The mosquito midgut harbors stable bacteria that enhance host hemolymph immunity

BACKGROUND: Mosquito symbionts have the potential to control mosquito-borne diseases by reducing vector competence through direct or indirect interactions with pathogens. However, the microbiome of field-collected mosquitoes is often unstable, and it remains unclear whether certain symbiont species can both colonize their hosts stably and modulate host immunity. In this study, we collected second-instar Aedes albopictus and Culex pipiens larvae from field water sources in Hong Kong and reared them to fourth-instar larvae and adults under laboratory conditions. We investigated microbiome changes from water to mosquito midguts and identified stable bacterial species (≥ 0.01% relative abundance) across mosquito stages using 16S rRNA-based bacteriome analysis. We further isolated symbiotic bacteria on culture plates, screened stable species by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, and evaluated their functional potential. RESULTS: Mosquito microbiomes were influenced by water source, developmental stage, and host species. Taxonomically, Proteobacteria and Bacteroidetes dominated mosquito midguts. Each mosquito species maintained four stable bacterial species (≥ 0.01% relative abundance) throughout development. We confirmed culturable Comamonas thiooxydans as a stable symbiont of Culex pipiens and Vibrionimonas magnilacihabitans in Aedes albopictus. Genomic predictions suggested their involvement in antimicrobial peptide synthesis. Functionally, these bacteria enhanced host survival and increased hemolymph antimicrobial activity against Erwinia carotovora subspecies carotovora 15 (ECC15), but not across mosquito species. CONCLUSION: Our findings suggest that mosquitoes harbor generally unstable bacterial communities with only a few species-specific stable symbionts, which may contribute to host survival and immune function. © 2026 Society of Chemical Industry.