Metabolic interactions between disease-transmitting vectors and their microbiota

Vectors transmit a variety of infectious pathogens that threaten animal and human health.The different life cycles, feeding habits, and habitats of these vectors lead to their association with diverse symbiotic microorganisms.Metabolic interactions between symbiotic microbes and vectors profoundly influence multiple aspects of vector biology.Harnessing and manipulating vector–microbe symbioses can provide potential targets for vector control. Disease-transmitting vectors are living organisms that pass infectious agents from one animal/human to another. The epidemiologically important vectors are usually hematophagous arthropods, including mosquitoes, ticks, triatome bugs, sand flies, and tsetse flies. All of them harbor an endogenous microbiota that functionally complements their host’s biology. Different arthropod vectors are ecologically and behaviorally distinct, and as such, their relationships with symbiotic microbes vary. In this review, we summarize the recent discoveries that reveal how bacterial metabolic activities influence development, nutrition, and pathogen defense in mosquitoes, ticks, triatome bugs, and sand flies. These studies provide a foundation for a systematic understanding of vector–microbiota interactions and for the development of integrated approaches to control vector-borne diseases. Disease-transmitting vectors are living organisms that pass infectious agents from one animal/human to another. The epidemiologically important vectors are usually hematophagous arthropods, including mosquitoes, ticks, triatome bugs, sand flies, and tsetse flies. All of them harbor an endogenous microbiota that functionally complements their host’s biology. Different arthropod vectors are ecologically and behaviorally distinct, and as such, their relationships with symbiotic microbes vary. In this review, we summarize the recent discoveries that reveal how bacterial metabolic activities influence development, nutrition, and pathogen defense in mosquitoes, ticks, triatome bugs, and sand flies. These studies provide a foundation for a systematic understanding of vector–microbiota interactions and for the development of integrated approaches to control vector-borne diseases. Vector-borne diseases, which account for more than 17% of all infectious diseases, kill more than 700 000 people annually [1.World Health Organization Global Vector Control Response (2017–2030). WHO, 2017Google Scholar]. Many vectors are hematophagous arthropods that transmit infectious agents during a blood meal [1.World Health Organization Global Vector Control Response (2017–2030). WHO, 2017Google Scholar]. Mosquitoes are the deadliest animals in the world. Three genera of mosquitoes, Anopheles, Aedes, and Culex, are the main disease-transmitting vectors. Anopheles mosquitoes vector malaria parasites that infect 241 million people globally [2.World Health Organization World Malaria Report 2021. WHO, 2022Google Scholar]. Aedes mosquitoes mainly transmit arboviruses, including dengue virus, chikungunya virus, Rift Valley virus, Zika virus, and yellow fever virus. Other viruses, including Japanese encephalitis virus and West Nile virus are transmitted by Culex [1.World Health Organization Global Vector Control Response (2017–2030). WHO, 2017Google Scholar]. Ticks are another important group of vectors that comprise two primary families, Ixodidae (hard ticks) and Argasidae (soft ticks). Both families spread a broad diversity of pathogens, including viruses, bacteria, and parasites. Ticks often carry more than one pathogen simultaneously [3.Rochlin I. Toledo A. Emerging tick-borne pathogens of public health importance: a mini-review.J. Med. Microbiol. 