countries, facilitating local investigation of An. coustani vector competence and insecticide resistance using the colony as a reference.

Innate immunity in Aedes mosquitoes: from pathogen resistance to shaping the microbiota

Discussions of host-microbe interactions in mosquito vectors are frequently dominated by a focus on the human pathogens they transmit (e.g. Plasmodium parasites and arboviruses). Underlying the interactions between a vector and its transmissible pathogens, however, is the physiology of an insect living and interacting with a world of bacteria and fungi including commensals, mutualists and primary and opportunistic pathogens. Here we review what is known about the bacteria and fungi associated with mosquitoes, with an emphasis on the members of the Aedes genus. We explore the reciprocal effects of microbe on mosquito, and mosquito on microbe. We analyse the roles of bacterial and fungal symbionts in mosquito development, their effects on vector competence, and their potential uses as biocontrol agents and vectors for paratransgenesis. We explore the compartments of the mosquito gut, uncovering the regionalization of immune effectors and modulators, which create the zones of resistance and immune tolerance with which the mosquito host controls and corrals its microbial symbionts. We examine the anatomical patterning of basally expressed antimicrobial peptides. Finally, we review the relationships between inducible antimicrobial peptides and canonical immune signalling pathways, comparing and contrasting current knowledge on each pathway in mosquitoes to the model insect Drosophila melanogaster. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

Introducing an environmental microbiome to axenic Aedes aegypti mosquitoes documents bacterial responses to a blood meal

IMPORTANCE

The blood meal of the female mosquito serves as a nutrition source to support egg development, so is an important aspect of its biology. Yet, the roles the microbiome may play in blood digestion are poorly characterized. We employed axenic mosquitoes to investigate how the microbiome differs between mosquitoes reared in the insectary versus mosquitoes that acquire their microbiome from the environment. Environmental microbiomes were more diverse and showed larger temporal shifts over the course of blood digestion. Importantly, only bacteria from the environmental microbiome performed hemolysis in culture, pointing to functional differences between bacterial populations. These data highlight that taxonomic differences between the microbiomes of insectary-reared and wild mosquitoes are potentially also related to their functional ecology. Thus, axenic mosquitoes colonized with environmental bacteria offer a way to investigate the role of bacteria from the wild in mosquito processes such as blood digestion, under controlled laboratory conditions.

Rodents as vehicle for delivery of transgenic bacteria to make paratransgenic sand fly vectors of cutaneous leishmaniasis in field condition

Vector-borne diseases, among them leishmaniasis, cause more than 700,000 deaths annually. The lack of an effective vaccination and the increasing resistance of sand flies to insecticides require the urgent development of innovative approaches to contain the disease. The use of engineered bacteria that express anti-parasite molecules (paratransgenesis) shows much promise. However, a challenge for implementation of this strategy is to devise means to introduce modified bacteria into sand flies in the field. In this study, we use rodent food bait as a delivery strategy to introduce two mCherry-fluorescent bacteria, Serratia AS1 and Enterobacter cloacae, into adult sand flies in field settings. Bacteria-infected food was provided to Rhombomys opimus rodents. These bacteria transiently pass through the rodent alimentary tract and are delivered to larval habitats with the rodent feces. The feces are ingested by sand fly larvae and, in the case of Serratia AS1, are trans-stadially transmitted to adults. This is the first report of targeting delivery of Serratia AS1 in a paratransgenic system to control transmission of leishmaniasis under field condition. This novel strategy shows promise for delivering transgenic bacteria to Leishmania vectors in the field.

Microbiota in disease-transmitting vectors

Haematophagous arthropods, including mosquitoes, ticks, flies, triatomine bugs and lice (here referred to as vectors), are involved in the transmission of various pathogens to mammals on whom they blood feed. The diseases caused by these pathogens, collectively known as vector-borne diseases (VBDs), threaten the health of humans and animals. Although the vector arthropods differ in life histories, feeding behaviour as well as reproductive strategies, they all harbour symbiotic microorganisms, known as microbiota, on which they depend for completing essential aspects of their biology, such as development and reproduction. In this Review, we summarize the shared and unique key features of the symbiotic associations that have been characterized in the major vector taxa. We discuss the crosstalks between microbiota and their arthropod hosts that influence vector metabolism and immune responses relevant for pathogen transmission success, known as vector competence. Finally, we highlight how current knowledge on symbiotic associations is being explored to develop non-chemical-based alternative control methods that aim to reduce vector populations, or reduce vector competence. We conclude by highlighting the remaining knowledge gaps that stand to advance basic and translational aspects of vector-microbiota interactions.

Outer membrane vesicles from a mosquito commensal mediate targeted killing of Plasmodium parasites via the phosphatidylcholine scavenging pathway

The gut microbiota is a crucial modulator of Plasmodium infection in mosquitoes, including the production of anti-Plasmodium effector proteins. But how the commensal-derived effectors are translocated into Plasmodium parasites remains obscure. Here we show that a natural Plasmodium blocking symbiotic bacterium Serratia ureilytica Su_YN1 delivers the effector lipase AmLip to Plasmodium parasites via outer membrane vesicles (OMVs). After a blood meal, host serum strongly induces Su_YN1 to release OMVs and the antimalarial effector protein AmLip into the mosquito gut. AmLip is first secreted into the extracellular space via the T1SS and then preferentially loaded on the OMVs that selectively target the malaria parasite, leading to targeted killing of the parasites. Notably, these serum-induced OMVs incorporate certain serum-derived lipids, such as phosphatidylcholine, which is critical for OMV uptake by Plasmodium via the phosphatidylcholine scavenging pathway. These findings reveal that this gut symbiotic bacterium evolved to deliver secreted effector molecules in the form of extracellular vesicles to selectively attack parasites and render mosquitoes refractory to Plasmodium infection. The discovery of the role of gut commensal-derived OMVs as carriers in cross-kingdom communication between mosquito microbiota and Plasmodium parasites offers a potential innovative strategy for blocking malaria transmission.

Delftia tsuruhatensis TC1 symbiont suppresses malaria transmission by anopheline mosquitoes

Malaria control demands the development of a wide range of complementary strategies. We describe the properties of a naturally occurring, non-genetically modified symbiotic bacterium, Delftia tsuruhatensis TC1, which was isolated from mosquitoes incapable of sustaining the development of Plasmodium falciparum parasites. D. tsuruhatensis TC1 inhibits early stages of Plasmodium development and subsequent transmission by the Anopheles mosquito through secretion of a small-molecule inhibitor. We have identified this inhibitor to be the hydrophobic molecule harmane. We also found that, on mosquito contact, harmane penetrates the cuticle, inhibiting Plasmodium development. D. tsuruhatensis TC1 stably populates the mosquito gut, does not impose a fitness cost on the mosquito, and inhibits Plasmodium development for the mosquito's life. Contained field studies in Burkina Faso and modeling showed that D. tsuruhatensis TC1 has the potential to complement mosquito-targeted malaria transmission control.

