Natural microbe-mediated refractoriness to Plasmodium infection in Anopheles gambiae

How are arbovirus vectors able to tolerate infection?

One of the defining features of mosquito vectors of arboviruses such as Dengue and Zika is their ability to tolerate high levels of virus proliferation without suffering significant pathology. This adaptation is central to vector competence and disease spread. The molecular mechanisms, pathways, cellular and metabolic adaptations responsible for mosquito disease tolerance are still largely unknown and may represent effective ways to control mosquito populations and prevent arboviral diseases. In this review article, we describe the key link between disease tolerance and pathogen transmission, and how vector control methods may benefit by focusing efforts on dissecting the mechanisms underlying mosquito tolerance of arboviral infections. We briefly review recent work investigating tolerance mechanisms in other insects, describe the state of the art regarding the mechanisms of disease tolerance in mosquitos, and highlight the emerging role of gut microbiota in mosquito immunity and disease tolerance.

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.

Schistosomiasis Vector Snails and Their Microbiota Display a Phylosymbiosis Pattern

Planorbidae snails are the intermediate host for the trematode parasite of the Schistosoma genus, which is responsible for schistosomiasis, a disease that affects both humans and cattle. The microbiota for Schistosoma has already been described as having an effect on host/parasite interactions, specifically through immunological interactions. Here, we sought to characterize the microbiota composition of seven Planorbidae species and strains. Individual snail microbiota was determined using 16S ribosomal DNA amplicon sequencing. The bacterial composition was highly specific to the host strain with limited interindividual variation. In addition, it displayed complete congruence with host phylogeny, revealing a phylosymbiosis pattern. These results were confirmed in a common garden, suggesting that the host highly constrains microbial composition. This study presents the first comparison of bacterial communities between several intermediate snail hosts of Schistosoma parasites, paving the way for further studies on the understanding of this tripartite interaction.

Coculturing of Mosquito-Microbiome Bacteria Promotes Heme Degradation in Elizabethkingia anophelis

Anopheles mosquito microbiomes are intriguing ecological niches. Within the gut, microbes adapt to oxidative stress due to heme and iron after blood meals. Although metagenomic sequencing has illuminated spatial and temporal fluxes of microbiome populations, limited data exist on microbial growth dynamics. Here, we analyze growth interactions between a dominant microbiome species, Elizabethkingia anophelis, and other Anopheles-associated bacteria. We find E. anophelis inhibits a Pseudomonas sp. via an antimicrobial-independent mechanism and observe biliverdins, heme degradation products, upregulated in cocultures. Purification and characterization of E. anophelis HemS demonstrates heme degradation, and we observe hemS expression is upregulated when cocultured with Pseudomonas sp. This study reveals a competitive microbial interaction between mosquito-associated bacteria and characterizes the stimulation of heme degradation in E. anophelis when grown with Pseudomonas sp.

Gut microbiota is essential in PGRP-LA regulated immune protection against Plasmodium berghei infection

BACKGROUND

Malaria remains to be one of the deadliest infectious diseases and imposes substantial financial and social costs in the world. Mosquitoes rely on the immune system to control parasite infection. Peptidoglycan recognition proteins (PGRPs), a family of pattern-recognition receptors (PRR), are responsible for initiating and regulating immune signaling pathways. PGRP-LA is involved in the regulation of immune defense against the Plasmodium parasite, however, the underlying mechanism needs to be further elucidated.

METHODS

The spatial and temporal expression patterns of pgrp-la in Anopheles stephensi were analyzed by qPCR. The function of PGRP-LA was examined using a dsRNA-based RNA interference strategy. Western blot and periodic acid schiff (PAS) staining were used to assess the structural integrity of peritrophic matrix (PM).

RESULTS

The expression of pgrp-la in An. stephensi was induced in the midgut in response to the rapid proliferating gut microbiota post-blood meal. Knocking down of pgrp-la led to the downregulation of immune effectors that control gut microbiota growth. The decreased expression of these immune genes also facilitated P. berghei infection. However, such dsLA treatment did not influence the structural integrity of PM. When gut microbiota was removed by antibiotic treatment, the regulation of PGRP-LA on immune effectors was abolished and the knock down of pgrp-la failed to increase susceptibility of mosquitoes to parasite infection.

CONCLUSIONS

PGRP-LA regulates the immune responses by sensing the dynamics of gut microbiota. A mutual interaction between gut microbiota and PGRP-LA contributes to the immune defense against Plasmodium parasites in An. stephensi.

Draft Genome Sequence of Enterobacter hormaechei ENT5, a Component of the Symbiotic Community of Tephritid Flies

Enterobacter strains are among the dominant symbiotic bacteria in the gastrointestinal tract of insects, with the ability to fulfill diverse roles. In this announcement, we describe the draft genome sequence of Enterobacter hormaechei strain ENT5, isolated from wild adult Zeugodacus cucurbitae flies.

Enterobacter strains are among the dominant symbiotic bacteria in the gastrointestinal tract of insects, with the ability to fulfill diverse roles. In this announcement, we describe the draft genome sequence of Enterobacter hormaechei strain ENT5, isolated from wild adult Zeugodacus cucurbitae flies.

Composition and structure of the culturable gut bacterial communities in Anopheles albimanus from Colombia

The understanding of factors affecting the gut bacterial communities in malaria vectors is essential for the design of vector control interventions, such as those based on a paratransgenic approach. One of the requirements of this method is the availability of bacteria from the mosquito susceptible to culture. Thus, the aim of this study was to evaluate the composition and structure of the culturable gut bacterial communities in field mosquitoes Anopheles albimanus from Colombia, in addition to generate a bacterial collection to further analyze microbial functional activity. Gut bacteria were isolated from An. albimanus larvae and adult mosquitoes collected in localities of the Atlantic and Pacific Coasts. The bacterial isolates were grouped in 28 morphospecies that corresponded to three phyla, three classes, nine families and 14 genera. The larvae guts from San Antero (Atlantic Coast) and Buenaventura (Pacific Coast) shared the genera Bacillus and Lysinibacillus and in adults, Bacillus and Bacillus cereus Group were registered in both localities. Gut bacterial richness was higher in adults from the Pacific with respect to the Atlantic Coast, while larval richness was similar in samples of both coasts. The Shannon index indicated uniformity in morphospecies abundances in both localities. Finally, the characterization of morphospecies from the gut of Anopheles albimanus mosquitoes from Colombia by culture-dependent methods complemented with 16S rRNA gene sequencing allowed the definition, at a finer resolution, of the composition and structure of these microbial communities. In addition, the obtained bacterial culture collection will allow further evaluation of the microorganisms for their potential as biocontrol agents.

Temperature dependence of parasitic infection and gut bacterial communities in bumble bees

High temperatures (e.g., fever) and gut microbiota can both influence host resistance to infection. However, effects of temperature-driven changes in gut microbiota on resistance to parasites remain unexplored. We examined the temperature dependence of infection and gut bacterial communities in bumble bees infected with the trypanosomatid parasite Crithidia bombi. Infection intensity decreased by over 80% between 21 and 37°C. Temperatures of peak infection were lower than predicted based on parasite growth in vitro, consistent with mismatches in thermal performance curves of hosts, parasites and gut symbionts. Gut bacterial community size and composition exhibited slight but significant, non-linear, and taxon-specific responses to temperature. Abundance of total gut bacteria and of Orbaceae, both negatively correlated with infection in previous studies, were positively correlated with infection here. Prevalence of the bee pathogen-containing family Enterobacteriaceae declined with temperature, suggesting that high temperature may confer protection against diverse gut pathogens. Our results indicate that resistance to infection reflects not only the temperature dependence of host and parasite performance, but also temperature-dependent activity of gut bacteria. The thermal ecology of gut parasite-symbiont interactions may be broadly relevant to infectious disease, both in ectothermic organisms that inhabit changing climates, and in endotherms that exhibit fever-based immunity.

