D3042: Investigating the impact of microbiome-derived acids on insect immunity using a Drosophila infection model

Association with the lactic acid bacterium Lactiplantibacillus plantarum (Lp) improves the response of Drosophila melanogaster to pathogenic infection. Lactic acid produced by Lp in the gut has the potential to affect host defenses in two distinct ways. First, it lowers pH, which can impose a barrier against pathogens by directly inhibiting their growth. Second, it induces intestinal stem cell proliferation, which could support gut epithelial repair during infection. This work investigates the hypothesis that the improved immunity conferred by Lp during infection is due to the production of lactic acid, which can act both as a pH modulator and a host signaling molecule. The inhibitory effect of Lp-derived lactic acid on pathogens was investigated using a strain of Lp that lacks lactate dehydrogenase and thus is incapable of lactic acid fermentation. The impacts of lactic acid on the host were also explored by supplementing the diet with lactic acid. We found that pH modulation through fermentative activity is largely responsible for the ability of Lp to counter pathogen proliferation. Preliminary findings suggest that lactic acid supplementation also improves the ability of the host to respond to pathogens in a pH independent manner. This shows a dual role for microbe-derived acids in affecting the outcome of insect pathogenesis by targeting both the pathogen and the host. Studying the impact of acids on infection adds to our understanding of how bacterial fermentation can shape microbe-microbe interactions in insect systems.