The effect of bacterial feeding on the midgut morphology and development of the sand fly Lutzomyia longipalpis

Phlebotomine sand flies are vectors of many pathogens, and best known for their role in Leishmania transmission. Sand fly control is hampered by many factors, including the possibility of insecticide resistance. Renewed interest has emerged for new approaches, including paratransgenesis, to control sand flies in endemic areas. Our research focuses on identifying aspects of sand fly larvae midgut physiology and innate immune response to bacterial challenge. It has been previously demonstrated that midgut infection in Drosophila leads to changes in physical morphology of the gut as well as changes in the innate immune and homeostasis responses. Here, 3^rd instar Lu. longipalpis sand fly larvae were fed with GFP-labeled Gram(+) Bacillus subtilis and Gram(-) Pantoea agglomerans. These bacteria differentially localize within the gut, and this effect is driven by the gut pH gradient. Infection with P. agglomerans led to an apoptotic response in midgut epithelial cells while with B. subtilis this effect was not observed. Both bacteria are cleared form the gut if not maintained during continuous feeding. Analysis of innate immune and homeostasis transcripts revealed that larvae differentially responded to bacterial challenge. Interestingly, the larvae suppressed their immune response and effector molecule production via the negative regulator of immunity, Pirk. Gene-specific knockdown targeting Pirk is currently underway to assess possible effects on larval development and immune responses. Our studies provide insights in understanding bacterial distribution in insect gut and physiology and innate immunity of immature sand fly stages, with significant potential for translational research applied to vector control.