f Bacterial adherence determines the virulence of many human-pathogenic bacteria. Experimental approaches elucidating this early infection event in greater detail have been performed using mainly methods of cellular microbiology. However, in vitro infections of cell monolayers reflect the in vivo situation only partially, and animal infection models are not available for many human-pathogenic bacteria. Therefore, ex vivo infection of human organs might represent an attractive method to overcome these limitations. We infected whole human umbilical cords ex vivo with Bartonella henselae or Acinetobacter baumannii under dynamic flow conditions mimicking the in vivo infection situation of human endothelium. For this purpose, methods for quantifying endotheliumadherent wild-type and trimeric autotransporter adhesin (TAA)-deficient bacteria were set up. Data revealed that (i) A. baumannii binds in a TAA-dependent manner to endothelial cells, (ii) this organ infection model led to highly reproducible adherence rates, and furthermore, (iii) this model allowed to dissect the biological function of TAAs in the natural course of human infections. These findings indicate that infection models using ex vivo human tissue samples ("organ microbiology") might be a valuable tool in analyzing bacterial pathogenicity with the capacity to replace animal infection models at least partially.T he analysis of adhesion to host cells is crucial for the elucidation of bacterial infection mechanisms. Animal infection models or cellular microbiology-based approaches are well established, and many important findings have thereby been gathered. By way of example, the protein injection machineries and the autotransporter adhesins (TAA) of Bartonella henselae and Acinetobacter baumannii have been elucidated in that manner (1-3). Although these methods are well established and widely accepted, they still are hampered by several limitations: animal models might reflect the course of human infections only partially (4) or might not even be available for many pathogens (e.g., B. henselae). Cellular infection models allow very detailed analyses of the interaction of bacteria with host cells under standardized conditions. However, they are usually done statically using cell monolayers, and this mimics the complexity of in vivo infection situation only to some extent. Methods to overcome the species barrier in infection models and to analyze host-pathogen interactions close to the dynamic human situation are urgently needed. Ex vivo infection models, using human organ grafts, might represent an attractive alternative or addition to animal or cellular microbiology experiments.B. henselae causes cat scratch disease and endocarditis, whereas immunosuppressed individuals can suffer from vasculoproliferative disorders such as bacillary angiomatosis. B. henselae is believed to be an endotheliotropic pathogen. Bartonella adhesin A (BadA) has been identified as the key factor involved in the adherence to endothelial cells (ECs) and to extracellular matrix (EC...