2020; 69: 781-791Crossref PubMed Scopus (92) Google Scholar]. Other vectors, including sand flies, tsetse flies, and triatome bugs, transmit pathogens (that cause leishmaniasis, human and animal African trypanosomiases, and Chagas' disease, respectively) which are responsible for high levels of morbidity and mortality [1.World Health Organization Global Vector Control Response (2017–2030). WHO, 2017Google Scholar]. Most vector-borne diseases lack effective vaccines and drugs, and disease prevention relies primarily on vector control. Understanding endogenous regulation of vector biology will help to develop effective approaches for vector control. Hematophagous vectors include obligate and non-obligate blood feeders [4.Beaty B.J. Marquardt W.C. The Biology of Disease Vectors. The University Press of Colorado, 1996Google Scholar]. Obligate blood feeders, including ticks, tsetse flies, and triatome bugs, feed exclusively on vertebrate blood during all life stages. Conversely, non-obligate blood feeders, such as mosquitoes and sand flies, consume organic materials during immature stages and, in addition to blood, they ingest sugars for energy provisioning during adulthood. An increasing number of studies have demonstrated that vector metabolism changes profoundly during development, blood feeding, and during environmental stress [5.Horvath T.D. et al.Unraveling mosquito metabolism with mass spectrometry-based metabolomics.Trends Parasitol. 2021; 37: 747-761Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar, 6.Martins R. et al.The dynamics of energy metabolism in the tick embryo.Rev. Bras. Parasitol. Vet. 2018; 27: 259-266PubMed Google Scholar, 7.Cabezas-Cruz A. et al.Tick–pathogen interactions: the metabolic perspective.Trends Parasitol. 2019; 35: 316-328Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar]. Nutritional status in turn influences the development, fecundity, and vector competence of these arthropods [8.Galay R.L. et al.Iron metabolism in hard ticks (Acari: Ixodidae): The antidote to their toxic diet.Parasitol. Int. 2015; 64: 182-189Crossref PubMed Scopus (45) Google Scholar, 9.Weger-Lucarelli J. et al.Taking a bite out of nutrition and arbovirus infection.PLoS Negl. Trop. Dis. 2018; 12e0006247Crossref Scopus (22) Google Scholar, 10.Carvajal-Lago L. et al.Implications of diet on mosquito life history traits and pathogen transmission.Environ. Res. 2021; 195110893Crossref PubMed Scopus (12) Google Scholar, 11.Shaw W.R. et al.Plasmodium development in Anopheles: a tale of shared resources.Trends Parasitol. 2022; 38: 124-135Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar]. These bloodsucking arthropods harbor a broad spectrum of symbiotic bacteria, viruses, and eukaryotic organisms – collectively known as their microbiota (see Glossary) [12.de Almeida J.P. et al.The virome of vector mosquitoes.Curr. Opin. Virol. 2021; 49: 7-12Crossref PubMed Scopus (17) Google Scholar,13.Gao H. et al.Mosquito microbiota and implications for disease control.Trends Parasitol. 2020; 36: 98-111Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar]. In this review, we focus on the bacterial component of the vector microbiota, and we detail the functional role these microbes have on their host’s biology. The relationship between symbiotic bacteria and arthropod vectors is usually obligate or facultative in nature [14.Fisher R.M. et al.The evolution of host–symbiont dependence.Nat. Commun. 2017; 8: 15973Crossref PubMed Scopus (142) Google Scholar,15.Elston K.M. et al.Engineering insects from the endosymbiont out.Trends Microbiol. 2022; 30: 79-96Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar]. Obligate symbionts feature conspicuously reduced genomes, yet many have retained the capacity to provide their vector hosts with nutrients missing from vertebrate blood. Obligate symbionts are vertically transmitted and reside intracellularly within cells that usually collectively form a specialized niche referred to as a ‘bacteriome’. Conversely, facultative symbionts often have free-living stages, are usually transiently acquired by their vector host (although they can also be vertically transmitted), and can thereafter reside intracellularly or extracellularly. Facultative symbionts generally benefit their host but are not required for host survival. Obligate and facultative symbionts often live in or immediately adjacent to (as in the case of bacteriome-residing symbionts) their host’s gut in close proximity to pathogens. Pathogens undergo complex, metabolically intensive developmental programs while residing within their arthropod vector hosts. Like their arthropod vectors, these pathogens lack the ability to synthesize all necessary nutrients de novo, and as such, they too often depend on provisioning by vectors or symbiotic bacteria [7.Cabezas-Cruz A. et al.Tick–pathogen interactions: the metabolic perspective.Trends Parasitol. 