Thermal stress responses of Sodalis glossinidius, an indigenous bacterial symbiont of hematophagous tsetse flies

Tsetse flies (Diptera: Glossinidae) house a taxonomically diverse microbiota that includes environmentally acquired bacteria, maternally transmitted symbiotic bacteria, and pathogenic African trypanosomes. Sodalis glossinidius, which is a facultative symbiont that resides intra and extracellularly within multiple tsetse tissues, has been implicated as a mediator of trypanosome infection establishment in the fly’s gut. Tsetse’s gut-associated population of Sodalis are subjected to marked temperature fluctuations each time their ectothermic fly host imbibes vertebrate blood. The molecular mechanisms that Sodalis employs to deal with this heat stress are unknown. In this study, we examined the thermal tolerance and heat shock response of Sodalis. When grown on BHI agar plates, the bacterium exhibited the most prolific growth at 25^oC, and did not grow at temperatures above 30^oC. Growth on BHI agar plates at 31°C was dependent on either the addition of blood to the agar or reduction in oxygen levels. Sodalis was viable in liquid cultures for 24 hours at 30^oC, but began to die upon further exposure. The rate of death increased with increased temperature. Similarly, Sodalis was able to survive for 48 hours within tsetse flies housed at 30^oC, while a higher temperature (37^oC) was lethal. Sodalis’ genome contains homologues of the heat shock chaperone protein-encoding genes dnaK, dnaJ, and grpE, and their expression was up-regulated in thermally stressed Sodalis, both in vitro and in vivo within tsetse fly midguts. Arrested growth of E. coli dnaK, dnaJ, or grpE mutants under thermal stress was reversed when the cells were transformed with a low copy plasmid that encoded the Sodalis homologues of these genes. The information contained in this study provides insight into how arthropod vector enteric commensals, many of which mediate their host’s ability to transmit pathogens, mitigate heat shock associated with the ingestion of a blood meal.

Microorganisms associated with insects must cope with fluctuating temperatures. Because symbiotic bacteria influence the biology of their host, how they respond to temperature changes will have an impact on the host and other microorganisms in the host. The tsetse fly and its symbionts represent an important model system for studying thermal tolerance because the fly feeds exclusively on vertebrate blood and is thus exposed to dramatic temperature shifts. Tsetse flies house a microbial community that can consist of symbiotic and environmentally acquired bacteria, viruses, and parasitic African trypanosomes. This work, which makes use of tsetse’s commensal endosymbiont, Sodalis glossinidius, is significance because it represents the only examination of thermal tolerance mechanisms in a bacterium that resides indigenously within an arthropod disease vector. A better understanding of the biology of thermal tolerance in Sodalis provides insight into thermal stress survival in other insect symbionts and may yield information to help control vector-borne disease.

Isolation and characterization of Pseudomonas cedrina infecting Plutella xylostella (Lepidoptera: Plutellidae)

The diamondback moth, Plutella xylostella, is one of the most destructive pests worldwide and its management relies exclusively on frequent application of chemical insecticides. Resistance to common insecticides is now widespread, and novel classes of insecticides are needed. Entomopathogenic bacteria and their related products play an important role in the management of this pest. In the present work, one bacterial strain was separated from infected pupae of P. xylostella collected from field and its pathogenicity was evaluated. On the basis of the 16S ribosomal RNA sequencing, BLASTN, and phylogenetic analysis, this bacterial isolate was identified as Pseudomonas cedrina. Oral administration of P. cedrina at levels above 10,000 CFU/ml gave significant mortality to P. xylostella larvae. The pathogenicity was also observed by reduced longevity and fecundity in adult females. However, when live bacterial cells were removed, the cultured broth lost any pathogenicity. In response to the bacterial infection, P. xylostella expressed antimicrobial and stress-associated genes. A mixture treatment of P. cedrina and Bacillus thuringiensis showed an additive effect on larval mortality of P. xylostella. These results indicated that P. cedrina is an opportunistic entomopathogen without secretion of toxins. Furthermore, the additive effect of P. cedrina and B. thuringiensis provide a new insight to develop new strategy for controlling P. xylostella.

Characterization of Bacterial Communities in Breeding Waters of Anopheles darlingi in Manaus in the Amazon Basin Malaria-Endemic Area

The microbiota in mosquito breeding waters can affect ovipositing mosquitoes, have effects on larval development, and can modify adult mosquito-gut bacterial composition. This, in turn, can affect transmission of human pathogens such as malaria parasites. Here, we explore the microbiota of four breeding sites for Anopheles darlingi, the most important malaria vector in Latin America. The sites are located in Manaus in the Amazon basin in Brazil, an area of active malaria transmission. Using 16S rRNA gene sequencing by MiSeq, we found that all sites were dominated by Proteobacteria and Firmicutes and that 94% of the total number of reads belonged to 36 operational taxonomic units (OTUs) identified in all sites. Of these, the most common OTUs belonged to Escherichia/Shigella, Staphylococcus, and Pseudomonas. Of the remaining 6% of the reads, the OTUs found to differentiate between the four sites belonged to the orders Burkholderiales, Actinomycetales, and Clostridiales. We conclude that An. darlingi can develop in breeding waters with different surface-water bacteria, but that the common microbiota found in all breeding sites might indicate or contribute to a suitable habitat for this important malaria vector.

Bacterial communities associated with the midgut microbiota of wild Anopheles gambiae complex in Burkina Faso

Plasmodium falciparum is transmitted by mosquitoes from the Anopheles gambiae sensu lato (s.l) species complex and is responsible for severe forms of malaria. The composition of the mosquitoes' microbiota plays a role in P. falciparum transmission, so we studied midgut bacterial communities of An. gambiae s.l from Burkina Faso. DNA was extracted from 17 pools of midgut of mosquitoes from the Anopheles gambiae complex from six localities in three climatic areas, including cotton-growing and cotton-free localities to include potential differences in insecticide selection pressure. The v3-v4 region of the 16S rRNA gene was targeted and sequenced using Illumina Miseq (2 × 250 nt). Diversity analysis was performed using QIIME and R software programs. The major bacterial phylum was Proteobacteria (97.2%) in all samples. The most abundant genera were Enterobacter (32.8%) and Aeromonas (29.8%), followed by Pseudomonas (11.8%), Acinetobacter (5.9%) and Thorsellia (2.2%). No statistical difference in operational taxonomic units (OTUs) was found (Kruskal-Wallis FDR-p > 0.05) among the different areas, fields or localities. Richness and diversity indexes (observed OTUs, Chao1, Simpson and Shannon indexes) showed significant differences in the cotton-growing fields and in the agroclimatic zones, mainly in the Sudano-Sahelian area. OTUs from seven bacterial species that mediate refractoriness to Plasmodium infection in An. gambiae s.l were detected. The beta diversity analysis did not show any significant difference. Therefore, a same control strategy of using bacterial species refractoriness to Plasmodium to target mosquito midgut bacterial community and affect their fitness in malaria transmission may be valuable tool for future malaria control efforts in Burkina Faso.

Intestinal Microbiota Confer Protection by Priming the Immune System of Red Palm Weevil Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae)

The immune system of animals, including insects, is the vital factor to maintain the symbiotic interactions between animals and their associated microbes. However, the effects of gut microbiota on insect immunity remain mostly elusive. Red palm weevil (RPW), Rhynchophorus ferrugineus Olivier, is a destructive pest of palm trees worldwide, which has forged alliances with its gut microbiota. Here, we found that the aposymbiotic insects succumbed at a significantly faster rate than conventionally reared (CR) ones upon bacterial infection. Physiological assays confirmed that CR insects had stronger antimicrobial activity and higher phenoloxidase activity in contrast to germfree (GF) ones, indicating that the systemic immune responses of GF individuals were compromised markedly. Interestingly, under the bacterial challenge conditions, the reassociation of gut microbiota with GF insects could enhance their survival rate by rescuing their immunocompetence. Furthermore, comparative transcriptome analysis uncovered that 35 immune-related genes, including pathogen recognition receptors, effectors and immune signaling pathway, were significantly downregulated in GF insects as compared to CR ones. Collectively, our findings corrobate that intestinal commensal bacteria have profound immunostimulatory effects on RPW larvae. Therefore, knowledge on the effects of gut microbiota on RPW immune defenses may contribute to of set up efficient control strategies of this pest.

Mosquito adaptations to hematophagia impact pathogen transmission

Mosquito-borne diseases such as Dengue fever, Chikungunya, and Malaria are critical threats to public health in many parts of the world. Female mosquitoes have evolved multiple adaptive mechanisms to hematophagy, including the ability to efficiently draw and digest blood, as well as the ability to eliminate excess fluids and toxic by-products of blood digestion. Pathogenic agents enter the mosquito digestive tract with the blood meal and need to travel through the midgut and into the hemocele in order to reach the salivary glands and infect a new host. Pathogens need to adjust to these hostile gut, hemocele, and salivary gland environments, and when possible influence the physiology and behavior of their hosts to enhance transmission.

