Re-introducing bacteria in mosquitoes--a method for determination of mosquito feeding preferences based on coloured sugar solutions

Two promising candidates for paratransgenesis, Elizabethkingia and Asaia, increase in both sexes of Anopheles gambiae mosquitoes after feeding

BACKGROUND

The male mosquito microbiome may be important for identifying ideal candidates for disease control. Among other criteria, mosquito-associated symbionts that have high localization in both male and female mosquitoes and are transmissible through both vertical and sexual routes are desirable. However, mosquito microbiome studies have mainly been female-focused. In this study, the microbiota of male and female Anopheles gambiae sensu lato (s.l.) were compared to identify shared or unique bacteria.

METHODS

Late larval instars of Anopheles mosquitoes were collected from the field and raised to adults. Equal numbers of males and females of 1-day-old non-sugar-fed, 4-5-day-old sugar-fed and post-blood-fed females were randomly selected for whole-body analyses of bacteria 16S rRNA.

RESULTS

Results revealed that male and female mosquitoes generally share similar microbiota except when females were blood-fed. Compared to newly emerged unfed mosquitoes, feeding on sugar and/or blood increased variability in microbial composition (⍺-diversity), with a higher disparity among females (39% P = 0.01) than in males (29% P = 0.03). Elizabethkingia meningoseptica and Asaia siamensis were common discriminants between feeding statuses in both males and females. While E. meningoseptica was particularly associated with sugar-fed mosquitoes of both sexes and sustained after blood feeding in females, A. siamensis was also increased in sugar-fed mosquitoes but decreased significantly in blood-fed females (LDA score > 4.0, P < 0.05). Among males, A. siamensis did not differ significantly after sugar meals.

CONCLUSIONS

Results indicate the opportunities for stable infection in mosquitoes should these species be used in bacteria-mediated disease control. Further studies are recommended to investigate possible host-specific tissue tropism of bacteria species which will inform selection of the most appropriate microbes for effective transmission-blocking strategies.

A recombinant Aspergillus oryzae fungus transmitted from larvae to adults of Anopheles stephensi mosquitoes inhibits malaria parasite oocyst development

The control of malaria parasite transmission from mosquitoes to humans is hampered by decreasing efficacies of insecticides, development of drug resistance against the last-resort antimalarials, and the absence of effective vaccines. Herein, the anti-plasmodial transmission blocking activity of a recombinant Aspergillus oryzae (A. oryzae-R) fungus strain, which is used in human food industry, was investigated in laboratory-reared Anopheles stephensi mosquitoes. The recombinant fungus strain was genetically modified to secrete two anti-plasmodial effector peptides, MP2 (midgut peptide 2) and EPIP (enolase-plasminogen interaction peptide) peptides. The transstadial transmission of the fungus from larvae to adult mosquitoes was confirmed following inoculation of A. oryzae-R in the water trays used for larval rearing. Secretion of the anti-plasmodial effector peptides inside the mosquito midguts inhibited oocyst formation of P. berghei parasites. These results indicate that A. oryzae can be used as a paratransgenesis model carrying effector proteins to inhibit malaria parasite development in An. stephensi. Further studies are needed to determine if this recombinant fungus can be adapted under natural conditions, with a minimal or no impact on the environment, to target mosquito-borne infectious disease agents inside their vectors.

Maximizing the Potential of Attractive Targeted Sugar Baits (ATSBs) for Integrated Vector Management

