|Beschreibung:||In the present study sponge-bacteria associations as well as the antimicrobial potential of sponge-associated bacteria was investigated. Culture independent methods were applied to examine diversity, specificity and temporal consistency of sponge-associated bacterial communities while culture-based methods were used to address the antimicrobial potential of bacteria associated with different Mediterranean sponges. For molecular studies, Chondrilla nucula and Tethya aurantium were used as model organisms for sponges containing an apparent high number of associated bacteria (bacteriosponges) and sponges without apparent associated bacteria (bacteria-free sponges), respectively. Stable and specific bacterial associations were demonstrated for both sponges suggesting that associations between sponges and bacteria are a common occurrence and bacteria-free sponges might not exist. Sponge-microbe associations are proposed as microbial consortia inhabiting the micro-environment sponge. The association of Bdellovibrio-like Deltaproteobacteria as well as putatively ammonia-oxidizing archaea with C. nucula indicate a high complexity of possible interactions not only between sponge and associated bacteria but also within the associated bacterial community. High sequence similarities between sponge-derived and seawater-derived 16S rDNA sequences reflect filter-feeding by sponges and emphasise the need for comparative controls. However, distinct bacterial communities associated with two different sponge-species living in close proximity in their natural habitat demonstrate low impact of the ambient seawater for sponge-bacteria associations. The bacterial populations in the two model organisms differed in community structure and diversity. Bacterial sequences obtained from C. nucula were affiliated to other sponge-derived sequences in large parts (79%), while only a minor fraction (22%) of T. aurantium-associated phylotypes shared similarity with known sponge-associated bacteria. Further, comparison of cortex and endosome revealed spatial differences in the T. aurantium-associated bacterial community. Antimicrobially active bacteria were isolated from different sponges, demonstrating that cultivable sponge-associated bacteria exhibit a high potential for the search of new antimicrobial substances with biotechnological application. The cultivable bacterial communities associated with at least six different sponges were found to be dominated by a single alphaproteobacterial phylotype revealing antimicrobial activity. Kind of association, specificity as well as way of acquisition is still uncertain and critically discussed in this study.