The Tsetse Fly Displays an Attenuated Immune Response to Its Secondary Symbiont, Sodalis glossinidius

Sodalis glossinidius, a vertically transmitted facultative symbiont of the tsetse fly, is a bacterium in the early/intermediate state of its transition toward symbiosis, representing an important model for investigating how the insect host immune defense response is regulated to allow endosymbionts to establish a chronic infection within their hosts without being eliminated. In this study, we report on the establishment of a tsetse fly line devoid of S. glossinidius only, allowing us to experimentally investigate (i) the complex immunological interactions between a single bacterial species and its host, (ii) how the symbiont population is kept under control, and (iii) the impact of the symbiont on the vector competence of the tsetse fly to transmit the sleeping sickness parasite. Comparative transcriptome analysis showed no difference in the expression of genes involved in innate immune processes between symbiont-harboring (Gmm^Sod+) and S. glossinidius-free (Gmm^Sod–) flies. Re-exposure of (Gmm^Sod–) flies to the endosymbiotic bacterium resulted in a moderate immune response, whereas exposure to pathogenic E. coli or to a close non-insect associated relative of S. glossinidius, i.e., S. praecaptivus, resulted in full immune activation. We also showed that S. glossinidius densities are not affected by experimental activation or suppression of the host immune system, indicating that S. glossinidius is resistant to mounted immune attacks and that the host immune system does not play a major role in controlling S. glossinidius proliferation. Finally, we demonstrate that the absence or presence of S. glossinidius in the tsetse fly does not alter its capacity to mount an immune response to pathogens nor does it affect the fly’s susceptibility toward trypanosome infection.

A Gut Symbiotic Bacterium Serratia marcescens Renders Mosquito Resistance to Plasmodium Infection Through Activation of Mosquito Immune Responses

The malaria development in the mosquito midgut is a complex process that results in considerable parasite losses. The mosquito gut microbiota influences the outcome of pathogen infection in mosquitoes, but the underlying mechanisms through which gut symbiotic bacteria affect vector competence remain elusive. Here, we identified two Serratia strains (Y1 and J1) isolated from field-caught female Anopheles sinensis from China and assessed their effect on Plasmodium development in An. stephensi. Colonization of An. stephensi midgut by Serratia Y1 significantly renders the mosquito resistant to Plasmodium berghei infection, while Serratia J1 has no impact on parasite development. Parasite inhibition by Serratia Y1 is induced by the activation of the mosquito immune system. Genome-wide transcriptomic analysis by RNA-seq shows a similar pattern of midgut gene expression in response to Serratia Y1 and J1 in sugar-fed mosquitoes. However, 24 h after blood ingestion, Serratia Y1 modulates more midgut genes than Serratia J1 including the c-type lectins (CTLs), CLIP serine proteases and other immune effectors. Furthermore, silencing of several Serratia Y1-induced anti-Plasmodium factors like the thioester-containing protein 1 (TEP1), fibrinogen immunolectin 9 (FBN9) or leucine-rich repeat protein LRRD7 can rescue parasite oocyst development in the presence of Serratia Y1, suggesting that these factors modulate the Serratia Y1-mediated anti-Plasmodium effect. This study enhances our understanding of how gut bacteria influence mosquito-Plasmodium interactions.

Evidence for Gut-Associated Serratia symbiotica in Wild Aphids and Ants Provides New Perspectives on the Evolution of Bacterial Mutualism in Insects

Many insects engage in symbiotic associations with diverse assemblages of bacterial symbionts that can deeply impact on their ecology and evolution. The intraspecific variation of symbionts remains poorly assessed while phenotypic effects and transmission behaviors, which are key processes for the persistence and evolution of symbioses, may differ widely depending on the symbiont strains. Serratia symbiotica is one of the most frequent symbiont species in aphids and a valuable model to assess this intraspecific variation since it includes both facultative and obligate symbiotic strains. Despite evidence that some facultative S. symbiotica strains exhibit a free-living capacity, the presence of these strains in wild aphid populations, as well as in insects with which they maintain regular contact, has never been demonstrated. Here, we examined the prevalence, diversity, and tissue tropism of S. symbiotica in wild aphids and associated ants. We found a high occurrence of S. symbiotica infection in ant populations, especially when having tended infected aphid colonies. We also found that the S. symbiotica diversity includes strains found located within the gut of aphids and ants. In the latter, this tissue tropism was found restricted to the proventriculus. Altogether, these findings highlight the extraordinary diversity and versatility of an insect symbiont and suggest the existence of novel routes for symbiont acquisition in insects.

Distribution and Variation of Bacterial Endosymbiont and "Candidatus Liberibacter asiaticus" Titer in the Huanglongbing Insect Vector, Diaphorina citri Kuwayama

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is an economic insect pest in most citrus-growing regions and the vector of 'Candidatus Liberibacter asiaticus' (CLas), one of at least three known bacteria associated with Huanglongbing (HLB or citrus greening disease). D. citri harbors bacterial endosymbionts, including Wolbachia pipientis (strain Wolbachia wDi), 'Candidatus Carsonella ruddii,' and 'Candidatus Profftella armatura.' Many important functions of these bacteria can be inferred from their genome sequences, but their interactions with each other, CLas, and their D. citri host are poorly understood. In the present study, the titers of the endosymbionts in different tissues, in each sex, and in insects reared on healthy citrus (referred to as unexposed) and CLas-infected citrus (referred to as CLas-exposed) D. citri were investigated using real-time, quantitative PCR (qPCR) using two different quantitative approaches. Wolbachia and CLas were detected in all insect tissues. The titer of Wolbachia was higher in heads of CLas-exposed males as compared to unexposed males. In males and females, Wolbachia titer was highest in the Malpighian tubules. The highest titer of CLas was observed in the gut. Profftella and Carsonella titers were significantly reduced in the bacteriome of CLas-exposed males compared with that of unexposed males, but this effect was not observed in females. In ovaries of CLas-exposed females, the Profftella and Carsonella titers were increased as compared to non-exposed females. CLas appeared to influence the overall levels of the symbionts but did not drastically perturb the overall microbial community structure. In all the assessed tissues, CLas titer in males was significantly higher than that of females using absolute quantification. These data provide a better understanding of multi-trophic interactions regulating symbiont dynamics in the HLB pathosystem.

Microbial Communities in Different Developmental Stages of the Oriental Fruit Fly, Bactrocera dorsalis, Are Associated with Differentially Expressed Peptidoglycan Recognition Protein-Encoding Genes

The insect microbiota can change dramatically to enable adaptation of the host in different developmental stages and environments; however, little is known about how the host maintains its microbiota to achieve such adaptations. In this study, 16S rRNA sequencing revealed that the microorganisms in larvae and adults of the Oriental fruit fly, Bactrocera dorsalis, are primarily Gram-negative bacteria but that the major components in pupae are Gram-positive bacteria. Using suppression subtractive hybridization (SSH) and transcriptome analysis, we screened two specifically expressed genes encoding peptidoglycan recognition proteins (PGRP-LB and PGRP-SB1) and analyzed their relationship to B. dorsalis microbial communities. Knockdown of the PGRP-LB gene in larvae and adults led to increased ratios of Gram-positive bacteria; knockdown of the PGRP-SB1 gene in pupae led to increased ratios of Gram-negative bacteria. Our results suggest that maintenance of the microbiota in different developmental stages of B. dorsalis may be associated with the PGRP-LB and PGRP-SB1 genes.IMPORTANCE Microorganisms are ubiquitous in insects and have widespread impacts on multiple aspects of insect biology. However, the microorganisms present in insects can change dramatically in different developmental stages, and it is critical to maintain the appropriate microorganisms in specific host developmental stages. Therefore, analysis of the factors associated with the microbiota in specific development stages of the host is needed. In this study, we applied suppression subtractive hybridization (SSH) combined with transcriptome analysis to investigate whether the microbiota in development stages of the Oriental fruit fly, Bactrocera dorsalis, is associated with expression of PGRP genes. We found that two different PGRP genes were specifically expressed during development and that these genes may be associated with changes in microbial communities in different developmental stages of B. dorsalis.