Recent Advancements in Pathogenic Mechanisms, Applications and Strategies for Entomopathogenic Fungi in Mosquito Biocontrol

Fungal diseases are widespread among insects and play a crucial role in naturally regulating insect populations. Mosquitoes, known as vectors for numerous infectious diseases, pose a significant threat to human health. Entomopathogenic fungi (EPF) have emerged as highly promising alternative agents to chemical mosquitocides for controlling mosquitoes at all stages of their life cycle due to their unique infection pathway through direct contact with the insect's cuticle. In recent years, significant advancements have been made in understanding the infection pathways and pathogenic mechanisms of EPF against mosquitoes. Various strategies involving the use of EPF alone or combinations with other approaches have been employed to target mosquitoes at various developmental stages. Moreover, the application of genetic technologies in fungi has opened up new avenues for enhancing the mosquitocidal efficacy of EPF. This review presents a comprehensive summary of recent advancements in our understanding the pathogenic mechanisms of EPF, their applications in mosquito management, and the combination of EPF with other approaches and employment of transgenic technologies. The biosafety concerns associated with their use and the corresponding approaches are also discussed. The recent progress suggests that EPF have the potential to serve as a future biorational tool for controlling mosquito vectors.

Mosquito-Borne Diseases and Their Control Strategies: An Overview Focused on Green Synthesized Plant-Based Metallic Nanoparticles

Mosquitoes act as vectors of pathogens that cause most life-threatening diseases, such as malaria, Dengue, Chikungunya, Yellow fever, Zika, West Nile, Lymphatic filariasis, etc. To reduce the transmission of these mosquito-borne diseases in humans, several chemical, biological, mechanical, and pharmaceutical methods of control are used. However, these different strategies are facing important and timely challenges that include the rapid spread of highly invasive mosquitoes worldwide, the development of resistance in several mosquito species, and the recent outbreaks of novel arthropod-borne viruses (e.g., Dengue, Rift Valley fever, tick-borne encephalitis, West Nile, yellow fever, etc.). Therefore, the development of novel and effective methods of control is urgently needed to manage mosquito vectors. Adapting the principles of nanobiotechnology to mosquito vector control is one of the current approaches. As a single-step, eco-friendly, and biodegradable method that does not require the use of toxic chemicals, the green synthesis of nanoparticles using active toxic agents from plant extracts available since ancient times exhibits antagonistic responses and broad-spectrum target-specific activities against different species of vector mosquitoes. In this article, the current state of knowledge on the different mosquito control strategies in general, and on repellent and mosquitocidal plant-mediated synthesis of nanoparticles in particular, has been reviewed. By doing so, this review may open new doors for research on mosquito-borne diseases.

Internal and external microbiota of home-caught Anopheles coluzzii (Diptera: Culicidae) from Côte d'Ivoire, Africa: Mosquitoes are filthy

Over the past 10 years, studies using high-throughput 16S rRNA gene sequencing have shown that mosquitoes harbor diverse bacterial communities in their digestive system. However, no previous research has examined the total bacteria community inside versus outside of mosquitoes and whether bacteria found on the outside could represent a potential health threat through mechanical transfer. We examined the bacterial community of the external surface and internal body of female Anopheles coluzzii adults collected from homes in Côte d'Ivoire, Africa, by Illumina sequencing of the V3 to V4 region of 16S rRNA gene. Anopheles coluzzii is in the Anopheles gambiae sensu lato (s.l.) species complex and important in the transmission of malaria. The total 16S rRNA reads were assigned to 34 phyla, 73 orders, 325 families, and 700 genera. At the genus level, the most abundant genera inside and outside combined were Bacillus, Staphylococcus, Enterobacter, Corynebacterium, Kocuria, Providencia, and Sphingomonas. Mosquitoes had a greater diversity of bacterial taxa internally compared to the outside. The internal bacterial communities were similar between homes, while the external body samples were significantly different between homes. The bacteria on the external body were associated with plants, human and animal skin, and human and animal infections. Internally, Rickettsia bellii and Rickettsia typhi were found, potentially of importance, since this genus is associated with human diseases. Based on these findings, further research is warranted to assess the potential mechanical transmission of bacteria by mosquitoes moving into homes and the importance of the internal mosquito microbiota in human health.

Combining transgenesis with paratransgenesis to fight malaria

Malaria is among the deadliest infectious diseases, and Plasmodium, the causative agent, needs to complete a complex development cycle in its vector mosquito for transmission to occur. Two promising strategies to curb transmission are transgenesis, consisting of genetically engineering mosquitoes to express antimalarial effector molecules, and paratransgenesis, consisting of introducing into the mosquito commensal bacteria engineered to express antimalarial effector molecules. Although both approaches restrict parasite development in the mosquito, it is not known how their effectiveness compares. Here we provide an in-depth assessment of transgenesis and paratransgenesis and evaluate the combination of the two approaches. Using the Q-system to drive gene expression, we engineered mosquitoes to produce and secrete two effectors - scorpine and the MP2 peptide - into the mosquito gut and salivary glands. We also engineered Serratia, a commensal bacterium capable of spreading through mosquito populations to secrete effectors into the mosquito gut. Whereas both mosquito-based and bacteria-based approaches strongly reduced the oocyst and sporozoite intensity, a substantially stronger reduction of Plasmodium falciparum development was achieved when transgenesis and paratransgenesis were combined. Most importantly, transmission of Plasmodium berghei from infected to naïve mice was maximally inhibited by the combination of the two approaches. Combining these two strategies promises to become a powerful approach to combat malaria.

An evaluation of fusion partner proteins for paratransgenesis in Asaia bogorensis

Mosquitoes transmit many pathogens responsible for human diseases, such as malaria which is caused by parasites in the genus Plasmodium. Current strategies to control vector-transmitted diseases are increasingly undermined by mosquito and pathogen resistance, so additional methods of control are required. Paratransgenesis is a method whereby symbiotic bacteria are genetically modified to affect the mosquito’s phenotype by engineering them to deliver effector molecules into the midgut to kill parasites. One paratransgenesis candidate is Asaia bogorensis, a Gram-negative bacterium colonizing the midgut, ovaries, and salivary glands of Anopheles sp. mosquitoes. Previously, engineered Asaia strains using native signals to drive the release of the antimicrobial peptide, scorpine, fused to alkaline phosphatase were successful in significantly suppressing the number of oocysts formed after a blood meal containing P. berghei. However, these strains saw high fitness costs associated with the production of the recombinant protein. Here, we report evaluation of five different partner proteins fused to scorpine that were evaluated for effects on the growth and fitness of the transgenic bacteria. Three of the new partner proteins resulted in significant levels of protein released from the Asaia bacterium while also significantly reducing the prevalence of mosquitoes infected with P. berghei. Two partners performed as well as the previously tested Asaia strain that used alkaline phosphatase in the fitness analyses, but neither exceeded it. It may be that there is a maximum level of fitness and parasite inhibition that can be achieved with scorpine being driven constitutively, and that use of a Plasmodium specific effector molecule in place of scorpine would help to mitigate the stress on the symbionts.

