Characterization by 16S rRNA gene analysis and in situ hybridization of bacteria living in the hindgut of a deposit-feeding echinoid (Echinodermata)

The hindgut caecum of the deposit-feeding echinoid Echinocardium cordatam harbours a symbiotic bacterial microflora, organized into layered mats around detrital particles owing to the proliferation of filamentous bacteria. The bacterial community was analysed using 16S rRNA gene analysis and fluorescence in situ hybridization. The purpose was to characterize its biodiversity and to identify its predominant members. The majority of the 16S sequences belong to the delta-Proteobacteria (61.5%), the Bacteroidetes and the Firmicutes constitute the two other main bacterial groups (respectively 23.1% and 15.4%). A total of 41% of the delta-Proteobacteria clones isolated belong to a bacterium of the genus Desulfonema for which a specific oligonucleotide probe was designed, enabling Identification of its distribution in the nodules.