In critically ill patients Candida spp. are frequently isolated from respiratory tract secretions such as endotracheal aspirates and bronchoalveolar lavages (BAL) and are most often considered as colonizers of the respiratory tract. In contrast, pneumonia due to infection with Candida spp. is rare and is diagnosed by histological demonstration of the yeast in lung tissue with associated inflammation. In spite of this, preemptive antifungal therapy based on isolation of Candida spp. from the respiratory tract is often initiated in critically ill patients. The disadvantages of this approach include increased selective pressure for the development of antimicrobial resistance, potential risks of adverse drug reactions and high treatment costs. On the other hand, immediate administration of appropriate antifungal therapy has been shown to be an important predictor of favorable outcome for patients with invasive fungal infections. Therefore, the development of reliable diagnostic measures for the detection of invasive pulmonary candidiasis is crucial. The overall objective of the proposed research project is to identify diagnostic strategies to differentiate between Candida colonization and Candida infection of the lower respiratory tract in critically ill patients. The proposed projects intends to test the hypothesis that 1.) invasive Candida strains from the lower respiratory tract differ from colonizing Candida strains with regard to production and expression of putative virulence factors and/or that 2.) patients suffering from pulmonary invasive candidiasis differ from patients colonized by Candida spp. with regard to inflammatory markers, other serum markers (fungal antigen) and composition of indigenous pulmonary bacterial flora.