The effect of microbial load and autophagy on Drosophila immunity and life span

Aging is a complicated process which is affected by many factors. Here we want to check what bacteria are associated with the fruit fly Drosophila melanogaster and how the bacteria affect the fly life span. Microbial load was quantified inside the body and on the surface of adult flies. Both aerobic and anaerobic bacterial load increased dramatically during aging in both compartments. Structures resembling abundant small bacteria and bacterial biofilms were visualized on the surface of old flies by scanning electron microscopy and cell staining. Bacteria cultured from laboratory flies included aerobic species Acetobacter aceti, Acetobacter tropicalis, Acetobacter pasteurianus and anaerobic Lactobacillus plantarum and Lactobacillus MR-2. Additional species Lactobacillus homohiochii, Lactobacillus fructivorans and Lactobacillus brevis were identified by DNA sequencing. Bacterial load and anti-microbial-peptide gene expression were reduced or eliminated using axenic culture conditions and antibiotics, however life span was unaffected. The data demonstrate that Drosophila can tolerate a significant internal and external bacterial load and mount a large innate immune response without a detectable trade-off with life span, and suggest that microbes do not limit life span in the optimized laboratory assay.;Autophagy is a basic cellular function to autodigest contents of the cytoplasm for recycling or removal. Recently it was also found that it is involved in resistance to bacteria in cultured cells. In addition, autophagy genes are required for life span extension caused by reduced insulin/IGF1-like signaling and dietary restriction in C. elegans. Here I wanted to test if the autophagy pathway might be limiting for immunity and/or life span in adult Drosophila. The Geneswitch system was used to cause conditional inactivation of the Atg5, Atg7 and Atg12 genes by RNAi. Conditional inhibition of Atg genes in adult flies reduced resistance to injected E. coli, as evidenced by increased bacterial titers and reduced survival. However, survival of uninjected flies was unaffected by Atg gene inactivation. The data indicate that Atg gene activity is required for normal immune function in adult flies, and suggest that neither immune function nor autophagy are limiting for adult life span under typical laboratory conditions.