2019; 35: 316-328Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar,16.Zuzarte-Luis V. Mota M.M. Parasite sensing of host nutrients and environmental cues.Cell Host Microbe. 2018; 23: 749-758Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar,17.Attardo G.M. et al.Bacterial symbionts of tsetse flies: relationships and functional interactions between tsetse flies and their symbionts.Results Probl. Cell Differ. 2020; 69: 497-536Crossref PubMed Scopus (5) Google Scholar]. In this review we focus on four prominent disease vector systems (mosquito, tick, triatome bug, and sand fly) and provide an overview of how metabolites derived from their bacterial symbionts influence vector development, how bacterial symbionts react to invading pathogens, and how symbiont–symbiont interaction occurs within the arthropod vector. There are around 3500 species of mosquitoes in the world, but only members of the genera Anopheles, Aedes, and Culex serve as the main disease-transmitting vectors [4.Beaty B.J. Marquardt W.C. The Biology of Disease Vectors. The University Press of Colorado, 1996Google Scholar,18.Clements A.N. The Biology of Mosquitoes: Development, Nutrition and Reproduction. Chapman & Hall, 1992Google Scholar]. Immature mosquito larval and pupal stages are aquatic, and larvae acquire organic materials from their environment. Adults are terrestrial and can feed on plant saps and nectars, although females from disease-transmitting genera also ingest animal blood for egg development [18.Clements A.N. The Biology of Mosquitoes: Development, Nutrition and Reproduction. Chapman & Hall, 1992Google Scholar]. Therefore, mosquitoes are exposed to, and acquire, a variety of microbes from their habitats. The mosquito midgut, where digestion occurs, is in direct contact with ingested materials, including food, microbes, and toxins. As such, this tissue hosts a dynamic and diversified microbial community. Other organs, such as salivary glands and reproductive tracts, are also colonized with a variety of microbiota (Figure 1A , Key figure) [13.Gao H. et al.Mosquito microbiota and implications for disease control.Trends Parasitol. 2020; 36: 98-111Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar]. Microbes belonging to the phyla Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria are commonly found in midguts of different mosquito species and have a profound influence on mosquito biology [13.Gao H. et al.Mosquito microbiota and implications for disease control.Trends Parasitol. 2020; 36: 98-111Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar]. Here we focus on the recent research that highlights the metabolic contribution of bacterial symbionts to mosquito development, metabolism, and vector competence (Table 1).Table 1The influence of selected symbionts on vector metabolismVectorsSymbiontsMetabolic functionsImpact on pathogensRefsMosquitoSerratia sp.Prodigiosin ProductionInhibition of Plasmodium infection[60.Comandatore F. et al.Modeling the life cycle of the intramitochondrial bacterium "Candidatus Midichloria mitochondrii" using electron microscopy data.mBio. 2021; 12e0057421Crossref Scopus (7) Google Scholar]Blood digestion–[34.Gaio Ade O. et al.Contribution of midgut bacteria to blood digestion and egg production in Aedes aegypti (diptera: culicidae) (L.).Parasit. Vectors. 2011; 4: 105Crossref PubMed Scopus (179) Google Scholar,35.Chen S. et al.Genomic, physiologic, and symbiotic characterization of Serratia marcescens strains isolated from the mosquito Anopheles stephensi.Front. Microbiol. 2017; 8: 1483Crossref PubMed Scopus (32) Google Scholar]lipase secretionInhibition of Plasmodium infection[32.Guegan M. et al.Who is eating fructose within the Aedes albopictus gut microbiota?.Environ. Microbiol. 2020; 22: 1193-1206Crossref PubMed Scopus (15) Google Scholar]Enhancin secretionPromotion of dengue virus infection[50.Wu P. et al.A gut commensal bacterium promotes mosquito permissiveness to arboviruses.Cell Host Microbe. 