Due to the limitations of the human therapeutics and vaccines available to treat and prevent mosquito-borne diseases, the primary strategy for disease mitigation is through vector control. However, the current tools and approaches used for mosquito control have proven insufficient to prevent malaria and arboviral infections, such as dengue, Zika, and lymphatic filariasis, and hence, these diseases remain a global public health threat. The proven ability of mosquito vectors to adapt to various control strategies through insecticide resistance, invasive potential, and behavioral changes from indoor to outdoor biting, combined with human failures to comply with vector control requirements, challenge sustained malaria and arboviral disease control worldwide. To address these concerns, increased efforts to explore more varied and integrated control strategies have emerged. These include approaches that involve the behavioral management of vectors. Attractive targeted sugar baits (ATSBs) are a vector control approach that manipulates and exploits mosquito sugar-feeding behavior to deploy insecticides. Although traditional approaches have been effective in controlling malaria vectors indoors, preventing mosquito bites outdoors and around human dwellings is challenging. ATSBs, which can be used to curb outdoor biting mosquitoes, have the potential to reduce mosquito densities and clinical malaria incidence when used in conjunction with existing vector control strategies. This review examines the available literature regarding the utility of ATSBs for mosquito control, providing an overview of ATSB active ingredients (toxicants), attractants, modes of deployment, target organisms, and the potential for integrating ATSBs with existing vector control interventions.

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.

Delivery of a Genetically Marked Serratia AS1 to Medically Important Arthropods for Use in RNAi and Paratransgenic Control Strategies

Understanding how arthropod vectors acquire their bacteria is essential for implementation of paratransgenic and RNAi strategies using genetically modified bacteria to control vector-borne diseases. In this study, a genetically marked Serratia AS1 strain expressing the mCherry fluorescent protein (mCherry-Serratia) was used to test various acquisition routes in six arthropod vectors including Anopheles stephensi, Culex pipiens, Cx. quinquefaciatus, Cx. theileri, Phlebotomus papatasi, and Hyalomma dromedarii. Depending on the species, the bacteria were delivered to (i) mosquito larval breeding water, (ii) host skin, (iii) sugar bait, and (iv) males (paratransgenic). The arthropods were screened for the bacteria in their guts or other tissues. All the hematophagous arthropods were able to take the bacteria from the skin of their hosts while taking blood meal. The mosquitoes were able to take up the bacteria from the water at larval stages and to transfer them transstadially to adults and finally to transfer them to the water they laid eggs in. The mosquitoes were also able to acquire the bacteria from male sperm. The level of bacterial acquisition was influenced by blood feeding time and strategies (pool or vessel feeding), dipping in water and resting time of newly emerged adult mosquitoes, and the disseminated tissue/organ. Transstadial, vertical, and venereal bacterial acquisition would increase the sustainability of the modified bacteria in vector populations and decrease the need for supplementary release experiments whereas release of paratransgenic males that do not bite has fewer ethical issues. Furthermore, this study is required to determine if the modified bacteria can be introduced to arthropods in the same routes in nature.

Field assessment of potential sugar feeding stations for disseminating bacteria in a paratransgenic approach to control malaria

BACKGROUND

Using bacteria to express and deliver anti-parasite molecules in mosquitoes is among the list of genetic tools to control malaria. The introduction and spread of transgenic bacteria through wild adult mosquitoes is one of the major challenges of this strategy. In prospect of future field experiments, an open field study with blank (without bacteria) attractive sugar bait (ASB) was performed under the assumption that transgenic bacteria would be spread to all sugar fed mosquitoes.

METHODS

Two types of ASB stations were developed, one with clay pots (CP) placed at mosquito resting sites and one with window entry traps (WET) placed inside inhabited houses. The ASB consisted in either glucose, honey or fruit cocktail solutions. In addition, mark-release-recapture (MRR) experiment of mosquitoes after feeding them with glucose was also conducted to check the proportion of the mosquito population that can be reached by the two ASB stations as well as its suitability to complement the ASB stations for disseminating bacteria.

RESULTS

Overall, 88% of the mosquitoes were collected in the WET_ASB. The CP_ASB stations were much less attractive with the highest average of 82 ± 11 mosquitoes/day in the CP near the wood piles. The proportions of sugar fed mosquitoes upon ASB were low in both type of ASB stations, ~ 2% and ~ 14% in WET and CP, respectively. Honey solution was the most attractive solution compared to the glucose and the fruit cocktail solutions. The recapture rate in the MRR experiment was low: ~ 4.1% over 7 days.