Malaria vector species in Amazonian Peru co-occur in larval habitats but have distinct larval microbial communities

In Amazonian Peru, the primary malaria vector, Nyssorhynchus darlingi (formerly Anopheles darlingi), is difficult to target using standard vector control methods because it mainly feeds and rests outdoors. Larval source management could be a useful supplementary intervention, but to determine its feasibility, more detailed studies on the larval ecology of Ny. darlingi are essential. We conducted a multi-level study of the larval ecology of Anophelinae mosquitoes in the peri-Iquitos region of Amazonian Peru, examining the environmental characteristics of the larval habitats of four species, comparing the larval microbiota among species and habitats, and placing Ny. darlingi larval habitats in the context of spatial heterogeneity in human malaria transmission. We collected Ny. darlingi, Nyssorhynchus rangeli (formerly Anopheles rangeli), Nyssorhynchus triannulatus s.l. (formerly Anopheles triannulatus s.l.), and Nyssorhynchus sp. nr. konderi (formerly Anopheles sp. nr. konderi) from natural and artificial water bodies throughout the rainy and dry seasons. We found that, consistent with previous studies in this region and in Brazil, the presence of Ny. darlingi was significantly associated with water bodies in landscapes with more recent deforestation and lower light intensity. Nyssorhynchus darlingi presence was also significantly associated with a lower vegetation index, other Anophelinae species, and emergent vegetation. Though they were collected in the same water bodies, the microbial communities of Ny. darlingi larvae were distinct from those of Ny. rangeli and Ny. triannulatus s.l., providing evidence either for a species-specific larval microbiome or for segregation of these species in distinct microhabitats within each water body. We demonstrated that houses with more reported malaria cases were located closer to Ny. darlingi larval habitats; thus, targeted control of these sites could help ameliorate malaria risk. The co-occurrence of Ny. darlingi larvae in water bodies with other putative malaria vectors increases the potential impact of larval source management in this region.

Microbial evolutionary medicine: from theory to clinical practice

Medicine and clinical microbiology have traditionally attempted to identify and eliminate the agents that cause disease. However, this traditional approach is becoming inadequate for dealing with a changing disease landscape. Major challenges to human health are non-communicable chronic diseases, often driven by altered immunity and inflammation, and communicable infections from agents which harbour antibiotic resistance. This Review focuses on the so-called evolutionary medicine framework, to study how microbial communities influence human health. The evolutionary medicine framework aims to predict and manipulate microbial effects on human health by integrating ecology, evolutionary biology, microbiology, bioinformatics, and clinical expertise. We focus on the potential of evolutionary medicine to address three key challenges: detecting microbial transmission, predicting antimicrobial resistance, and understanding microbe-microbe and human-microbe interactions in health and disease, in the context of the microbiome.

Isolation and identification of microflora from the midgut and salivary glands of Anopheles species in malaria endemic areas of Ethiopia

Background

Anopheles mosquitoes are of great importance to human health. A number of studies have shown that midgut and salivary gland microflora have an impact on malaria parasite burden through colonization mechanisms, involving either direct Plasmodium microbiota interaction or bacterial-mediated induction of mosquito immune response. The objective of this study was to isolate and identify the microflora from the midgut and salivary glands of Anopheles species.

Methods

A total of 20 pools (ten per pool) from insectary-reared and 56 pools (five per pool) of field-collected Anopheles mosquitoes were anesthetized by chloroform and dissected. 70% of ethanol was used for surface sterilization of mosquitoes and laboratory equipment, followed by rinsing Anopheles mosquitoes four times with 1X PBS. Each pool of dissected midgut and salivary gland sample was transferred in 1X PBS and squashed, incubated in the water bath and enriched in tryptic soya broth for 24 h at 35 ± 2 °C. As a control, the PBS solutions used to rinse the mosquitoes were also incubated in tryptic soya broth in the same conditions as the sample. After enrichment, a loopful of each sample was taken and inoculated on Blood, Chocolate, MacConkey, and Sabouraud Dextrose agar. Finally, the microbiota was isolated by colony characteristics, biochemical tests, and automated VITEK 2 Compact Analyzer.

Results

From all field and laboratory mosquitoes, Pseudomonas was found to be the dominant microbiota identified from all species of Anopheles mosquitoes. Acinetobacter and Klebsiellapneumonia and other families of gram-positive and gram-negative bacteria were identified.

Conclusions

A number of bacteria were isolated and identified. This is the first report on isolation and identification of microbiota from midgut and salivary glands of Anopheles species in Ethiopia. It can be used as a baseline for studying the relationship between microbiota and mosquitoes, and for the development of a new malaria biological control.

The effect of silencing immunity related genes on longevity in a naturally occurring Anopheles arabiensis mosquito population from southwest Ethiopia

BACKGROUND

Vector control remains the most important tool to prevent malaria transmission. However, it is now severely constrained by the appearance of physiological and behavioral insecticide resistance. Therefore, the development of new vector control tools is warranted. Such tools could include immunization of blood hosts of vector mosquitoes with mosquito proteins involved in midgut homeostasis (anti-mosquito vaccines) or genetic engineering of mosquitoes that can drive population-wide knockout of genes producing such proteins to reduce mosquito lifespan and malaria transmission probability.

METHODS

To achieve this, candidate genes related to midgut homeostasis regulation need to be assessed for their effect on mosquito survival. Here, different such candidate genes were silenced through dsRNA injection in the naturally occurring Anopheles arabiensis mosquitoes and the effect on mosquito survival was evaluated.

RESULTS

Significantly higher mortality rates were observed in the mosquitoes silenced for FN3D1 (AARA003032), FN3D3 (AARA007751) and GPRGr9 (AARA003963) genes as compared to the control group injected with dsRNA against a non-related bacterial gene (LacZ). This observed difference in mortality rate between the candidate genes and the control disappeared when gene-silenced mosquitoes were treated with antibiotic mixtures, suggesting that gut microbiota play a key role in the observed reduction of mosquito survival.

CONCLUSIONS

We demonstrated that interference with the expression of the FN3D1, FN3D3 or GPRGr9 genes causes a significant reduction of the longevity of An. arabiensis mosquito in the wild.

[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.

Stochastic processes govern bacterial communities from the blood of pikas and from their arthropod vectors

Vector-borne microbes influence pathogen transmission and blood microbiomes, thereby affecting the emergence of infectious diseases. Thus, understanding the relationship between host and vector microbiomes is of importance. In this study, we investigated the bacterial community composition, diversity and assembly of the flea (Rhadinopsylla dahurica vicina), torsalo (Hypoderma curzonial), and the blood and gut of their shared pika host, Ochotona curzoniae. Bartonella, Sphingomonas and Bradyrhizobium were enriched in blood, while Wolbachia and Fusobacterium were more abundant in fleas and torsaloes. Most of potential pathogenic microbes (belonging to Fusobacterium, Rickettsia, Kingella, Porphyromonas, Bartonella and Mycoplasma) were present in the blood of pikas and their vectors. Blood communities were more similar to those from fleas than other sample types and were independent of host factors or geographical sites. Notably, blood microbes originate mainly from fleas rather than gut or torsaloes. Interestingly, the community assembly of blood, fleas or torsaloes was primarily governed by stochastic processes, while the gut microbiome was determined by deterministic processes. Ecological drift plays a dominant role in the assembly of blood and flea microbiomes. These results reflect the difficulty for predicting and regulating the microbial ecology of fleas for the prevention of potential microbiome-associated diseases.

Presence of Aedes and Anopheles mosquito larvae is correlated to bacteria found in domestic water-storage containers

Water-storage containers are common in households where access to water is scarce and often act as breeding sites for vector mosquitoes. Bacteria in these containers may be important for attracting or repelling ovipositing mosquitoes. We hypothesized that bacterial community composition in water-storage containers would represent either inhibitory or suitable environmental conditions for mosquito larvae. To investigate this, we characterized the bacterial community composition in water-storage containers and correlated these communities to Aedes and Anopheles larval densities. Water samples were collected over two years from 13 containers in an Indian village and analyzed by high throughput 16S rRNA gene amplicon sequencing. Comparisons of bacterial community composition between water with and without mosquito larvae showed that Xanthomonadaceae, Comamonadaceae and Burkholderiaceae were more common (P < 0.05) in absence of larvae, while Lachnospiraceae, Synechococcaceae, Alcaligenaceae and Cryomorphaceae were more common (P < 0.05) in presence of larvae. Indicator analysis identified operational taxonomic units designated as CL500-29 marine group (Acidimicrobiaceae) and FukuN101 (Microbacteriaceae) for absence and presence of larvae, respectively. These results contribute to the understanding of which bacteria, directly or indirectly, can be linked to absence or presence of mosquitoes around households and set the basis for potential measures to be taken against these vector mosquitoes.