2019; 25: 101-112. e5Abstract Full Text Full Text PDF PubMed Scopus (104) Google Scholar]Lelliottia and AmpullimonsFructose assimilation–[32.Guegan M. et al.Who is eating fructose within the Aedes albopictus gut microbiota?.Environ. Microbiol. 2020; 22: 1193-1206Crossref PubMed Scopus (15) Google Scholar]Acinetobacter sp.Blood digestion–[33.Minard G. et al.Prevalence, genomic and metabolic profiles of Acinetobacter and Asaia associated with field-caught Aedes albopictus from Madagascar.FEMS Microbiol. Ecol. 2013; 83: 63-73Crossref PubMed Scopus (63) Google Scholar]Asaia sp.Nitrogen metabolism–[36.Samaddar N. et al.Nitrogen fixation in Asaia sp. (family Acetobacteraceae).Curr. Microbiol. 2011; 63: 226-231Crossref PubMed Scopus (23) Google Scholar]Insecticide detoxification–[41.Comandatore F. et al.Phylogenomics reveals that Asaia symbionts from insects underwent convergent genome reduction, preserving an insecticide-degrading gene.mBio. 2021; 12: e00106-e00121Crossref PubMed Scopus (8) Google Scholar,42.Scates S.S. et al.Bacteria-mediated modification of insecticide toxicity in the yellow fever mosquito, Aedes aegypti.Pestic. Biochem. Physiol. 2019; 161: 77-85Crossref PubMed Scopus (11) Google Scholar]Midgut pH regulationPromotion of Plasmodium infection[29.Wang M. et al.Glucose-mediated proliferation of a gut commensal bacterium promotes Plasmodium infection by increasing mosquito midgut pH.Cell Rep. 2021; 35108992Abstract Full Text Full Text PDF Scopus (18) Google Scholar]Elizabethkingia anophelisBlood digestion and iron scavenging–[37.Kukutla P. et al.Insights from the genome annotation of Elizabethkingia anophelis from the malaria vector Anopheles gambiae.PLoS One. 2014; 9e97715Crossref PubMed Scopus (35) Google Scholar,38.Chen S. et al.Elizabethkingia anophelis: physiologic and transcriptomic responses to iron stress.Front. Microbiol. 2020; 11: 804Crossref PubMed Scopus (8) Google Scholar]Pseudomonas alcaligenes3-hydroxykynurenine metabolismInhibition of Plasmodium infection[48.Feng Y. et al.Anopheline mosquitoes are protected against parasite infection by tryptophan catabolism in gut microbiota.Nat. Microbiol. 2022; 7: 707-715Crossref PubMed Scopus (9) Google Scholar]TickCoxiella-like symbiontB vitamins provision–[55.Smith T.A. et al.A Coxiella-like endosymbiont is a potential vitamin source for the lone star tick.Genome Biol. Evol. 2015; 7: 831-838Crossref PubMed Scopus (156) Google Scholar]Chorismite production–[58.Gottlieb Y. et al.Distinctive genome reduction rates revealed by genomic analyses of two Coxiella-like endosymbionts in ticks.Genome Biol Evol. 2015; 7: 1779-1796Crossref PubMed Scopus (111) Google Scholar,65.Zhong Z. et al.Symbiont-regulated serotonin biosynthesis modulates tick feeding activity.Cell Host Microbe. 2021; 29: 1545-1557.e4Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar]Francisella-like symbiontB vitamins provision–[56.Duron O. et al.Tick–bacteria mutualism depends on B vitamin synthesis pathways.Curr. Biol. 2018; 28: 1896-1902.e5Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar]Rickettisa-like symbiontB vitamins provision–[57.Hunter D.J. et al.The Rickettsia endosymbiont of Ixodes pacificus contains all the genes of de novo folate biosynthesis.PLoS One. 2015; 10e0144552Crossref Scopus (79) Google Scholar]Candidatus Midichloria mitochondriiB vitamins provision, ATP exchange–[61.Sassera D. et al.Phylogenomic evidence for the presence of a flagellum and cbb3 oxidase in the free-living mitochondrial ancestor.Mol. Biol. Evol. 2011; 28: 3285-3296Crossref PubMed Scopus (98) Google Scholar]TriatomineNocardia rhodniiThiamine and folic acid provision–[77.Salcedo-Porras N. et al.The role of bacterial symbionts in triatomines: an evolutionary perspective.Microorganisms. 2020; 8: 1438Crossref PubMed Scopus (22) Google Scholar]Rhodococcus rhodniiB vitamins provision–[78.Pachebat J.A. et al.Draft genome sequence of Rhodococcus rhodnii strain LMG5362, a symbiont of Rhodnius prolixus (Hemiptera, Reduviidae, Triatominae), the principle vector of Trypanosoma cruzi.Genome Announc. 2013; 1e00329–13Crossref PubMed Scopus (26) Google Scholar,79.Tobias N.J. et al.Enzymatic biosynthesis of B-complex vitamins is supplied by diverse microbiota in the Rhodnius prolixus anterior midgut following Trypanosoma cruzi infection.Comput. Struct. Biotec. 