The Strategy of Paratransgenesis for the Control of Malaria Transmission

Insect-borne diseases are responsible for important burdens on health worldwide particularly in Africa. Malaria alone causes close to half a million deaths every year, mostly in developing, tropical and subtropical countries, with 94% of the global deaths in 2019 occurring in the WHO African region. With several decades, vector control measures have been fundamental to fight against malaria. Considering the spread of resistance to insecticides in mosquitoes and to drugs in parasites, the need for novel strategies to inhibit the transmission of the disease is pressing. In recent years, several studies have focused on the interaction of malaria parasites, bacteria and their insect vectors. Their findings suggested that the microbiota of mosquitoes could be used to block Plasmodium transmission. A strategy, termed paratransgenesis, aims to interfere with the development of malaria parasites within their vectors through genetically-modified microbes, which produce antimalarial effectors inside the insect host. Here we review the progress of the paratransgenesis approach. We provide a historical perspective and then focus on the choice of microbial strains and on genetic engineering strategies. We finally describe the different steps from laboratory design to field implementation to fight against malaria.

Lipases secreted by a gut bacterium inhibit arbovirus transmission in mosquitoes

Arboviruses are etiological agents of various severe human diseases that place a tremendous burden on global public health and the economy; compounding this issue is the fact that effective prophylactics and therapeutics are lacking for most arboviruses. Herein, we identified 2 bacterial lipases secreted by a Chromobacterium bacterium isolated from Aedes aegypti midgut, Chromobacterium antiviral effector-1 (CbAE-1) and CbAE-2, with broad-spectrum virucidal activity against mosquito-borne viruses, such as dengue virus (DENV), Zika virus (ZIKV), Japanese encephalitis virus (JEV), yellow fever virus (YFV) and Sindbis virus (SINV). The CbAEs potently blocked viral infection in the extracellular milieu through their lipase activity. Mechanistic studies showed that this lipase activity directly disrupted the viral envelope structure, thus inactivating infectivity. A mutation in the lipase motif of CbAE-1 fully abrogated the virucidal ability. Furthermore, CbAEs also exert lipase-dependent entomopathogenic activity in mosquitoes. The anti-arboviral and entomopathogenic properties of CbAEs render them potential candidates for the development of novel transmission control strategies against vector-borne diseases.

Mosquito-borne viruses are the etiological agents of severe human diseases and annually lead to a great number of deaths. These viruses have spread widely and raised major public health concerns throughout the world. Although effective vaccines have been developed for a few mosquito-borne viruses, such as JEV and yellow fever virus (YFV), vaccines or antiviral therapeutics against most mosquito-borne viruses are currently unavailable. In this study, we identified two virucidal and entomopathogenic effectors with lipase activity, CbAE-1 and CbAE-2, from a mosquito midgut derived bacterium Chromobacterium sp. Beijing. Both CbAEs showed potent virucidal activity against a variety of mosquito-borne viruses, including DENV, ZIKV, JEV, YFV, and SINV, as well as other enveloped viruses. Since CbAEs inactivate viruses through their lipase activity by directly disrupting the viral envelope structure, they may provide a novel option for genetically engineering microbiota symbiotic with mosquitoes for arboviral control. Overall, the anti-arboviral and entomopathogenic properties of Csp_BJ and CbAEs render them particularly interesting candidates for the development of novel transmission control strategies against vector-borne diseases.

Malaria-Transmitting Vectors Microbiota: Overview and Interactions With Anopheles Mosquito Biology

Malaria remains a vector-borne infectious disease that is still a major public health concern worldwide, especially in tropical regions. Malaria is caused by a protozoan parasite of the genus Plasmodium and transmitted through the bite of infected female Anopheles mosquitoes. The control interventions targeting mosquito vectors have achieved significant success during the last two decades and rely mainly on the use of chemical insecticides through the insecticide-treated nets (ITNs) and indoor residual spraying (IRS). Unfortunately, resistance to conventional insecticides currently being used in public health is spreading in the natural mosquito populations, hampering the long-term success of the current vector control strategies. Thus, to achieve the goal of malaria elimination, it appears necessary to improve vector control approaches through the development of novel environment-friendly tools. Mosquito microbiota has by now given rise to the expansion of innovative control tools, such as the use of endosymbionts to target insect vectors, known as "symbiotic control." In this review, we will present the viral, fungal and bacterial diversity of Anopheles mosquitoes, including the bacteriophages. This review discusses the likely interactions between the vector microbiota and its fitness and resistance to insecticides.

Vector microbiota and immunity: modulating arthropod susceptibility to vertebrate pathogens

Arthropods, including mosquitoes, sand flies, tsetse flies, and ticks are vectors of many bacterial, parasitic, and viral pathogens that cause serious disease in humans and animals. Their microbiota, that is, all microorganisms that dwell within their tissues, can impact vector immunity and susceptibility to pathogen infection. Historically, host-pathogen-microbiota interactions have not been well described, with little known about mechanism. In this review, we highlight recent advances in understanding how individual microorganisms and microbial communities interact with vectors and human pathogens, the mechanisms they utilize to achieve these effects, and the potential for exploiting these interactions to control pathogen transmission. These studies fill important knowledge gaps and further our understanding of the roles that the vector microbiota plays in pathogen transmission.

Overview of paratransgenesis as a strategy to control pathogen transmission by insect vectors

This article presents an overview of paratransgenesis as a strategy to control pathogen transmission by insect vectors. It first briefly summarises some of the disease-causing pathogens vectored by insects and emphasises the need for innovative control methods to counter the threat of resistance by both the vector insect to pesticides and the pathogens to therapeutic drugs. Subsequently, the state of art of paratransgenesis is described, which is a particularly ingenious method currently under development in many important vector insects that could provide an additional powerful tool for use in integrated pest control programmes. The requirements and recent advances of the paratransgenesis technique are detailed and an overview is given of the microorganisms selected for genetic modification, the effector molecules to be expressed and the environmental spread of the transgenic bacteria into wild insect populations. The results of experimental models of paratransgenesis developed with triatomines, mosquitoes, sandflies and tsetse flies are analysed. Finally, the regulatory and safety rules to be satisfied for the successful environmental release of the genetically engineered organisms produced in paratransgenesis are considered.