CONCLUSION

The WET_ASB looks promising to disseminate transgenic bacteria to endophilic West Africa Anopheles mosquito. However, this feeding station may not be fully effective and could be combined with the CP_ASB to also target outdoor resting mosquitoes. Overall, efforts are needed to improve the mosquito-feeding rates upon ASB.

The potential for flower nectar to allow mosquito to mosquito transmission of Francisella tularensis

Francisella tularensis is disseminated in nature by biting arthropods such as mosquitoes. The relationship between mosquitoes and F. tularensis in nature is highly ambiguous, due in part to the fact that mosquitoes have caused significant tularemia outbreaks despite being classified as a mechanical vector of F. tularensis. One possible explanation for mosquitoes being a prominent, yet mechanical vector is that these insects feed on flower nectar between blood meals, allowing for transmission of F. tularensis between mosquitoes. Here, we aimed to assess whether F. tularensis could survive in flower nectar. Moreover, we examined if mosquitoes could interact with or ingest and transmit F. tularensis from one source of nectar to another. F. tularensis exhibited robust survivability in flower nectar with concentrations of viable bacteria remaining consistent with the rich growth medium. Furthermore, F. tularensis was able to survive (albeit to a lesser extent) in 30% sucrose (a nectar surrogate) over a period of time consistent with that of a typical flower bloom. Although we observed diminished bacterial survival in the nectar surrogate, mosquitoes that fed on this material became colonized with F. tularensis. Finally, colonized mosquitoes were capable of transferring F. tularensis to a sterile nectar surrogate. These data suggest that flower nectar may be capable of serving as a temporary source of F. tularensis that could contribute to the amplification of outbreaks. Mosquitoes that feed on an infected mammalian host and subsequently feed on flower nectar could deposit some F. tularensis bacteria into the nectar in the process. Mosquitoes subsequently feeding on this nectar source could potentially become colonized by F. tularensis. Thus, the possibility exists that flower nectar may allow for vector-vector transmission of F. tularensis.

Attractive Toxic Sugar Bait (ATSB) For Control of Mosquitoes and Its Impact on Non-Target Organisms: A Review

Mosquito abatement programs contend with mosquito-borne diseases, insecticidal resistance, and environmental impacts to non-target organisms. However, chemical resources are limited to a few chemical classes with similar modes of action, which has led to insecticide resistance in mosquito populations. To develop a new tool for mosquito abatement programs that control mosquitoes while combating the issues of insecticidal resistance, and has low impacts of non-target organisms, novel methods of mosquito control, such as attractive toxic sugar baits (ATSBs), are being developed. Whereas insect baiting to dissuade a behavior, or induce mortality, is not a novel concept, as it was first introduced in writings from 77 AD, mosquito baiting through toxic sugar baits (TSBs) had been quickly developing over the last 60 years. This review addresses the current body of research of ATSB by providing an overview of active ingredients (toxins) include in TSBs, attractants combined in ATSB, lethal effects on mosquito adults and larvae, impact on non-target insects, and prospects for the use of ATSB.

Evans blue as a simple method to discriminate mosquitoes' feeding choice on small laboratory animals

Background

Temperature, humidity, vision, and particularly odor, are external cues that play essential roles to mosquito blood feeding and oviposition. Entomological and behavioral studies employ well-established methods to evaluate mosquito attraction or repellency and to identify the source of the blood meal. Despite the efficacy of such methods, the costs involved in the production or acquisition of all parts, components and the chemical reagents involved are unaffordable for most researchers from poor countries. Thus, a simple and relatively low-cost method capable of evaluating mosquito preferences and the blood volume ingested is desirable.

Principal Findings

By using Evans blue (EB) vital dye and few standard laboratory supplies, we developed and validated a system capable of evaluating mosquito’s choice between two different host sources of blood. EB-injected and PBS-injected mice submitted to a number of situations were placed side by side on the top of a rounded recipient covered with tulle fabric and containing Aedes aegypti mosquitoes. Homogenates from engorged mosquitoes clearly revealed the blood source (EB- or PBS-injected host), either visually or spectrometrically. This method was able to estimate the number of engorded mosquitoes, the volume of blood ingested, the efficacy of a commercial repellent and the attractant effects of black color and human sweat.