2020; 18: 3395-3401Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar]Serratia sp.Prodigiosin productionInhibition of trypanosome infection[77.Salcedo-Porras N. et al.The role of bacterial symbionts in triatomines: an evolutionary perspective.Microorganisms. 2020; 8: 1438Crossref PubMed Scopus (22) Google Scholar]Sand flyRahnella aquatilisSucrose catabolismPromotion of Leishmania infection[84.Louradour I. et al.The midgut microbiota plays an essential role in sand fly vector competence for Leishmania major.Cell. Microbiol. 2017; 19https://doi.org/10.1111/cmi.12755Crossref PubMed Scopus (53) Google Scholar] Open table in a new tab In an effort to understand the role of microbiota on mosquito biology, researchers began generating microbe-free (axenic) mosquitoes in the 1930s [19.Steven B. et al.The axenic and gnotobiotic mosquito: emerging models for microbiome host interactions.Front. Microbiol. 2021; 12714222Crossref Scopus (9) Google Scholar]. Until recently, axenic mosquito larvae were generated by surface-sterilizing eggs, rearing all developmental stages in sterile environments, and using a sterilized diet. However, these axenic individuals fail to develop past the larval stage unless recolonized with live endogenous bacteria or non-endogenous Escherichia coli [20.Coon K.L. et al.Mosquitoes rely on their gut microbiota for development.Mol. Ecol. 2014; 23: 2727-2739Crossref PubMed Scopus (301) Google Scholar]. Mechanistic studies further revealed that bacteria-induced hypoxia plays a role in larva development [21.Coon K.L. et al.Bacteria-mediated hypoxia functions as a signal for mosquito development.Proc. Natl. Acad. Sci. U. S. A. 2017; 114: E5362-E5369Crossref PubMed Scopus (91) Google Scholar,22.Valzania L. et al.Hypoxia-induced transcription factor signaling is essential for larval growth of the mosquito Aedes aegypti.Proc. Natl. Acad. Sci. U. S. A. 2018; 115: 457-465Crossref PubMed Scopus (55) Google Scholar]. More so, generation of the hypoxic midgut environment relies on the presence of riboflavin derived from resident bacteria. Riboflavin influences mosquito respiratory metabolism through its byproducts flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) [23.Wang Y. et al.Riboflavin instability is a key factor underlying the requirement of a gut microbiota for mosquito development.Proc. Natl. Acad. Sci. U. S. A. 2021; 118e2101080118Google Scholar]. Another study revealed that nutrition, especially bacteria-derived nutrients, is another key factor that influences larval development [24.Correa M.A. et al.Generation of axenic Aedes aegypti demonstrate live bacteria are not required for mosquito development.Nat. Commun. 2018; 9: 4464Crossref PubMed Scopus (53) Google Scholar]. Altogether, the results from different groups suggest that gut microbiota and their nutritional contributions are essential factors in larval development [20.Coon K.L. et al.Mosquitoes rely on their gut microbiota for development.Mol. Ecol. 2014; 23: 2727-2739Crossref PubMed Scopus (301) Google Scholar,25.Martinson V.G. Strand M.R. Diet-microbiota interactions alter mosquito development.Front. Microbiol. 2021; 12650743Crossref PubMed Scopus (10) Google Scholar]. Recently, a novel approach was developed to produce axenic adult mosquitoes. Specifically, axenic larva are transiently colonized with genetically modified E. coli auxotrophic for the two nonstandard amino acids – meso-diaminopimelic acid (m-DAP) and D-alanine (D-Ala) [26.Romoli O. et al.Production of germ-free mosquitoes via transient colonisation allows stage-specific investigation of host–microbiota interactions.Nat. Commun. 2021; 12: 942Crossref PubMed Scopus (27) Google Scholar]. The bacterium can be eliminated at any point during larval development by removing the two amino acids from the diet. The use of this technique also demonstrated that larval resident bacteria play an important role in providing folate and increasing energy storage. In addition to influencing larval development, bacterial colonization also influences lipid metabolism and starvation resistance in adults, suggesting the carry-over effects of bacteria from larva to adult [27.Giraud E. et al.Mosquito–bacteria interactions during larval development trigger metabolic changes with carry-over effects on adult fitness.Mol. Ecol. 2021; 31: 1444-1460Crossref Scopus (7) Google Scholar]. Some symbionts are also larvicidal. For example, Serratia marcescens impairs larval survival of multiple mosquito species by producing the metabolite prodigiosin [28.Suryawanshi R.K. et al.Mosquito larvicidal and pupaecidal potential of prodigiosin from Serratia marcescens and understanding its mechanism of action.Pestic. Biochem. Physiol. 