Cross-Domain and Viral Interactions in the Microbiome

The importance of the microbiome to human health is increasingly recognized and has become a major focus of recent research. However, much of the work has focused on a few aspects, particularly the bacterial component of the microbiome, most frequently in the gastrointestinal tract. Yet humans and other animals can be colonized by a wide array of organisms spanning all domains of life, including bacteria and archaea, unicellular eukaryotes such as fungi, multicellular eukaryotes such as helminths, and viruses. As they share the same host niches, they can compete with, synergize with, and antagonize each other, with potential impacts on their host. Here, we discuss these major groups making up the human microbiome, with a focus on how they interact with each other and their multicellular host.

Molecular characterisation of immunological memory following homologous or heterologous challenges in the schistosomiasis vector snail, Biomphalaria glabrata

Invertebrate immune response may be primed by a current infection in a sustained manner, leading to the failure of a secondary infection with the same pathogen. The present study focuses on the Schistosomiasis vector snail Biomphalaria glabrata, in which a specific genotype-dependent immunological memory was demonstrated as a shift from a cellular to a humoral immune response. Herein, we investigate the complex molecular bases associated with this genotype-dependant immunological memory response. We demonstrate that Biomphalaria regulates a polymorphic set of immune recognition molecules and immune effector repertoires to respond to different strains of Schistosoma parasites. These results suggest a combinatorial usage of pathogen recognition receptors (PRRs) that distinguish different strains of parasites during the acquisition of immunological memory. Immunizations also show that snails become resistant after exposure to parasite extracts. Hemolymph transfer and a label-free proteomic analysis proved that circulating hemolymph compounds can be produced and released to more efficiently kill the newly encountered parasite of the same genetic lineage.

A large-scale field study of bacterial communities in cereal aphid populations across Morocco

Insects are frequently associated with bacteria that can have significant ecological and evolutionary impacts on their hosts. To date, few studies have examined the influence of environmental factors to microbiome composition of aphids. The current work assessed the diversity of bacterial communities of five cereal aphid species (Sitobion avenae, Rhopalosiphum padi, R. maidis, Sipha maydis and Diuraphis noxia) collected across Morocco, covering a wide range of environmental conditions. We aimed to test whether symbiont combinations are host or environment specific. Deep 16S rRNA sequencing enabled us to identify 17 bacterial operational taxonomic units (OTUs). The obligate symbiont Buchnera aphidicola was represented by five OTUs with multiple haplotypes in many single samples. Facultative endosymbionts were presented by a high prevalence of Regiella insecticola and Serratia symbiotica in S. avenae and Si. maydis, respectively. In addition to these symbiotic partners, Pseudomonas, Acinetobacter, Pantoea, Erwinia and Staphyloccocus were also identified in aphids, suggesting that the aphid microbiome is not limited to the presence of endosymbiotic bacteria. Beside a significant association between host species and bacterial communities, an inverse correlation was also found between altitude and α-diversity. Overall, our results support that symbiont combinations are mainly host specific.

Green, Yellow, and Red Fluorescent Proteins as Markers for Bacterial Isolates from Mosquito Midguts

The growing awareness that microbial symbionts residing in mosquito midguts can interrupt transmission of vector-borne diseases has stimulated interest in understanding their potential role in mosquito biology. Fluorescent proteins are powerful molecular markers that provide for detailed analysis of the function and behavior of specific midgut bacterial isolates without disturbing the normal gut microbiota. The aim of this study was to label bacterial isolates from the midgut of Ochlerotatus triseriatus, the primary vector of La Crosse virus, with green, yellow, and red fluorescent proteins (GFP, YFP, RFP) via electroporation. We also assessed the stability of GFP-, YFP-, and RFP-bearing plasmids and their effect on bacterial growth. Seven of eleven bacterial species could not be labeled despite several attempts. Labeling of Escherichia coli and Enterobacter cloacae was successfully achieved with all three fluorescent proteins. In contrast, labeling of Aerococcus viridans was achieved with GFP only and labeling of Aeromonas hydrophila was achieved with GFP and YFP only. The stability of GFP plasmid varied among bacterial species with A. hydrophila followed by E. cloacae having the most stable GFP label. In contrast, YFP and RFP plasmids were very stable in all bacterial species possessing these labels. GFP plasmid reduced the growth of labeled strains relative to wild type but this effect was not evident in YFP and RFP plasmids. These findings suggest that some mosquito midgut bacterial isolates can effectively be labeled with GFP, YFP and RFP plasmids allowing non-destructive studies on their functions within the vector.

Colonization of the tsetse fly midgut with commensal Kosakonia cowanii Zambiae inhibits trypanosome infection establishment

Tsetse flies (Glossina spp.) vector pathogenic trypanosomes (Trypanosoma spp.) in sub-Saharan Africa. These parasites cause human and animal African trypanosomiases, which are debilitating diseases that inflict an enormous socio-economic burden on inhabitants of endemic regions. Current disease control strategies rely primarily on treating infected animals and reducing tsetse population densities. However, relevant programs are costly, labor intensive and difficult to sustain. As such, novel strategies aimed at reducing tsetse vector competence require development. Herein we investigated whether Kosakonia cowanii Zambiae (Kco_Z), which confers Anopheles gambiae with resistance to Plasmodium, is able to colonize tsetse and induce a trypanosome refractory phenotype in the fly. Kco_Z established stable infections in tsetse's gut and exhibited no adverse effect on the fly's survival. Flies with established Kco_Z infections in their gut were significantly more refractory to infection with two distinct trypanosome species (T. congolense, 6% infection; T. brucei, 32% infection) than were age-matched flies that did not house the exogenous bacterium (T. congolense, 36% infected; T. brucei, 70% infected). Additionally, 52% of Kco_Z colonized tsetse survived infection with entomopathogenic Serratia marcescens, compared with only 9% of their wild-type counterparts. These parasite and pathogen refractory phenotypes result from the fact that Kco_Z acidifies tsetse's midgut environment, which inhibits trypanosome and Serratia growth and thus infection establishment. Finally, we determined that Kco_Z infection does not impact the fecundity of male or female tsetse, nor the ability of male flies to compete with their wild-type counterparts for mates. We propose that Kco_Z could be used as one component of an integrated strategy aimed at reducing the ability of tsetse to transmit pathogenic trypanosomes.

In vitro Trypanocidal Activity, Genomic Analysis of Isolates, and in vivo Transcription of Type VI Secretion System of Serratia marcescens Belonging to the Microbiota of Rhodnius prolixus Digestive Tract

Serratia marcescens is a bacterium with the ability to colonize several niches, including some eukaryotic hosts. S. marcescens have been recently found in the gut of hematophagous insects that act as parasite vectors, such as Anopheles, Rhodnius, and Triatoma. While some S. marcescens strains have been reported as symbiotic or pathogenic to other insects, the role of S. marcescens populations from the gut microbiota of Rhodnius prolixus, a vector of Chagas’ disease, remains unknown. Bacterial colonies from R. prolixus gut were isolated on BHI agar. After BOX-PCR fingerprinting, the genomic sequences of two isolates RPA1 and RPH1 were compared to others S. marcescens from the NCBI database in other to estimate their evolutionary divergence. The in vitro trypanolytic activity of these two bacterial isolates against Trypanosoma cruzi (DM28c clone and Y strain) was assessed by microscopy. In addition, the gene expression of type VI secretion system (T6SS) was detected in vivo by RT-PCR. Comparative genomics of RPA1 and RPH1 revealed, besides plasmid presence and genomic islands, genes related to motility, attachment, and quorum sensing in both genomes while genes for urea hydrolysis and type II secretion system (T2SS) were found only in the RPA1 genome. The in vitro trypanolytic activity of both S. marcescens strains was stronger in their stationary phases of growth than in their exponential ones, with 65–70 and 85–90% of epimastigotes (Dm28c clone and Y strain, respectively) being lysed after incubation with RPA1 or RPH1 in stationary phase. Although T6SS transcripts were detected in guts up to 40 days after feeding (DAF), R. prolixus morbidity or mortality did not appear to be affected. In this report, we made available two trypanolytic S. marcescens strains from R. prolixus gut to the scientific community together with their genomic sequences. Here, we describe their genomic features with the purpose of bringing new insights into the S. marcescens adaptations for colonization of the specific niche of triatomine guts. This study provides the basis for a better understanding of the role of S. marcescens in the microbiota of R. prolixus gut as a potential antagonist of T. cruzi in this complex system.