Symbionts and gene drive: two strategies to combat vector-borne disease

Mosquitoes bring global health problems by transmitting parasites and viruses such as malaria and dengue. Unfortunately, current insecticide-based control strategies are only moderately effective because of high cost and resistance. Thus, scalable, sustainable, and cost-effective strategies are needed for mosquito-borne disease control. Symbiont-based and genome engineering-based approaches provide new tools that show promise for meeting these criteria, enabling modification or suppression approaches. Symbiotic bacteria like Wolbachia are maternally inherited and manipulate mosquito host reproduction to enhance their vertical transmission. Genome engineering-based gene drive methods, in which mosquitoes are genetically altered to spread drive alleles throughout wild populations, are also proving to be a potentially powerful approach in the laboratory. Here, we review the latest developments in both symbionts and gene drive-based methods. We describe some notable similarities, as well as distinctions and obstacles, relating to these promising technologies.

Wolbachia in Aedes koreicus: Rare Detections and Possible Implications

The emerging distribution of new alien mosquito species was recently described in Europe. In addition to the invasion of Aedes albopictus, several studies have focused on monitoring and controlling other invasive Aedes species, as Aedes koreicus and Aedes japonicus. Considering the increasing development of insecticide resistance in Aedes mosquitoes, new control strategies, including the use of bacterial host symbionts, are proposed. However, little is known about the bacterial communities associated with these species, thus the identification of possible candidates for Symbiotic Control is currently limited. The characterization of the natural microbiota of field-collected Ae. koreicus mosquitoes from North-East Italy through PCR screening, identified native infections of Wolbachia in this species that is also largely colonized by Asaia bacteria. Since Asaia and Wolbachia are proposed as novel tools for Symbiotic Control, our study supports their use for innovative control strategies against new invasive species. Although the presence of Asaia was previously characterized in Ae. koreicus, our study characterized this Wolbachia strain, also inferring its phylogenetic position. The co-presence of Wolbachia and Asaia may provide additional information about microbial competition in mosquito, and to select suitable phenotypes for the suppression of pathogen transmission and for the manipulation of host reproduction in Ae. koreicus.

The influence of culture-dependent native microbiota in Zika virus infection in Aedes aegypti

Background

Emerging and re-emerging vector-borne diseases (VBDs) pose a recurring threat to tropical countries, mainly due to the abundance and distribution of the Aedes aegypti mosquito, which is a vector of the Zika, dengue, chikungunya, and yellow fever arboviruses.

Methods

Female 3–5 day-old Ae. aegypti were distributed into two experimental groups: group I—survey of cultivable bacteria; sucrose group: fed only on sucrose, i.e., non-blood-fed (UF); blood-fed group: (i) fed with non-infected blood (BF); (ii) fed with blood infected with the Zika virus (BZIKV); (iii) pretreated with penicillin/streptomycin (pen/strep), and fed with non-infected blood (TBF); (iv) pretreated with pen/strep and fed blood infected with ZIKV, i.e., gravid with developed ovaries, (TGZIKV); group II—experimental co-infections: bacteria genera isolated from the group fed on sucrose, i.e., non-blood-fed (UF).

Results

Using the cultivable method and the same mosquito colony and ZIKV strain described by in a previous work, our results reveled 11 isolates (Acinetobacter, Aeromonas, Cedecea, Cellulosimicrobium, Elizabethkingia, Enterobacter, Lysinibacillus, Pantoea, Pseudomonas, Serratia, and Staphylococcus). Enterobacter was present in all evaluated groups (i.e., UF, BF, BZIKV, TBF, and TGZIKV), whereas Elizabethkingia was present in the UF, BZIKV, and TBF groups. Pseudomonas was present in the BZIKV and TBF groups, whereas Staphylococcus was present in the TBF and TGZIKV groups. The only genera of bacteria that were found to be present in only one group were Aeromonas, Lysinibacillus, and Serratia (UF); Cedacea, Pantoea and Acinetobacter (BF); and Cellulosimicrobium (BZIKV). The mosquitoes co-infected with ZIKV plus the isolates group fed on sucrose (UF) showed interference in the outcome of infection.

Conclusions

We demonstrate that the distinct feeding aspects assessed herein influence the composition of bacterial diversity. In the co-infection, among ZIKV, Ae. aegypti and the bacterial isolates, the ZIKV/Lysinibacillus–Ae. aegypti had the lowest number of viral copies in the head-SG, which means that it negatively affects vector competence. However, when the saliva was analyzed after forced feeding, no virus was detected in the mosquito groups ZIKV/Lysinibacillus–Lu. longipalpis and Ae. aegypti; the combination of ZIKV/Serratia may interfere in salivation. This indicates that the combinations do not produce viable viruses and may have great potential as a method of biological control.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05160-7.

Roles of Bacterial Symbionts in Transmission of Plant Virus by Hemipteran Vectors

The majority of plant viruses are transmitted by hemipteran insects. Bacterial symbionts in hemipteran hosts have a significant impact on the host life, physiology and ecology. Recently, the involvement of bacterial symbionts in hemipteran vector-virus and vector-plant interactions has been documented. Thus, the exploitation and manipulation of bacterial symbionts have great potential for plant viral disease control. Herein, we review the studies performed on the impact of symbiotic bacteria on plant virus transmission, including insect-bacterial symbiont associations, the role of these bacterial symbionts in viral acquisition, stability and release during viral circulation in insect bodies, and in viral vertical transmission. Besides, we prospect further studies aimed to understand tripartite interactions of the virus-symbiotic microorganisms-insect vector.

Microbial Diversity of Adult Aedes aegypti and Water Collected from Different Mosquito Aquatic Habitats in Puerto Rico

Mosquitoes, the major vectors of viruses like dengue, are naturally host to diverse microorganisms, which play an important role in their development, fecundity, immunity, and vector competence. The composition of their microbiota is strongly influenced by the environment, particularly their aquatic larval habitat. In this study, we used 2×300 bp 16s Illumina sequencing to compare the microbial profiles of emerging adult Aedes aegypti mosquitoes and the water collected from common types of aquatic habitat containers in Puerto Rico, which has endemic dengue transmission. We sequenced 141 mosquito and 46 water samples collected from plastic containers, septic tanks, discarded tires, underground trash cans, tree holes, or water meters. We identified 9 bacterial genera that were highly prevalent in the mosquito microbiome, and 77 for the microbiome of the aquatic habitat. The most abundant mosquito-associated bacterial OTUs were from the families Burkholderiaceae, Pseudomonadaceae, Comamonadaceae, and Xanthomonadaceae. Microbial profiles varied greatly between mosquitoes, and there were few major differences explained by container type; however, the microbiome of mosquitoes from plastic containers was more diverse and contained more unique taxa than the other groups. Container water was significantly more diverse than mosquitoes, and our data suggest that mosquitoes filter out many bacteria, with Alphaproteobacteria in particular being far more abundant in water. These findings provide novel insight into the microbiome of mosquitoes in the region and provide a platform to improve our understanding of the fundamental mosquito-microbe interactions.

