An obligate microsporidian parasite modulates defense against opportunistic bacterial infection in the yellow fever mosquito.

Unlabelled: The ability of mosquitoes to transmit vertebrate pathogens depends on multiple factors, including the mosquitoes' life history traits, immune response, and microbiota (i.e., the microbes associated with the mosquito throughout its life). The microsporidium is an obligate intracellular parasite that specifically infects mosquitoes and severely affects mosquito survival and other life history traits critical for pathogen transmission. In this work, we investigated how impacts bacterial infection with in mosquitoes. We measured development, survival, and bacterial load in both larval and adult stages of mosquitoes. In larvae, exposure was either horizontal or vertical and was introduced orally. Regardless of the route of transmission, exposure resulted in significantly higher loads in larvae. exposure also significantly reduced larval survival but subsequent exposure to had no effect. In adult females, exposure was only horizontal and was introduced orally or via intrathoracic injection. In both cases, infection significantly increased bacterial loads in adult female mosquitoes. In addition, females infected with and subsequently injected with suffered 100% mortality which corresponded with a rapid increase in bacterial load. These findings suggest that exposure to can influence the establishment and/or replication of other microbes in the mosquito. This has implications for understanding the ecology of mosquito immune defense and potentially disease transmission by mosquito vector species.

Importance: The microsporidium is a parasite of the yellow fever mosquito, . This mosquito transmits multiple viruses to humans in the United States and around the world, including dengue, yellow fever, and Zika viruses. Hundreds of millions of people worldwide will become infected with one of these viruses each year. infection significantly reduces the lifespan of and is therefore a promising novel biocontrol agent. Here, we show that when the mosquito is infected with this parasite, it is also significantly more susceptible to infection by an opportunistic bacterial pathogen, . This novel discovery suggests the mosquito's ability to control infection by other microbes is impacted by the presence of the parasite.