Generational conservation of composition and diversity of field-acquired midgut microbiota in Anopheles gambiae (sensu lato) during colonization in the laboratory

Background

The gut microbiota is known to play a role in a mosquito vector’s life history, a subject of increasing research. Laboratory experiments are essential for such studies and require laboratory colonies. In this study, the conservation of field-obtained midgut microbiota was evaluated in laboratory-reared Anopheles gambiae (s.l.) mosquitoes continuously hatched in water from field breeding habitats.

Methods

Pupae and late instars were obtained from the field and reared, and the emerged adults were blood-fed. The eggs obtained from them were hatched in either water from the field or in dechlorinated tap water. The mosquito colonies were maintained for 10 generations. Midguts of female adults from unfed F₀ (emerging from field-caught pupae and larvae), F₅ and F₁₀ were dissected out and genomic DNA was extracted for 16S metagenomic sequencing. The sequences were compared to investigate the diversity and bacterial compositional differences using ANCOM and correlation clustering methods.

Results

Less than 10% of the bacterial families identified had differential relative abundances between generational groups and accounted for 46% of the variation observed. Although diversity reduced in F₁₀ mosquitoes during laboratory colonization (Shannon-Weaver; P-value < 0.05), 50% of bacterial genera were conserved in those bred continuously in field-water compared to 38% in those bred in dechlorinated tap water.

Conclusions

To our knowledge, this study is the first report on the assessment of gut bacterial community of mosquitoes during laboratory colonization and recommends the use of water from the natural breeding habitats if they are intended for microbiota research.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3287-0) contains supplementary material, which is available to authorized users.

Intestinal probiotics restore the ecological fitness decline of Bactrocera dorsalis by irradiation

The sterile insect technique (SIT) as an eco-friendly and reliable strategy has been used to control populations of insect pests of agricultural, veterinary and human health importance. Successful applications of SIT rely on the high-level ecological fitness of sterile males. A suitable and stable gut microbiome can contribute to the ecological fitness of insect by influencing their physiology, biochemistry and development processes. Here, we show that a shift in the gut bacterial composition and structure by sterilizing irradiation, characterized by a decrease in the major gut microbiota community Enterobacteriaceae, an expansion of the minor members (e.g., Bacillaceae) and a higher richness and diversity, is tightly linked to radiation-induced ecological fitness (male mating competitiveness, flight capacity, survival rate and life span) decline in Bactrocera dorsalis (Hendel) sterile males. Function prediction of gut microbiota indicated that changes in microbiome taxonomy tend to drive microbiome functional shifts. A higher nutrient consumption of the flourishing minor gut microbiota may cause a decline in nutrients and energy metabolic activity of host and then result in the reduced ecological fitness of irradiated flies. Furthermore, we found that a gut bacterial strain Klebsiella oxytoca (BD177) can restore ecological fitness by improving food intake and increasing haemolymph sugar and amino acid levels of irradiated B. dorsalis flies. Our findings suggest that gut symbiont-based probiotics can be used as agents for reversing radiation-induced ecological fitness decrease.

Novel Wolbachia strains in Anopheles malaria vectors from Sub-Saharan Africa

Background:  Wolbachia, a common insect endosymbiotic bacterium that can influence pathogen transmission and manipulate host reproduction, has historically been considered absent from the  Anopheles (An.) genera, but has recently been found in  An. gambiae s.l. populations in West Africa.  As there are numerous  Anopheles species that have the capacity to transmit malaria, we analysed a range of species across five malaria endemic countries to determine  Wolbachia prevalence rates, characterise novel  Wolbachia strains and determine any correlation between the presence of  Plasmodium,  Wolbachia and the competing bacterium  Asaia. Methods:  Anopheles adult mosquitoes were collected from five malaria-endemic countries: Guinea, Democratic Republic of the Congo (DRC), Ghana, Uganda and Madagascar, between 2013 and 2017.  Molecular analysis was undertaken using quantitative PCR, Sanger sequencing,  Wolbachia multilocus sequence typing (MLST) and high-throughput amplicon sequencing of the bacterial  16S rRNA gene.  Results: Novel  Wolbachia strains were discovered in five species:  An. coluzzii,  An. gambiae s.s.,  An. arabiensis,  An. moucheti and  An. species A, increasing the number of  Anopheles species known to be naturally infected. Variable prevalence rates in different locations were observed and novel strains were phylogenetically diverse, clustering with  Wolbachia supergroup B strains.  We also provide evidence for resident strain variants within  An. species A.  Wolbachia is the dominant member of the microbiome in  An. moucheti and  An. species A but present at lower densities in  An. coluzzii.  Interestingly, no evidence of  Wolbachia/Asaia co-infections was seen and  Asaia infection densities were shown to be variable and location dependent.  Conclusions: The important discovery of novel  Wolbachia strains in  Anopheles provides greater insight into the prevalence of resident  Wolbachia strains in diverse malaria vectors.  Novel  Wolbachia strains (particularly high-density strains) are ideal candidate strains for transinfection to create stable infections in other  Anopheles mosquito species, which could be used for population replacement or suppression control strategies.

Analysis of the gut-specific microbiome from field-captured tsetse flies, and its potential relevance to host trypanosome vector competence

Background

The tsetse fly (Glossina sp.) midgut is colonized by maternally transmitted and environmentally acquired bacteria. Additionally, the midgut serves as a niche in which pathogenic African trypanosomes reside within infected flies. Tsetse’s bacterial microbiota impacts many aspects of the fly’s physiology. However, little is known about the structure of tsetse’s midgut-associated bacterial communities as they relate to geographically distinct fly habitats in east Africa and their contributions to parasite infection outcomes. We utilized culture dependent and independent methods to characterize the taxonomic structure and density of bacterial communities that reside within the midgut of tsetse flies collected at geographically distinct locations in Kenya and Uganda.

Results

Using culture dependent methods, we isolated 34 strains of bacteria from four different tsetse species (G. pallidipes, G. brevipalpis, G. fuscipes and G. fuscipleuris) captured at three distinct locations in Kenya. To increase the depth of this study, we deep sequenced midguts from individual uninfected and trypanosome infected G. pallidipes captured at two distinct locations in Kenya and one in Uganda. We found that tsetse’s obligate endosymbiont, Wigglesworthia, was the most abundant bacterium present in the midgut of G. pallidipes, and the density of this bacterium remained largely consistent regardless of whether or not its tsetse host was infected with trypanosomes. These fly populations also housed the commensal symbiont Sodalis, which was found at significantly higher densities in trypanosome infected compared to uninfected flies. Finally, midguts of field-captured G. pallidipes were colonized with distinct, low density communities of environmentally acquired microbes that differed in taxonomic structure depending on parasite infection status and the geographic location from which the flies were collected.

Conclusions

The results of this study will enhance our understanding of the tripartite relationship between tsetse, its microbiota and trypanosome vector competence. This information may be useful for developing novel disease control strategies or enhancing the efficacy of those already in use.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1284-7) contains supplementary material, which is available to authorized users.

Blood feeding tsetse flies as hosts and vectors of mammals-pre-adapted African Trypanosoma: current and expected research directions

Research on the zoo-anthropophilic blood feeding tsetse flies' biology conducted, by different teams, in laboratory settings and at the level of the ecosystems- where also co-perpetuate African Trypanosoma- has allowed to unveil and characterize key features of tsetse flies' bacterial symbionts on which rely both (a) the perpetuation of the tsetse fly populations and (b) the completion of the developmental program of the African Trypanosoma. Transcriptomic analyses have already provided much information on tsetse fly genes as well as on genes of the fly symbiotic partners Sodalis glossinidius and Wigglesworthia, which account for the successful onset or not of the African Trypanosoma developmental program. In parallel, identification of the non- symbiotic bacterial communities hosted in the tsetse fly gut has recently been initiated: are briefly introduced those bacteria genera and species common to tsetse flies collected from distinct ecosystems, that could be further studied as potential biologicals preventing the onset of the African Trypanosoma developmental program. Finally, future work will need to concentrate on how to render tsetse flies refractory, and the best means to disseminate them in the field in order to establish an overall refractory fly population.