Diversity of midgut microbiota in laboratory-colonized and field-collected Aedes albopictus (Diptera: Culicidae): A preliminary study

Aedes (Ae.) albopictus is an important vector for many pathogens. Previous studies have revealed a role for midgut bacteria during pathogen infection in mosquitoes; however, studies of Ae. albopictus midgut bacteria are limited. We examined the diversity of midgut bacteria in female laboratory-colonized and field-collected Ae. albopictus. A total of 31 bacterial genera were identified representing 10 and 28 genera of laboratory-colonized and field-collected Ae. albopictus, respectively. The predominant bacterial genera in the laboratory-colonized Ae. albopictus were Staphylococcus and Micrococcus, whereas the bacterial diversity in the field-collected Ae. albopictus exhibited a higher proportion of Rhizobium and Agrobacterium as the dominant genera. However, only Staphylococcus showed a significant difference between laboratory-colonized and field-collected Ae. albopictus. The midgut bacterial species were identified from 30 laboratory-colonized Ae. albopictus mosquitoes. A total of 16 bacterial species were identified and the predominant bacterial species was Micrococcus luteus, followed by Staphylococcus epidermidis and Agrobacterium tumefaciens. Field mosquitoes were collected from the Sing Buri, Chumphon, and Yala Provinces of Thailand. The midgut bacterial species identified from the 10 Ae. albopictus collected from the Sing Buri Province included Bacillus subtilis, Staphylococcus haemolyticus, Staphylococcus hominis, and Serratia marcescens. Serratia marcescens was the only bacteria identified from this area. Midgut bacterial species were identified from 40 filed-collected Ae. albopictus from Chumphon Province. A total of 25 bacterial species were identified and the predominant species were Enterobacter cloacae, Micrococcus luteus, and Providencia rettgeri. Only 15 bacterial species were identified from the mosquitoes collected from Chumphon Province. A total of 18 bacterial species were identified from 30 Ae. albopictus collected from Yala Province and the predominant species were Rhizobium pusense and Agrobacterium tumefaciens. Only 12 bacterial species were found in mosquitoes collected from Yala Province. These findings indicate changes in the midgut bacteria population in Ae. albopictus from various locales, which may result from variability in the blood-meal source, diet, or habitat. A comprehensive survey of the midgut bacteria community prevalence in wild populations is critical for not only gaining a better understanding of the role of this bacterium in shaping the microbial community in Ae. albopictus, but also for informing current and future mosquito and disease control programs.

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.

Paratransgenic manipulation of a tsetse microRNA alters the physiological homeostasis of the fly's midgut environment

Tsetse flies are vectors of parasitic African trypanosomes, the etiological agents of human and animal African trypanosomoses. Current disease control methods include fly-repelling pesticides, fly trapping, and chemotherapeutic treatment of infected people and animals. Inhibiting tsetse's ability to transmit trypanosomes by strengthening the fly's natural barriers can serve as an alternative approach to reduce disease. The peritrophic matrix (PM) is a chitinous and proteinaceous barrier that lines the insect midgut and serves as a protective barrier that inhibits infection with pathogens. African trypanosomes must cross tsetse's PM in order to establish an infection in the fly, and PM structural integrity negatively correlates with trypanosome infection outcomes. Bloodstream form trypanosomes shed variant surface glycoproteins (VSG) into tsetse's gut lumen early during the infection establishment, and free VSG molecules are internalized by the fly's PM-producing cardia. This process results in a reduction in the expression of a tsetse microRNA (miR275) and a sequential molecular cascade that compromises PM integrity. miRNAs are small non-coding RNAs that are critical in regulating many physiological processes. In the present study, we investigated the role(s) of tsetse miR275 by developing a paratransgenic expression system that employs tsetse's facultative bacterial endosymbiont, Sodalis glossinidius, to express tandem antagomir-275 repeats (or miR275 sponges). This system induces a constitutive, 40% reduction in miR275 transcript abundance in the fly's midgut and results in obstructed blood digestion (gut weights increased by 52%), a significant increase (p-value < 0.0001) in fly survival following infection with an entomopathogenic bacteria, and a 78% increase in trypanosome infection prevalence. RNA sequencing of cardia and midgut tissues from paratransgenic tsetse confirmed that miR275 regulates processes related to the expression of PM-associated proteins and digestive enzymes as well as genes that encode abundant secretory proteins. Our study demonstrates that paratransgenesis can be employed to study microRNA regulated pathways in arthropods that house symbiotic bacteria.

The microbiome and mosquito vectorial capacity: rich potential for discovery and translation

Microbiome research has gained considerable interest due to the emerging evidence of its impact on human and animal health. As in other animals, the gut-associated microbiota of mosquitoes affect host fitness and other phenotypes. It is now well established that microbes can alter pathogen transmission in mosquitoes, either positively or negatively, and avenues are being explored to exploit microbes for vector control. However, less attention has been paid to how microbiota affect phenotypes that impact vectorial capacity. Several mosquito and pathogen components, such as vector density, biting rate, survival, vector competence, and the pathogen extrinsic incubation period all influence pathogen transmission. Recent studies also indicate that mosquito gut-associated microbes can impact each of these components, and therefore ultimately modulate vectorial capacity. Promisingly, this expands the options available to exploit microbes for vector control by also targeting parameters that affect vectorial capacity. However, there are still many knowledge gaps regarding  mosquito-microbe interactions  that need to be addressed in order to exploit them efficiently. Here, we review current evidence of impacts of the microbiome on aspects of vectorial capacity, and we highlight likely opportunities for novel vector control strategies and areas where further studies are required. Video abstract.

Comprehensive Ecological and Geographic Characterization of Eukaryotic and Prokaryotic Microbiomes in African Anopheles

Exposure of mosquitoes to numerous eukaryotic and prokaryotic microbes in their associated microbiomes has probably helped drive the evolution of the innate immune system. To our knowledge, a metagenomic catalog of the eukaryotic microbiome has not been reported from any insect. Here we employ a novel approach to preferentially deplete host 18S ribosomal RNA gene amplicons to reveal the composition of the eukaryotic microbial communities of Anopheles larvae sampled in Kenya, Burkina Faso and Republic of Guinea (Conakry). We identified 453 eukaryotic operational taxonomic units (OTUs) associated with Anopheles larvae in nature, but an average of 45% of the 18S rRNA sequences clustered into OTUs that lacked a taxonomic assignment in the Silva database. Thus, the Anopheles microbiome contains a striking proportion of novel eukaryotic taxa. Using sequence similarity matching and de novo phylogenetic placement, the fraction of unassigned sequences was reduced to an average of 4%, and many unclassified OTUs were assigned as relatives of known taxa. A novel taxon of the genus Ophryocystis in the phylum Apicomplexa (which also includes Plasmodium) is widespread in Anopheles larvae from East and West Africa. Notably, Ophryocystis is present at fluctuating abundance among larval breeding sites, consistent with the expected pattern of an epidemic pathogen. Species richness of the eukaryotic microbiome was not significantly different across sites from East to West Africa, while species richness of the prokaryotic microbiome was significantly lower in West Africa. Laboratory colonies of Anopheles coluzzii harbor 26 eukaryotic OTUs, of which 38% (n = 10) are shared with wild populations, while 16 OTUs are unique to the laboratory colonies. Genetically distinct An. coluzzii colonies co-housed in the same facility maintain different prokaryotic microbiome profiles, suggesting a persistent host genetic influence on microbiome composition. These results provide a foundation to understand the role of the Anopheles eukaryotic microbiome in vector immunity and pathogen transmission. We hypothesize that prevalent apicomplexans such as Ophryocystis associated with Anopheles could induce interference or competition against Plasmodium within the vector. This and other members of the eukaryotic microbiome may offer candidates for new vector control tools.

