In this paper, a knowledge representation infrastructure for semantic multimedia content analysis and reasoning is presented. This is one of the major objectives of the aceMedia Integrated Project where ontologies are being extended and enriched to include low-level audiovisual features, descriptors and behavioural models in order to support automatic content annotation. More specifically, the developed infrastructure consists of the core ontology based on extensions of the DOLCE core ontology and the multimedia-specific infrastructure components. These are, the Visual Descriptors Ontology, which is based on an RDFS representation of the MPEG-7 Visual Descriptors and the Multimedia Structure Ontology, based on the MPEG-7 MDS. Furthermore, the developed Visual Descriptor Extraction tool is presented, which will support the initialization of domain ontologies with multimedia features.