Paraquat-Mediated Oxidative Stress in Anopheles gambiae Mosquitoes Is Regulated by An Endoplasmic Reticulum (ER) Stress Response

Paraquat is a potent superoxide (O₂-)-inducing agent that is capable of inducing an oxidative imbalance in the mosquito midgut. This oxidative imbalance can super-stress the malaria parasite, leading to arrested development in the mosquito midgut and reduced transmission. While several studies have explored the effect of paraquat on malaria parasites, a fundamental understanding of the mosquito response to this compound remains unknown. Here, we quantified the mosquito midgut proteomic response to a paraquat-laced sugar meal, and found that An. gambiae midguts were enriched in proteins that are indicative of cells under endoplasmic reticulum (ER) stress. We also carried out qRT-PCR analyses for nine prominent thioredoxin (Trx) and glutathione (GSH)-dependent genes in mosquito midguts post P. falciparum blood meal ingestion to evaluate the concordance between transcripts and proteins under different oxidative stress conditions. Our data revealed an absence of significant upregulation in the Trx and GSH-dependent genes following infected blood meal ingestion. These data suggest that the intrinsic tolerance of the mosquito midgut to paraquat-mediated oxidative stress is through an ER stress response. These data indicate that mosquitoes have at least two divergent pathways of managing the oxidative stress that is induced by exogenous compounds, and outline the potential application of paraquat-like drugs to act selectively against malaria parasite development in mosquito midguts, thereby blocking mosquito-to-human transmission.

Ixodes scapularis does not harbor a stable midgut microbiome

Hard ticks of the order Ixodidae serve as vectors for numerous human pathogens, including the causative agent of Lyme Disease Borrelia burgdorferi. Tick-associated microbes can influence pathogen colonization, offering the potential to inhibit disease transmission through engineering of the tick microbiota. Here, we investigate whether B. burgdorferi encounters abundant bacteria within the midgut of wild adult Ixodes scapularis, its primary vector. Through the use of controlled sequencing methods and confocal microscopy, we find that the majority of field-collected adult I. scapularis harbor limited internal microbial communities that are dominated by endosymbionts. A minority of I. scapularis ticks harbor abundant midgut bacteria and lack B. burgdorferi. We find that the lack of a stable resident midgut microbiota is not restricted to I. scapularis since extension of our studies to I. pacificus, Amblyomma maculatum, and Dermacentor spp showed similar patterns. Finally, bioinformatic examination of the B. burgdorferi genome revealed the absence of genes encoding known interbacterial interaction pathways, a feature unique to the Borrelia genus within the phylum Spirochaetes. Our results suggest that reduced selective pressure from limited microbial populations within ticks may have facilitated the evolutionary loss of genes encoding interbacterial competition pathways from Borrelia.

Reduced diversity of gut microbiota in two Aedes mosquitoes species in areas of recent invasion

Aedes mosquitoes are considered highly successful global invasive species and vectors of several pathogens of relevance for public health. Their midgut's microbiota can play an important role in affecting not only their vectorial competence but also their fitness, physiology, food digestion, metabolism, immunity and adaptation to new environmental conditions. Using high-throughput sequencing we compared the microbiota of Aedes albopictus collected in Italy with those reported in populations from France and Vietnam. We also analysed Aedes koreicus gut microbiota for the first time. We found remarkable individual difference along with common bacterial taxa in both species. Ae. albopictus collected in Italy had a lower richness and a different composition of microbiota in respect to specimens collected in France and Vietnam. It also showed a core microbiota formed mainly of bacteria of the genus Pseudomonas. Overall, the two Aedes species (Ae. albopictus and Ae. koreicus) collected in Italy, showed a large core microbiota with 75.98% of the identified Operational Taxonomic Units. Furthermore, Ae. albopictus had 2.5% prevalence of Wolbachia and 0.07% of Asaia spp, while Ae. koreicus had 14.42% of Asaia spp. and no Wolbachia. This study provides new informations on the spatial variation of the midgut bacterial communities in mosquitoes of medical relevance within areas of recent invasion and provide the basis for further studies aimed at assessing the effects of such variation on vectorial capacity for a range of pathogens.

Influence of midgut microbiota in Anopheles stephensi on Plasmodium berghei infections

Background

The native gut microbiota of Anopheles mosquitoes is known to play a key role in the physiological function of its host. Interestingly, this microbiota can also influence the development of Plasmodium in its host mosquitoes. In recent years, much interest has been shown in the employment of gut symbionts derived from vectors in the control of vector-borne disease transmission. In this study, the midgut microbial diversity has been characterized among laboratory-reared adult Anopheles stephensi mosquitoes, from the colony created by rearing progeny of wild-caught mosquitoes (obtained from three different locations in southern India) for multiple generations, using 16S ribosomal RNA (rRNA) gene sequencing approach. Further, the influence of native midgut microbiota of mosquitoes on the development of rodent malaria parasite Plasmodium berghei in its host has been studied.

Methods

The microbial diversity associated with the midgut of An. stephensi mosquitoes was studied by sequencing V3 region of 16S ribosomal RNA (rRNA) gene. The influence of native midgut microbiota of An. stephensi mosquitoes on the susceptibility of the mosquitoes to rodent malaria parasite P. berghei was studied by comparing the intensity and prevalence of P. berghei infection among the antibiotic treated and untreated cohorts of mosquitoes.

Results

The analysis of bacterial diversity from the midguts of An. stephensi showed Proteobacteria as the most dominant population among the three laboratory-reared strains of An. stephensi studied. Major genera identified among these mosquito strains were Acinetobacter, Pseudomonas, Prevotella, Corynebacterium, Veillonella, and Bacillus. The mosquito infectivity studies carried out to determine the implication of total midgut microbiota on P. berghei infection showed that mosquitoes whose native microbiota cleared with antibiotics had increased susceptibility to P. berghei infection compared to the antibiotic untreated mosquitoes with its natural native microbiota.

Conclusions

The use of microbial symbiont to reduce the competence of vectors involved in disease transmission has gained much importance in recent years as an emerging alternative approach towards disease control. In this context, the present study was aimed to identify the midgut microbiota composition of An. stephensi, and its effect on the development of P. berghei. Interestingly, the analysis of midgut microbiota from An. stephensi revealed the presence of genus Veillonella in Anopheles species for the first time. Importantly, the study also revealed the negative influence of total midgut microbiota on the development of P. berghei in three laboratory strains of An. stephensi, emphasizing the importance of understanding the gut microbiota in malaria vectors, and its relationship with parasite development in designing strategies to control malaria transmission.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2535-7) contains supplementary material, which is available to authorized users.

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

Vector-borne diseases are a substantial portion of the global disease burden; one of the deadliest of these is malaria. Vector control strategies have been hindered by mosquito and pathogen resistances, and population alteration approaches using transgenic mosquitos still have many hurdles to overcome before they can be implemented in the field. Here we report a paratransgenic control strategy in which the microbiota of Anopheles stephensi was engineered to produce an antiplasmodial effector causing the mosquito to become refractory to Plasmodium berghei. The midgut symbiont Asaia was used to conditionally express the antiplasmodial protein scorpine only when a blood meal was present. These blood meal inducible Asaia strains significantly inhibit pathogen infection, and display improved fitness compared to strains that constitutively express the antiplasmodial effector. This strategy may allow the antiplasmodial bacterial strains to survive and be transmitted through mosquito populations, creating an easily implemented and enduring vector control strategy.

Bacterial symbionts in human blood-feeding arthropods: Patterns, general mechanisms and effects of global ecological changes

Due to their high impact on public health, human blood-feeding arthropods are one of the most relevant animal groups. Bacterial symbionts have been long known to play a role in the metabolism, and reproduction of these arthropod vectors. Nowadays, we have a more complete picture of their functions, acknowledging the wide influence of bacterial symbionts on processes ranging from the immune response of the arthropod host to the possible establishment of pathogens and parasites. One or two primary symbiont species have been found to co-evolve along with their host in each taxon (being ticks an exception), leading to various kinds of symbiosis, mostly mutualistic in nature. Moreover, several secondary symbiont species are shared by all arthropod groups. With respect to gut microbiota, several bacterial symbionts genera are hosted in common, indicating that these bacterial groups are prone to invade several hematophagous arthropod species feeding on humans. The main mechanisms underlying bacterium-arthropod symbiosis are discussed, highlighting that even primary symbionts elicit an immune response from the host. Bacterial groups in the gut microbiota play a key role in immune homeostasis, and in some cases symbiont bacteria could be competing directly or indirectly with pathogens and parasites. Finally, the effects climate change, great human migrations, and the increasingly frequent interactions of wild and domestic animal species are analyzed, along with their implications on microbiota alteration and their possible impacts on public health and the control of pathogens and parasites harbored in arthropod vectors of human parasites and pathogens.

