The sustained health of an individual animal depends on the composition and activities of its resident microbiota. A major challenge is to identify the processes by which the microbiota and animal interact, recognizing that this research should lead ultimately to novel strategies to promote human health. Drosophila is emerging as a tractable model system to investigate these interactions. New evidence reveals that the gut microbiota promotes insulin signaling in Drosophila, leading to increased growth and development rates. Different gut bacteria and mechanisms [corrected] were implicated: acetic acid produced by Acetobacter pomorum and the promotion of amino acid nutrition by Lactobacillus plantarum. [corrected]. These findings raise the possibility that multiple bacterial effectors may interact with signaling networks to shape animal health.