Stable Isotope Probing (SIP) using cellulose and urea as trophic links pointed towards the bacterial families Lachnospiraceae and Enterococcaceae in larvae and Enterobacteriaceae in adults as involved in cellulose processing. The genus Burkholderia in larvae and the genus Parabacteroides in adults were labeled with 15N urea (Article I). These shifts may be consequence to differences in carbon and nitrogen needs. 454-pyrosequencing revealed that the hindgut wall community was composed mainly by the Caulobacteraceae and Pseudomonadaceae families in second instar larvae, while in third instar the dominant families were Porphyromonadaceae and Bacteroidales-related. The hindgut wall community of adults was fairly similar to third instar larvae (Article III). Previous studies determined that the gut bacterial community of S. littoralis is not very rich. As E. mundtii had been targeted as a metabolically active symbiont by 13C-glucose SIP, this bacterium was isolated and its genome sequenced. E. mundtii genome presented an elevated percentage of coding capacity (12%) devoted to carbohydrate uptake (Article II). Altogether, the genome of E. mundtii suggests that this symbiont plays a crucial role in the digestion of host diet. Previous observations of M. hippocastani hindgut spotted specialized organs of unknown function (pockets). 454 pyrosequencing allowed identifying pocket bacteria as Achromobacter sp. and Micrococcaceae-related. Gas Chromatograhy Mass Spectrometry analysis revealed that Achromobacter sp. accumulated poly-β-hydroxybutyrate (PHB) in vitro. Presence of PHB was also detected in whole pocket tissue by Raman microspectroscopy. PHB might be crucial for the successful symbiotic colonization of the pockets (Article III).