Disentangling host-microbiome-pathogen interactions in Aedes aegypti, transitions to pathogenesis and the role of viral infections

The gut microbiota is a promising tool for controlling mosquito-borne diseases given its potential to influence vectorial capacity. Bacterial symbionts residing within the gut of Aedes mosquitoes can activate the host immune responses, thereby facilitating the clearance of both symbionts and human pathogens such as dengue, Zika (ZIKV), and yellow fever viruses. Recent investigations have revealed the activation of Drosophila DUOX-mediated immunity by gut symbionts that catabolise uridine and generate uracil, leading to the selective elimination of pathogenic bacteria while preserving commensal symbionts. However, whether this process occurs in mosquitoes, what factors instigate uracil production and their effect on the interactions between commensals and human pathogens in the mosquito gut remain unexplored. This thesis investigates the interactions between the Aedes microbiota and the host DUOX-mediated immunity alongside the interplay between the Aedes microbiota and ZIKV. I examined the impact of uracil on the enzymes of the DUOX mediated immunity, reactive oxygen species (ROS) production and mosquito fitness to evaluate the potential selective activation of this pathway by uracil (Chapter 4). I also studied the uridine catabolic pathway in bacteria and the conditions that can trigger it to elucidate how we can induce pathogenesis in symbionts in order to exploit them for vector control (Chapter 5). Finally, I explored the interactions between ZIKV and the mosquito microbiome in laboratory-reared and field-collected Aedes aegypti mosquitoes to identify the factors shaping these interactions (Chapter 6). The findings of this thesis revealed a sexually dimorphic response of the mosquito immune system to uracil and identified uridine, blood and ZIKV as potential triggers for pathogenesis in symbionts. Uridine catabolism appeared to facilitate gut colonisation and influenced DUOX-mediated immunity although it had minimal impact on ZIKV infection. Furthermore, microbiome-ZIKV interactions were found to be influenced by host genetic background and environmental conditions, with certain bacterial taxa in the Aedes gut correlating with refractoriness to ZIKV. I hope that the insights from this thesis will be used as a foundation for designing vector competence studies and contribute to the development of biological control tools to fight vector-borne diseases