2015; 123: 49-55Crossref PubMed Scopus (38) Google Scholar]. Altogether, these studies demonstrate that metabolites derived from resident bacteria play essential roles in either positively or negatively influencing mosquito development. Sugar and blood are the two major food sources for adult mosquitoes. Sugar ingested by mosquitoes is first stored in the crop and then transferred to the midgut for digestion [18.Clements A.N. The Biology of Mosquitoes: Development, Nutrition and Reproduction. Chapman & Hall, 1992Google Scholar]. Sugar composition influences the metabolic activity of mosquito microbiota [29.Wang M. et al.Glucose-mediated proliferation of a gut commensal bacterium promotes Plasmodium infection by increasing mosquito midgut pH.Cell Rep. 2021; 35108992Abstract Full Text Full Text PDF Scopus (18) Google Scholar]. Vertebrate blood is stored and digested in the mosquito midgut [18.Clements A.N. The Biology of Mosquitoes: Development, Nutrition and Reproduction. Chapman & Hall, 1992Google Scholar]. Blood ingestion elicits exponential bacterial growth but reduces bacterial diversity [30.Wang Y. et al.Dynamic gut microbiome across life history of the malaria mosquito Anopheles gambiae in Kenya.PLoS One. 2011; 6e24767Google Scholar]. A metabolomic comparison between antibiotic-treated and control Anopheles coluzzii revealed that antibiotic treatment significantly influences the tricarboxylic acid cycle and amino acid metabolism, suggesting that gut microbiota is involved in sugar metabolism and protein digestion [31.Chabanol E. et al.Antibiotic treatment in Anopheles coluzzii affects carbon and nitrogen metabolism.Pathogens. 2020; 9: 679Crossref Scopus (11) Google Scholar]. For example, the gut microbes Lelliottia and Ampullimons in male and female Asian tiger mosquitoes, respectively, are actively involved in assimilating fructose and possibly converting fructose into other nutrients for the mosquito [32.Guegan M. et al.Who is eating fructose within the Aedes albopictus gut microbiota?.Environ. Microbiol. 2020; 22: 1193-1206Crossref PubMed Scopus (15) Google Scholar]. Acinetobacter sp. isolated from Aedes albopictus adapts to specifically digesting blood-derived metabolites, such as α-keto-valeric acid and glycine, and nectar containing 4-hydroxy-benzoic acid and xylose [33.Minard G. et al.Prevalence, genomic and metabolic profiles of Acinetobacter and Asaia associated with field-caught Aedes albopictus from Madagascar.FEMS Microbiol. Ecol. 2013; 83: 63-73Crossref PubMed Scopus (63) Google Scholar]. Serratia sp. isolated from Anopheles stephensi and Aedes aegypti facilitate blood digestion through their hemolytic activity [34.Gaio Ade O. et al.Contribution of midgut bacteria to blood digestion and egg production in Aedes aegypti (diptera: culicidae) (L.).Parasit. Vectors. 2011; 4: 105Crossref PubMed Scopus (179) Google Scholar,35.Chen S. et al.Genomic, physiologic, and symbiotic characterization of Serratia marcescens strains isolated from the mosquito Anopheles stephensi.Front. Microbiol. 2017; 8: 1483Crossref PubMed Scopus (32) Google Scholar]. Asaia bogorensis might be responsible for nitrogen metabolism through its functional nitrogenase in mosquitoes [36.Samaddar N. et al.Nitrogen fixation in Asaia sp. (family Acetobacteraceae).Curr. Microbiol. 2011; 63: 226-231Crossref PubMed Scopus (23) Google Scholar]. The bacterial symbiont Elizabethkingia anophelis regulates the expression of multiple genes that code for hemolysin and for the synthesis of iron-binding siderophores when Anopheles gambiae consumes a blood meal. This process helps the mosquito digest erythrocytes and alleviate iron stress [37.Kukutla P. et al.Insights from the genome annotation of Elizabethkingia anophelis from the malaria vector Anopheles gambiae.PLoS One. 2014; 9e97715Crossref PubMed Scopus (35) Google Scholar,38.Chen S. et al.Elizabethkingia anophelis: physiologic and transcriptomic responses to iron stress.Front. Microbiol. 