Impact of the mosquito microbiome on vector competence
Mosquitoes harbor bacterial communities that have diverse impacts on nutrition, digestion, metabolism, development, immunity, and other aspects of insect biology [103, 104]. The adult mosquito gut, in particular, is a site of complex reciprocal interactions between the natural gut microbiota, the mosquito host response, and bloodmeal-acquired pathogens such as DENV. Importantly, the gut microbiome is known to influence vector competence for DENV and other mosquito-borne pathogens (reviewed in [105]).
Removal of native gut bacteria by antibiotic treatment has been reported to render Ae. aegypti more susceptible to DENV infection; these aseptic mosquitoes also display reduced levels of AMP expression [65]. In addition, several bacterial isolates derived from field-collected mosquitoes have the ability to inhibit DENV replication when reintroduced into aseptic mosquito midguts [102, 106]. In some cases, bacteria are thought to activate basal level production of immune effectors such as AMPs, and thus prime the mosquito against subsequent viral infection [65, 70, 102]. This is consistent with known functional overlaps between the mosquito antibacterial and antiviral responses [65, 66, 70, 102]. Other bacteria have been shown to inhibit DENV independently of the mosquito, and are thought to produce secondary metabolites that have direct antiviral activity [106].
Bacteria of the genus Wolbachia are maternally inherited, intracellular endosymbionts that naturally infect a wide range of insects, including Drosophila and Ae. albopictus, but not Ae. aegypti. Stable trans-infection of Ae. aegypti has been achieved through embryo microinjection [107, 108], producing mosquitoes that are more resistant to a range of pathogens, including DENV, chikungunya virus (CHIKV), yellow fever virus (YFV), and Plasmodium [109–111]. Microarray analyses indicate that Wolbachia induces the expression of Toll pathway and other immunity-related genes in stably trans-infected Ae. aegypti [70, 112, 113]. However, as Wolbachia restricts DENV in Drosophila and Ae. albopictus (two species with a long natural history of Wolbachia infection) in the absence of immune activation, it has been suggested that immune priming is not the fundamental mechanism of virus restriction, although it may enhance the trait in heterologous mosquito hosts [113, 114]. Wolbachia has also been shown to compete with the virus for crucial host resources [115], and to modulate the expression of certain mosquito microRNAs, thereby altering host gene expression to facilitate its own replication [116, 117].
In mosquitoes, Wolbachia is particularly suited for use in a population-replacement transmission-blocking strategy because of its ability to induce cytoplasmic incompatibility (CI), a phenomenon (maintained in stably trans-infected Ae. aegypti) in which crosses between uninfected females and infected males result in embryonic lethality (reviewed in [118]). This increases the reproductive success of infected females and allows Wolbachia to spread rapidly through insect populations despite possible fitness costs.
Sequencing-based, culture-independent approaches are increasingly being used to obtain comprehensive profiles of field mosquito microbiomes [119–122]. In Anopheles gambiae, the major African vector of malaria, targeted deep sequencing of microbial 16S ribosomal RNA revealed distinct gut microbiome communities at the aquatic larval and pupal stages and the terrestrial adult stage [119]. This finding is consistent with the fact that gut contents are usually cleared upon metamorphosis during the larvae-to-pupae and pupae-to-adult transitions [123], and implies that repopulation of the microbiome occurs at each stage. Bloodmeals drastically reduced gut microbiome diversity and led to an expansion of members of the Enterobacteriaceae family. These bacteria possess antioxidant mechanisms that may allow them to cope with the oxidative and nitrosative stresses associated with bloodmeal catabolism, suggesting that they benefit the mosquito by helping to maintain gut redox homeostasis [119].
1A study characterizing the microbiomes of wild-caught Aedes, Anopheles, and Culex mosquitoes from Kenya found that the gut microbiome of an individual adult mosquito was typically dominated by one bacterial taxon, while also containing many other much less abundant taxa. Although different mosquito species shared remarkably similar gut bacteria, there was enormous variation within individuals of the same species [120].
The composition and dynamics of endogenous mosquito gut microbiota may affect natural rates of disease transmission, as well as the success of transmission-blocking strategies that involve the introduction of native or non-native bacterial species into mosquito populations. Recent studies, for example, suggest that vertical transmission of Wolbachia in An. gambiae (another non-naturally infected mosquito species) is inhibited by native Asaia [124, 125]. The development of improved 16S sequencing methods that allow species-level identification [126], as well as metagenomic sequencing approaches that yield information on microbial function in addition to identity [127, 128], should help us understand complex relationships between bacterial communities and their insect hosts.