Significance

Despite the obvious limitations due to its simplicity and to the dependence of a live source of blood, the present method can be used to assess a number of host variables (diet, aging, immunity, etc) and optimized for several aspects of mosquito blood feeding and vector-host interactions. Thus, it is proposed as an alternative to field studies, and it could be used for initial screenings of chemical compound candidates for repellents or attractants, since it replicates natural conditions of exposure to mosquitoes in a laboratory environment.

Arranging the bouquet of disease: floral traits and the transmission of plant and animal pathogens

Several floral microbes are known to be pathogenic to plants or floral visitors such as pollinators. Despite the ecological and economic importance of pathogens deposited in flowers, we often lack a basic understanding of how floral traits influence disease transmission. Here, we provide the first systematic review regarding how floral traits attract vectors (for plant pathogens) or hosts (for animal pathogens), mediate disease establishment and evolve under complex interactions with plant mutualists that can be vectors for microbial antagonists. Attraction of floral visitors is influenced by numerous phenological, morphological and chemical traits, and several plant pathogens manipulate floral traits to attract vectors. There is rapidly growing interest in how floral secondary compounds and antimicrobial enzymes influence disease establishment in plant hosts. Similarly, new research suggests that consumption of floral secondary compounds can reduce pathogen loads in animal pollinators. Given recent concerns about pollinator declines caused in part by pathogens, the role of floral traits in mediating pathogen transmission is a key area for further research. We conclude by discussing important implications of floral transmission of pathogens for agriculture, conservation and human health, suggesting promising avenues for future research in both basic and applied biology.

Proof of concept for a novel insecticide bioassay based on sugar feeding by adult Aedes aegypti (Stegomyia aegypti)

Aedes aegypti L. (Stegomyia aegypti) (Diptera: Culicidae) is the principal vector of dengue and yellow fever viruses in tropical and subtropical regions of the world. Disease management is largely based on mosquito control achieved by insecticides applied to interior resting surfaces and through space sprays. Population monitoring to detect insecticide resistance is a significant component of integrated disease management programmes. We developed a bioassay method for assessing insecticide susceptibility based on the feeding activity of mosquitoes on plant sugars. Our prototype sugar-insecticide feeding bioassay system was composed of inexpensive, disposable components, contained minimal volumes of insecticide, and was compact and highly transportable. Individual mosquitoes were assayed in a plastic cup that contained a sucrose-permethrin solution. Trypan blue dye was added to create a visual marker in the mosquito's abdomen for ingested sucrose-permethrin solution. Blue faecal spots provided further evidence of solution ingestion. With the sugar-insecticide feeding bioassay, the permethrin susceptibility of Ae. aegypti females from two field-collected strains was characterized by probit analysis of dosage-response data. The field strains were also tested by forced contact of females with permethrin residues on filter paper. Dosage-response patterns were similar, indicating that the sugar-insecticide feeding bioassay had appropriately characterized the permethrin susceptibility of the two strains.

Effects of Bacillus thuringiensis israelensis on Anopheles arabiensis

Studies have shown that Culex quinquefasciatus oviposits fewer eggs in water treated with Bacillus thuringiensis var. israelensis (Bti). The present study examined the effects of Bti on adults of Anopheles arabiensis. Anopheles arabiensis oviposited in both treated and untreated water with a similar frequency. The number of eggs laid did not significantly differ between the treatments. Adult mosquitoes ingested Bti solution, but it did not significantly shorten their survival time. The neutral effects of Bti on ovipositing An. arabiensis do not reduce its effectiveness as a larvicide for malaria vector control.

