Antibiotics resistance development in European hospitals has increased alarmingly in recent years. To counteract this danger, a semantic web based IT solution is proposed which intends to integrate the access to relevant clinical data repositories from different European hospitals. This endeavor relies on formalized and shared models of the clinical domain. We describe the development process of the DebugIT Core Ontology, which is a key-mediator for semantic as well as syntactic clinical data integration in the mentioned endeavor. We show how UML diagrams can be used to illustrate the ontology engineering phase and the ontologies use case. Some domain statements are given which are then converted into more human friendly representations to be verified by medical experts.