The increasing appearance of resistant bacterial pathogens requires the continuous search for new antiinfectives with new targets and, ideally, a probably reduced rate of resistance development. To address new targets the knowledge about host – pathogen interactions should be exploited. As response to conditions in the host bacteria frequently express proteins, which allow survival and proliferation and which are virulence factors. Novel anti-infective strategies aim at the inhibition of either virulence factor expression or virulence factor activity. Thus, in this PhD project host - pathogen interactions will be mimicked by suitable in vitro cultivation conditions and cellular co-culture systems. Target pathogens will be Staphylococcus aureus and E. coli – strains. Suitable screening assays have to be established, which could be based either on the pathogen alone, but also on co-culture systems between host cells and bacteria. The physiological state of the pathogen in these conditions will be validated by appropriate analytical methods. Screening campaigns can use compound libraries comprising in total roughly 30000 small molecules. Identified active compounds will be developed further with respect to mode of action and SAR studies. State-of-the art bioanalytical methods are available for in-depth molecular analysis of the biological systems.