Functional genomic analyses of Enterobacter, Anopheles and Plasmodium reciprocal interactions that impact vector competence

Background: Malaria exerts a tremendous socioeconomic impact worldwide despite current control efforts, and novel disease transmission-blocking strategies are urgently needed. The Enterobacter bacterium Esp-Z, which is naturally harboured in the mosquito midgut, can inhibit the development of Plasmodium parasites prior to their invasion of the midgut epithelium through a mechanism that involves oxidative stress. Here, a multifaceted approach is used to study the tripartite interactions between the mosquito, Esp-Z and Plasmodium, towards addressing the feasibility of using sugar-baited exposure of mosquitoes to the Esp-Z bacterium for interruption of malaria transmission. Methods: The ability of Esp-Z to colonize Anopheles gambiae midguts harbouring microbiota derived from wild mosquitoes was determined by qPCR. Upon introduction of Esp-Z via nectar feeding, the permissiveness of colonized mosquitoes to Plasmodium falciparum infection was determined, as well as the impact of Esp-Z on mosquito fitness parameters, such as longevity, number of eggs laid and number of larvae hatched. The genome of Esp-Z was sequenced, and transcriptome analyses were performed to identify bacterial genes that are important for colonization of the mosquito midgut, as well as for ROS-production. A gene expression analysis of members of the oxidative defence pathway of Plasmodium berghei was also conducted to assess the parasite's oxidative defence response to Esp-Z exposure. Results: Esp-Z persisted for up to 4 days in the An. gambiae midgut after introduction via nectar feeding, and was able to significantly inhibit Plasmodium sporogonic development. Introduction of this bacterium did not adversely affect mosquito fitness. Candidate genes involved in the selection of a better fit Esp-Z to the mosquito midgut environment and in its ability to condition oxidative status of its surroundings were identified, and parasite expression data indicated that Esp-Z is able to induce a partial and temporary shutdown of the ookinetes antioxidant response. Conclusions: Esp-Z is capable of inhibiting sporogonic development of Plasmodium in the presence of the mosquito's native microbiota without affecting mosquito fitness. Several candidate bacterial genes are likely mediating midgut colonization and ROS production, and inhibition of Plasmodium development appears to involve a shutdown of the parasite's oxidative defence system. A better understanding of the complex reciprocal tripartite interactions can facilitate the development and optimization of an Esp-Z-based malaria control strategy.