2020; 11: 804Crossref PubMed Scopus (8) Google Scholar]. Oral administration of two siderophores, serratiochelin A and pyochelin, to An. gambiae significantly reduces blood-feeding propensity and fecundity [39.Ganley J.G. et al.A systematic analysis of mosquito-microbiome biosynthetic gene clusters reveals antimalarial siderophores that reduce mosquito reproduction capacity.Cell Chem. Biol. 2020; 27: 817-826. e5Abstract Full Text Full Text PDF PubMed Scopus (9) Google Scholar]. These studies suggest that mosquito symbionts adjust their gene expression to cope with iron-mediated stress during blood digestion. The expression of these siderophore genes could in turn be manipulated to antagonize mosquito blood feeding. In addition to nutrient assimilation, the microbiota also influences other facets of mosquito physiology. For example, the microbiota of Culex pipiens pipiens helps its host to prepare for diapause possibly by assisting with carbohydrate and lipid metabolism [40.Didion E.M. et al.Microbiome reduction prevents lipid accumulation during early diapause in the northern house mosquito, Culex pipiens.J. Insect Physiol. 2021; 134104295Crossref PubMed Scopus (8) Google Scholar]. Gut microbes, such as Asaia sp. isolated from multiple mosquitoes and insect pest species, help their host to detoxify insecticides, possibly contributing to the emergence and spread of insecticide resistance [41.Comandatore F. et al.Phylogenomics reveals that Asaia symbionts from insects underwent convergent genome reduction, preserving an insecticide-degrading gene.mBio. 2021; 12: e00106-e00121Crossref PubMed Scopus (8) Google Scholar,42.Scates S.S. et al.Bacteria-mediated modification of insecticide toxicity in the yellow fever mosquito, Aedes aegypti.Pestic. Biochem. Physiol. 2019; 161: 77-85Crossref PubMed Scopus (11) Google Scholar]. Gut microbes play crucial roles in determining mosquito vector competence. One of their well known functions is to increase the refractoriness of mosquitoes to pathogen infection by priming basal immunity and innate immunity memory [13.Gao H. et al.Mosquito microbiota and implications for disease control.Trends Parasitol. 2020; 36: 98-111Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar,43.Rodrigues J. et al.Hemocyte differentiation mediates innate immune memory in Anopheles gambiae mosquitoes.Science. 2010; 329: 1353-1355Crossref PubMed Scopus (336) Google Scholar]. Here we detail knowledge of bacterial metabolic contributions that impact mosquito vector competence. Wolbachia pipientis is well known for its ability to induce cytoplasmic incompatibility and inhibit transmission of a variety of arboviruses by enhancing its host’s immune response. As such, this symbiont is currently being used to control mosquito-borne diseases [44.Caragata E.P. et al.Wolbachia as translational science: controlling mosquito-borne pathogens.Trends Parasitol. 2021; 37: 1050-1067Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar]. In addition to manipulating mosquito immune activity, Wolbachia infection changes lipid metabolism. The competition of lipid resources between Wolbachia and viruses might play a role in blocking virus transmission [45.Molloy J.C. et al.Wolbachia modulates lipid metabolism in Aedes albopictus mosquito cells.Appl. Environ. Microbiol. 2016; 82: 3109-3120Crossref PubMed Scopus (76) Google Scholar,46.Manokaran G. et al.Modulation of acyl-carnitines, the broad mechanism behind Wolbachia-mediated inhibition of medically important flaviviruses in Aedes aegypti.Proc. Natl. Acad. Sci. U. S. A. 2020; 117: 24475-24483Crossref PubMed Scopus (19) Google Scholar]. This bacterium also downregulates mosquito insulin receptor production that is required for dengue and Zika virus replication. This process inhibits the ability of these arboviruses to successfully infect their host [47.Haqshenas G. et al.A role for the insulin receptor in the suppression of dengue virus and zika virus in Wolbachia-infected mosquito cells.Cell Rep. 2019; 26