Interaction of Viruses with the Insect Intestine

In nature, insects face a constant threat of infection by numerous exogeneous viruses, and their intestinal tracts are the predominant ports of entry. Insects can acquire these viruses orally during either blood feeding by hematophagous insects or sap sucking and foliage feeding by insect herbivores. However, the insect intestinal tract forms several physical and immunological barriers to defend against viral invasion, including cell intrinsic antiviral immunity, the peritrophic matrix and the mucin layer, and local symbiotic microorganisms. Whether an infection can be successfully established in the intestinal tract depends on the complex interactions between viruses and those barriers. In this review, we summarize recent progress on virus-intestinal tract interplay in insects, in which various underlying mechanisms derived from nutritional status, dynamics of symbiotic microorganisms, and virus-encoded components play intricate roles in the regulation of virus invasion in the intestinal tract, either directly or indirectly. Expected final online publication date for the Annual Review of Virology, Volume 8 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Converting endogenous genes of the malaria mosquito into simple non-autonomous gene drives for population replacement

Gene drives for mosquito population replacement are promising tools for malaria control. However, there is currently no clear pathway for safely testing such tools in endemic countries. The lack of well-characterized promoters for infection-relevant tissues and regulatory hurdles are further obstacles for their design and use. Here we explore how minimal genetic modifications of endogenous mosquito genes can convert them directly into non-autonomous gene drives without disrupting their expression. We co-opted the native regulatory sequences of three midgut-specific loci of the malaria vector Anopheles gambiae to host a prototypical antimalarial molecule and guide-RNAs encoded within artificial introns that support efficient gene drive. We assess the propensity of these modifications to interfere with the development of Plasmodium falciparum and their effect on fitness. Because of their inherent simplicity and passive mode of drive such traits could form part of an acceptable testing pathway of gene drives for malaria eradication.

Mosquito Trilogy: Microbiota, Immunity and Pathogens, and Their Implications for the Control of Disease Transmission

In mosquitoes, the interaction between the gut microbiota, the immune system, and the pathogens that these insects transmit to humans and animals is regarded as a key component toward the development of control strategies, aimed at reducing the burden of severe diseases, such as malaria and dengue fever. Indeed, different microorganisms from the mosquito microbiota have been investigated for their ability to affect important traits of the biology of the host insect, related with its survival, development and reproduction. Furthermore, some microorganisms have been shown to modulate the immune response of mosquito females, significantly shaping their vector competence. Here, we will review current knowledge in this field, focusing on i) the complex interaction between the intestinal microbiota and mosquito females defenses, both in the gut and at humoral level; ii) how knowledge on these issues contributes to the development of novel and targeted strategies for the control of mosquito-borne diseases such as the use of paratransgenesis or taking advantage of the relationship between Wolbachia and mosquito hosts. We conclude by providing a brief overview of available knowledge on microbiota-immune system interplay in major insect vectors.

Novel Asaia bogorensis Signal Sequences for Plasmodium Inhibition in Anopheles stephensi

Mosquitoes vector many pathogens that cause human disease, such as malaria that is caused by parasites in the genus Plasmodium. Current strategies to control vector-transmitted diseases are hindered by mosquito and pathogen resistance, so research has turned to altering the microbiota of the vectors. In this strategy, called paratransgenesis, symbiotic bacteria are genetically modified to affect the mosquito's phenotype by engineering them to deliver antiplasmodial effector molecules into the midgut to kill parasites. One paratransgenesis candidate is Asaia bogorensis, a Gram-negative, rod-shaped bacterium colonizing the midgut, ovaries, and salivary glands of Anopheles sp. mosquitoes. However, common secretion signals from E. coli and closely related species do not function in Asaia. Here, we report evaluation of 20 native Asaia N-terminal signal sequences predicted from bioinformatics for their ability to mediate increased levels of antiplasmodial effector molecules directed to the periplasm and ultimately outside the cell. We tested the hypothesis that by increasing the amount of antiplasmodials released from the cell we would also increase parasite killing power. We scanned the Asaia bogorensis SF2.1 genome to identify signal sequences from extra-cytoplasmic proteins and fused these to the reporter protein alkaline phosphatase. Six signals resulted in significant levels of protein released from the Asaia bacterium. Three signals were successfully used to drive the release of the antimicrobial peptide, scorpine. Further testing in mosquitoes demonstrated that these three Asaia strains were able to suppress the number of oocysts formed after a blood meal containing P. berghei to a significantly greater degree than wild-type Asaia, although prevalence was not decreased beyond levels obtained with a previously isolated siderophore receptor signal sequence. We interpret these results to indicate that there is a maximum level of suppression that can be achieved when the effectors are constitutively driven due to stress on the symbionts. This suggests that simply increasing the amount of antiplasmodial effector molecules in the midgut is insufficient to create superior paratransgenic bacterial strains and that symbiont fitness must be considered as well.

Hidden genomic features of an invasive malaria vector, Anopheles stephensi, revealed by a chromosome-level genome assembly

BACKGROUND

The mosquito Anopheles stephensi is a vector of urban malaria in Asia that recently invaded Africa. Studying the genetic basis of vectorial capacity and engineering genetic interventions are both impeded by limitations of a vector's genome assembly. The existing assemblies of An. stephensi are draft-quality and contain thousands of sequence gaps, potentially missing genetic elements important for its biology and evolution.

RESULTS

To access previously intractable genomic regions, we generated a reference-grade genome assembly and full transcript annotations that achieve a new standard for reference genomes of disease vectors. Here, we report novel species-specific transposable element (TE) families and insertions in functional genetic elements, demonstrating the widespread role of TEs in genome evolution and phenotypic variation. We discovered 29 previously hidden members of insecticide resistance genes, uncovering new candidate genetic elements for the widespread insecticide resistance observed in An. stephensi. We identified 2.4 Mb of the Y chromosome and seven new male-linked gene candidates, representing the most extensive coverage of the Y chromosome in any mosquito. By tracking full-length mRNA for > 15 days following blood feeding, we discover distinct roles of previously uncharacterized genes in blood metabolism and female reproduction. The Y-linked heterochromatin landscape reveals extensive accumulation of long-terminal repeat retrotransposons throughout the evolution and degeneration of this chromosome. Finally, we identify a novel Y-linked putative transcription factor that is expressed constitutively throughout male development and adulthood, suggesting an important role.