Oviposition responses of Anopheles gambiae s.s. (Diptera: Culicidae) and identification of volatiles from bacteria-containing solutions

In this study, a dual-choice oviposition bioassay was used to screen responses of gravid An. gambiae toward 17 bacterial species, previously isolated from Anopheles gambiae s.l. (Diptera: Culicidae) midguts or oviposition sites. The 10 isolates from oviposition sites have been identified by phylogenetic analyses of their 16S rRNA genes. Eight of the 10 isolates were gram-positive, out of which six belonged to the Bacilli class. Solid phase microextraction and gas chromatography coupled to mass spectrometry (GC-MS) were used to identify the volatiles emitted from the bacterial isolates. Aromatic and aliphatic alcohols, aliphatic ketones, alkylpyrazines, dimethyl oligosulfides, and indole were among the chemical compounds identified from the headspace above bacteria-containing saline. The mosquitoes laid significantly more eggs in six of the bacteria-containing solutions compared with the sterile solution. These six bacteria did not emit any compounds in common that could explain the positive oviposition response. Instead, the bacteria were grouped according to principal component analysis (PCA) based on the relative amounts of volatiles emitted. The PCA-plots facilitated the identification of 13 putative oviposition attractants for An. gambiae mosquitoes.

Transstadial and horizontal transfer of bacteria within a colony of Anopheles gambiae (Diptera: Culicidae) and oviposition response to bacteria-containing water

In a paratransgenic approach, genetically modified bacteria are utilized to kill the parasite in the vector gut. A critical component for paratransgenics against malaria is how transgenic bacteria can be introduced and then kept in a mosquito population. Here, we investigated transstadial and horizontal transfer of bacteria within an Anopheles gambiae mosquito colony with the focus on spiked breeding sites as a possible means of introducing bacteria to mosquitoes. A Pantoea stewartii strain, previously isolated from An. gambiae, marked with a green fluorescent protein (GFP), was introduced to mosquitoes in different life stages. The following life stages or older mosquitoes in the case of adults were screened for bacteria in their guts. In addition to P. stewartii other bacteria were isolated from the guts: these were identified by 16S rRNA sequence analysis and temporal temperature gradient gel electrophoresis (TTGE). Bacteria were transferred from larvae to pupae but not from pupae to adults. The mosquitoes were able to take up bacteria from the water they emerged from and transfer the same bacteria to the water they laid eggs in. Elizabethkingia meningoseptica was more often isolated from adult mosquitoes than P. stewartii. A bioassay was used to examine An. gambiae oviposition responses towards bacteria-containing solutions. The volatiles emitted from the solutions were sampled by headspace-solid phase microextraction (SPME) and identified by gas chromatography and mass spectrometry (GC-MS) analysis. P. stewartii but not E. meningoseptica mediated a positive oviposition response. The volatiles emitted by P. stewartii include indole and 3-methyl-1-butanol, which previously have been shown to affect An. gambiae mosquito behaviour. E. meningoseptica emitted indole but not 3-methyl-1-butanol, when suspended in saline. Taken together, this indicates that it may be possible to create attractive breeding sites for distribution of genetically modified bacteria in the field in a paratransgenic approach against malaria. Further research is needed to determine if the bacteria are also transferred in the same way in nature.

Thorsellia anophelis is the dominant bacterium in a Kenyan population of adult Anopheles gambiae mosquitoes

Anopheles gambiae mosquitoes are not known to harbor endosymbiotic bacteria. Here we show, using nucleic acid-based methods, that 16S rRNA gene sequences specific to a recently described mosquito midgut bacterium, Thorsellia anophelis, is predominant in the midgut of adult An. gambiae s.l. mosquitoes captured in residences in central Kenya, and also occurs in the aquatic rice paddy environment nearby. PCR consistently detected T. anophelis in the surface microlayer of rice paddies, which is also consistent with the surface-feeding behavior of A. gambiae s.l. larvae. Phylogenetic analysis of cloned environmental 16S rRNA genes identified four major Thorsellia lineages, which are closely affiliated to an insect endosymbiont of the genus Arsenophonus. Physiological characterizations support the hypothesis that T. anophelis is well adapted to the female anopheline midgut by utilizing blood and tolerating the alkaline conditions in this environment. The results suggest that aquatically derived bacteria such as T. anophelis can persist through mosquito metamorphosis and become well-established in the adult mosquito midgut.