Novel Wolbachia strains in Anopheles malaria vectors from Sub-Saharan Africa

Background: Wolbachia, a common insect endosymbiotic bacterium that can influence pathogen transmission and manipulate host reproduction, has historically been considered absent from the Anopheles (An.) genera, but has recently been found in An. gambiae s.l. populations.  As there are numerous Anopheles species that have the capacity to transmit malaria, we analysed a range of species to determine Wolbachia prevalence rates, characterise novel Wolbachia strains and determine any correlation between the presence of Plasmodium, Wolbachia and the competing endosymbiotic bacterium Asaia. Methods: Anopheles adult mosquitoes were collected from five malaria-endemic countries: Guinea, Democratic Republic of the Congo (DRC), Ghana, Uganda and Madagascar, between 2013 and 2017.  Molecular analysis of samples was undertaken using quantitative PCR, Sanger sequencing, Wolbachia multilocus sequence typing (MLST) and high-throughput amplicon sequencing of the bacterial 16S rRNA gene.  Results: Novel Wolbachia strains were discovered in five species: An. coluzzii, An. gambiae s.s., An. arabiensis, An. moucheti and An. species ‘A’, increasing the number of Anopheles species known to be naturally infected. Variable prevalence rates in different locations were observed and novel strains were phylogenetically diverse, clustering with Wolbachia supergroup B strains.  We also provide evidence for resident strain variants within An. species ‘A’.  Wolbachia is the dominant member of the microbiome in An. moucheti and An. species ‘A’, but present at lower densities in An. coluzzii.  Interestingly, no evidence of Wolbachia/Asaia co-infections was seen and Asaia infection densities were also shown to be variable and location dependent.  Conclusions: The important discovery of novel Wolbachia strains in Anopheles provides greater insight into the prevalence of resident Wolbachia strains in diverse malaria vectors.  Novel Wolbachia strains (particularly high-density strains) are ideal candidate strains for transinfection to create stable infections in other Anopheles mosquito species, which could be used for population replacement or suppression control strategies.

Micromanaging Immunity in the Murine Host vs. the Mosquito Vector: Microbiota-Dependent Immune Responses to Intestinal Parasites

The digestive tract plays a central role in nutrient acquisition and harbors a vast and intricate community of bacteria, fungi, viruses and parasites, collectively known as the microbiota. In recent years, there has been increasing recognition of the complex and highly contextual involvement of this microbiota in the induction and education of host innate and adaptive immune responses under homeostasis, during infection and inflammation. The gut passage and colonization by unicellular and multicellular parasite species present an immense challenge to the host immune system and to the microbial communities that provide vital support for its proper functioning. In mammals, parasitic nematodes induce distinct shifts in the intestinal microbial composition. Vice versa, the commensal microbiota has been shown to serve as a molecular adjuvant and immunomodulator during intestinal parasite infections. Moreover, similar interactions occur within insect vectors of deadly human pathogens. The gut microbiota has emerged as a crucial factor affecting vector competence in Anopheles mosquitoes, where it modulates outcomes of infections with malaria parasites. In this review, we discuss currently known involvements of the host microbiota in the instruction, support or suppression of host immune responses to gastrointestinal nematodes and protozoan parasites in mice, as well as in the malaria mosquito vector. A deeper understanding of the mechanisms underlying microbiota-dependent modulation of host and vector immunity against parasites in mammals and mosquitoes is key to a better understanding of the host-parasite relationships and the identification of more efficient approaches for intervention and treatment of parasite infections of both clinical and veterinary importance.

The NF-κB Inhibitor, IMD-0354, Affects Immune Gene Expression, Bacterial Microbiota and Trypanosoma cruzi Infection in Rhodnius prolixus Midgut

Rhodnius prolixus is an insect vector of Trypanosoma cruzi, the causative agent of Chagas disease in Latin America. Nuclear factor-κB (NF-κB) transcription factors (TF) are conserved components of the innate immune system in several multicellular organisms including insects. The drug IMD-0354 [N-(3,5-bis-trifluoromethyl-phenyl)-5-chloro-2-hydroxy-benzamide] is a selective inhibitor of IκB kinases. It blocks IκBα phosphorylation thus preventing nuclear translocation of the NF-κb TF. In humans, NF-κB is involved in several biological processes such as inflammation, cell proliferation and immunity. In insects, the activation of the immune system upon microbial challenge can be controlled by signaling pathways such as the immune deficiency (IMD) and Toll, to combat infection. These activated pathways signal to downstream NF-κB TF to stimulate specific immune genes, triggering the synthesis of several molecules such as the antimicrobial peptides. In Drosophila melanogaster, the activation and regulation of NF-κB TF have been elucidated, while in triatomines these mechanisms are not fully understood Therefore, the present study investigated the effects of oral administration of the drug IMD-0354 on the R. prolixus immune response to challenge with bacteria and T. cruzi, as well as the impact on the gut bacterial microbiota. R. prolixus were fed with rabbit blood containing IMD-0354 and Escherichia coli, Staphylococcus aureus, or T. cruzi. The effects of IMD-0354 on insect mortality and antimicrobial activity in insect midgut samples, as well as the relative expression of R. prolixus immune genes were recorded. The bacterial microbiota was analyzed, and viable parasites were counted in insect midgut samples. The IMD-0354 treatment modulated antibacterial activity and the gene expression patterns of defensin A, defensin B, defensin C, and prolixicin, and the genes involved in the IMD and Toll pathways. Additionally, there was an increase of bacterial microbiota in treated insects. Insects treated with IMD-0354 and concomitantly infected with bacteria or T. cruzi through the blood meal had increased mortality, while the T. cruzi population in R. prolixus midgut was reduced. The inhibitory effect of IMD-0354 indicates the importance of NF-κB TF in the innate immune responses involved in the control of bacteria and parasite infections in the R. prolixus midgut.

Vector biology meets disease control: using basic research to fight vector-borne diseases

Human pathogens that are transmitted by insects are a global problem, particularly those vectored by mosquitoes; for example, malaria parasites transmitted by Anopheles species, and viruses such as dengue, Zika and chikungunya that are carried by Aedes mosquitoes. Over the past 15 years, the prevalence of malaria has been substantially reduced and virus outbreaks have been contained by controlling mosquito vectors using insecticide-based approaches. However, disease control is now threatened by alarming rates of insecticide resistance in insect populations, prompting the need to develop a new generation of specific strategies that can reduce vector-mediated transmission. Here, we review how increased knowledge in insect biology and insect-pathogen interactions is stimulating new concepts and tools for vector control. We focus on strategies that either interfere with the development of pathogens within their vectors or directly impact insect survival, including enhancement of vector-mediated immune control, manipulation of the insect microbiome, or use of powerful new genetic tools such as CRISPR-Cas systems to edit vector genomes. Finally, we offer a perspective on the implementation hurdles as well as the knowledge gaps that must be filled in the coming years to safely realize the potential of these novel strategies to eliminate the scourge of vector-borne disease.

Factors shaping the gut bacterial community assembly in two main Colombian malaria vectors

BACKGROUND

The understanding of the roles of gut bacteria in the fitness and vectorial capacity of mosquitoes that transmit malaria, is improving; however, the factors shaping the composition and structure of such bacterial communities remain elusive. In this study, a high-throughput 16S rRNA gene sequencing was conducted to understand the effect of developmental stage, feeding status, species, and geography on the composition of the gut bacterial microbiota of two main Colombian malaria vectors, Anopheles nuneztovari and Anopheles darlingi.

RESULTS

The results revealed that mosquito developmental stage, followed by geographical location, are more important determinants of the gut bacterial composition than mosquito species or adult feeding status. Further, they showed that mosquito gut is a major filter for environmental bacteria colonization.

CONCLUSIONS

The sampling design and analytical approach of this study allowed to untangle the influence of factors that are simultaneously shaping the microbiota composition of two Latin-American malaria vectors, essential aspect for the design of vector biocontrol strategies.