CONCLUSION

Collectively, these results and resources underscore the significance of previously hidden genomic elements in the biology of malaria mosquitoes and will accelerate the development of genetic control strategies of malaria transmission.

Considerations for mosquito microbiome research from the Mosquito Microbiome Consortium

In the past decade, there has been increasing interest in mosquito microbiome research, leading to large amounts of data on different mosquito species, with various underlying physiological characteristics, and from diverse geographical locations. However, guidelines and standardized methods for conducting mosquito microbiome research are lacking. To streamline methods in mosquito microbiome research and optimize data quality, reproducibility, and comparability, as well as facilitate data curation in a centralized location, we are establishing the Mosquito Microbiome Consortium, a collaborative initiative for the advancement of mosquito microbiome research. Our overall goal is to collectively work on unraveling the role of the mosquito microbiome in mosquito biology, while critically evaluating its potential for mosquito-borne disease control. This perspective serves to introduce the consortium and invite broader participation. It highlights the issues we view as most pressing to the community and proposes guidelines for conducting mosquito microbiome research. We focus on four broad areas in this piece: (1) sampling/experimental design for field, semi-field, or laboratory studies; (2) metadata collection; (3) sample processing, sequencing, and use of appropriate controls; and (4) data handling and analysis. We finally summarize current challenges and highlight future directions in mosquito microbiome research. We hope that this piece will spark discussions around this area of disease vector biology, as well as encourage careful considerations in the design and implementation of mosquito microbiome research. Video Abstract.

Self-limiting paratransgenesis

Presently, the principal tools to combat malaria are restricted to killing the parasite in infected people and killing the mosquito vector to thwart transmission. While successful, these approaches are losing effectiveness in view of parasite resistance to drugs and mosquito resistance to insecticides. Clearly, new approaches to fight this deadly disease need to be developed. Recently, one such approach–engineering mosquito resident bacteria to secrete anti-parasite compounds–has proven in the laboratory to be highly effective. However, implementation of this strategy requires approval from regulators as it involves introduction of recombinant bacteria into the field. A frequent argument by regulators is that if something unexpectedly goes wrong after release, there must be a recall mechanism. This report addresses this concern. Previously we have shown that a Serratia bacterium isolated from a mosquito ovary is able to spread through mosquito populations and is amenable to be engineered to secrete anti-plasmodial compounds. We have introduced a plasmid into this bacterium that carries a fluorescent protein gene and show that when cultured in the laboratory, the plasmid is completely lost in about 130 bacterial generations. Importantly, when these bacteria were introduced into mosquitoes, the bacteria were transmitted from one generation to the next, but the plasmid was lost after three mosquito generations, rendering the bacteria non-recombinant (wild type). Furthermore, no evidence was obtained for horizontal transfer of the plasmid to other bacteria either in culture or in the mosquito. Prior to release, it is imperative to demonstrate that the genes that thwart parasite development in the mosquito are safe to the environment. This report describes a methodology to safely achieve this goal, utilizing transient expression from a plasmid that is gradually lost, returning the bacterium to wild type status.

The intolerable burden of malaria demands the development of novel approaches to fight this deadly disease. Previously, the engineered symbiotic bacterium Serratia AS1 was shown in the laboratory to be able to spread through mosquito populations and render mosquitoes resistant to the parasite. However, a major concern of regulators for the release of such engineered bacteria into the field is that there is no option for “recall” in case something goes wrong. Here we show that Serratia AS1 loses plasmids as it replicates in mosquitoes and in culture, reverting to wild type. We also show that horizontal transfer of the plasmid from Serratia AS1 to other bacteria is extremely rare, as this was undetectable in the mosquito and in culture. Our results suggest that initial field trials can make use of this reversible system whereby released recombinant bacteria expressing anti-plasmodial compounds from a plasmid revert to wild type at a predictable rate.

A Systematic Analysis of Mosquito-Microbiome Biosynthetic Gene Clusters Reveals Antimalarial Siderophores that Reduce Mosquito Reproduction Capacity

Advances in infectious disease control strategies through genetic manipulation of insect microbiomes have heightened interest in microbially produced small molecules within mosquitoes. Herein, 33 mosquito-associated bacterial genomes were mined and over 700 putative biosynthetic gene clusters (BGCs) were identified, 135 of which belong to known classes of BGCs. After an in-depth analysis of the 135 BGCs, iron-binding siderophores were chosen for further investigation due to their high abundance and well-characterized bioactivities. Through various metabolomic strategies, eight siderophore scaffolds were identified in six strains of mosquito-associated bacteria. Among these, serratiochelin A and pyochelin were found to reduce female Anopheles gambiae overall fecundity likely by lowering their blood-feeding rate. Serratiochelin A and pyochelin were further found to inhibit the Plasmodium parasite asexual blood and liver stages in vitro. Our work supplies a bioinformatic resource for future mosquito-microbiome studies and highlights an understudied source of bioactive small molecules.

The need for new vector control approaches targeting outdoor biting Anopheline malaria vector communities

Since the implementation of Roll Back Malaria, the widespread use of insecticide-treated nets (ITNs) and indoor residual spraying (IRS) is thought to have played a major part in the decrease in mortality and morbidity achieved in malaria-endemic regions. In the past decade, resistance to major classes of insecticides recommended for public health has spread across many malaria vector populations. Increasingly, malaria vectors are also showing changes in vector behaviour in response to current indoor chemical vector control interventions. Changes in the time of biting and proportion of indoor biting of major vectors, as well as changes in the species composition of mosquito communities threaten the progress made to control malaria transmission. Outdoor biting mosquito populations contribute to malaria transmission in many parts of sub-Saharan Africa and pose new challenges as they cannot be reliably monitored or controlled using conventional tools. Here, we review existing and novel approaches that may be used to target outdoor communities of malaria vectors. We conclude that scalable tools designed specifically for the control and monitoring of outdoor biting and resting malaria vectors with increasingly complex and dynamic responses to intensifying malaria control interventions are urgently needed. These are crucial for integrated vector management programmes designed to challenge current and future vector populations.

Roles of Symbiotic Microorganisms in Arboviral Infection of Arthropod Vectors

Arthropod vectors serve as native reservoirs and transmitters of hundreds of arboviruses. In arthropod vectors, symbiotic microorganisms residing in the gut lumen and/or hemocoelic tissues maintain complicated relationships with their host and influence multiple aspects of vector physiology. Recently, accumulating evidence has established an important role for symbiotic microorganisms in vector-virus interactions which could potentially be used to control viral transmission. Herein, we review recent progress on symbiotic microbe-arbovirus interactions and summarize the molecular mechanisms by which commensal microbes act on hosts and arboviruses. Understanding the sophisticated interactions among arthropod vectors, microbiota, and arboviruses may offer new strategies for the prevention of arboviral diseases in the future.

Role of plants in the transmission of Asaia sp., which potentially inhibit the Plasmodium sporogenic cycle in Anopheles mosquitoes

Biological control against malaria and its transmission is currently a considerable challenge. Plant-associated bacteria of the genus Asaia are frequently found in nectarivorous arthropods, they thought to have a natural indirect action on the development of plasmodium in mosquitoes. However, virtually nothing is known about its natural cycle. Here, we show the role of nectar-producing plants in the hosting and dissemination of Asaia. We isolated Asaia strains from wild mosquitoes and flowers in Senegal and demonstrated the transmission of the bacteria from infected mosquitoes to sterile flowers and then to 26.6% of noninfected mosquitoes through nectar feeding. Thus, nectar-producing plants may naturally acquire Asaia and then colonize Anopheles mosquitoes through food-borne contamination. Finally, Asaia may play an indirect role in the reduction in the vectorial capacity of Anopheles mosquitoes in a natural environment (due to Plasmodium-antagonistic capacities of Asaia) and be used in the development of tools for Asaia-based paratransgenetic malaria control.

Mosquito-Borne Diseases Emergence/Resurgence and How to Effectively Control It Biologically

Deadly pathogens and parasites are transmitted by vectors and the mosquito is considered the most threatening vector in public health, transmitting these pathogens to humans and animals. We are currently witnessing the emergence/resurgence in new regions/populations of the most important mosquito-borne diseases, such as arboviruses and malaria. This resurgence may be the consequence of numerous complex parameters, but the major cause remains the mismanagement of insecticide use and the emergence of resistance. Biological control programmes have rendered promising results but several highly effective techniques, such as genetic manipulation, remain insufficiently considered as a control mechanism. Currently, new strategies based on attractive toxic sugar baits and new agents, such as Wolbachia and Asaia, are being intensively studied for potential use as alternatives to chemicals. Research into new insecticides, Insect Growth Regulators, and repellent compounds is pressing, and the improvement of biological strategies may provide key solutions to prevent outbreaks, decrease the danger to at-risk populations, and mitigate resistance.

Use of Microbiota to Fight Mosquito-Borne Disease

Mosquito-borne diseases cause more than 700 million people infected and one million people die (Caraballo and King, 2014). With the limitations of progress toward elimination imposed by insecticide- and drug-resistance, combined with the lack of vaccines, innovative strategies to fight mosquito-borne disease are urgently needed. In recent years, the use of mosquito microbiota has shown great potential for cutting down transmission of mosquito-borne pathogens. Here we review what is known about the mosquito microbiota and how this knowledge is being used to develop new ways to control mosquito-borne disease. We also discuss the challenges for the eventual release of genetically modified (GM) symbionts in the field.

Chimeric symbionts expressing a Wolbachia protein stimulate mosquito immunity and inhibit filarial parasite development

Wolbachia can reduce the capability of mosquitoes to transmit infectious diseases to humans and is currently exploited in campaigns for the control of arboviruses, like dengue and Zika. Under the assumption that Wolbachia-mediated activation of insect immunity plays a role in the reduction of mosquito vectorial capacity, we focused our attention on the Wolbachia surface protein (WSP), a potential inductor of innate immunity. We hypothesized that the heterologous expression of this protein in gut- and tissue-associated symbionts may reduce parasite transmission. We thus engineered the mosquito bacterial symbiont Asaia to express WSP (Asaia^WSP). Asaia^WSP induced activation of the host immune response in Aedes aegypti and Anopheles stephensi mosquitoes, and inhibited the development of the heartworm parasite Dirofilaria immitis in Ae. aegypti. These results consolidate previous evidence on the immune-stimulating property of WSP and make Asaia^WSP worth of further investigations as a potential tool for the control of mosquito-borne diseases.

Epis and Varotto-Boccazzi et al. show that Wolbachia surface protein (WSP) activates host innate immunity in mosquitoes, inhibiting the development of the heartworm parasite in its insect host. This study suggests the possibility that the WSP-expressing symbiont may be harnessed to control mosquito-borne diseases.

Mosquito Microbiota and Implications for Disease Control

Mosquito-transmitted diseases account for about 500 000 deaths every year. Blocking these pathogens in the mosquito vector before they are transmitted to humans is an effective strategy to prevent mosquito-borne diseases. Like most higher organisms, mosquitoes harbor a highly diverse and dynamic microbial flora that can be explored for prevention of pathogen transmission. Here we review the structure and function of the mosquito microbiota, including bacteria, fungi, and viruses, and discuss the potential of using components of the microbiota to thwart pathogen transmission.

Asaia Activates Immune Genes in Mosquito Eliciting an Anti-Plasmodium Response: Implications in Malaria Control

In mosquitoes, the discovery of the numerous interactions between components of the microbiota and the host immune response opens up the attractive possibility of the development of novel control strategies against mosquito borne diseases. We have focused our attention to Asaia, a symbiont of several mosquito vectors who has been proposed as one of the most potential tool for paratransgenic applications; although being extensively characterized, its interactions with the mosquito immune system has never been investigated. Here we report a study aimed at describing the interactions between Asaia and the immune system of two vectors of malaria, Anophelesstephensi and An. gambiae. The introduction of Asaia isolates induced the activation of the basal level of mosquito immunity and lower the development of malaria parasite in An. stephensi. These findings confirm and expand the potential of Asaia in mosquito borne diseases control, not only through paratransgenesis, but also as a natural effector for mosquito immune priming.

[Mosquito microbiota and its influence on disease vectorial transmission]

Mosquitoes (Diptera: Culicidae) are found worldwide. Around 100 among 3500 mosquito species are known to be vectors of parasites and viruses, responsible for infectious diseases including malaria and dengue. Mosquitoes host diverse microbial communities that influence disease transmission, either by direct interference or via affecting host immunity and physiology. These microbial communities are present within diverse tissues, including the digestive tract, and vary depending on the sex of the mosquito, its developmental stage, and ecological factors. This review summarizes the current knowledge about the mosquito microbiota, defined as a community of commensal, symbiotic or pathogenic microbes harboured by a host. We first describe the current knowledge on the diversity of the microbiota, that includes bacteria, fungi, parasites and viruses and on its modes of acquisition throughout the mosquito life cycle. We then focus on microbial interactions within the mosquito gut, which notably affect vector competence, and on host-microbe interactions affecting mosquito fitness. Finally, we discuss current or potential methods based on the use of microbes or microbial products to interfere with pathogen transmission or to reduce mosquito lifespan and reproduction.