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Staphylococcus aureus is not only an extracellular but also an intracellular pathogen. The host cell invasion of non-professional phagocytes essentially contributes to infection development. The fibronectin bridging between S. aureus FnBPs and alpha5beta1 integrins on the host cell side that facilitates the uptake of staphylococci is well investigated. In a recent in vitro study we showed that typical barrier cells such as endothelial and epithelial cells were highly invaded by bacteria, whereas primary human osteoblasts and fibroblasts took up S. aureus to a much lesser extent [1]. To find an explanation for the differences in the uptake ability of the cells we analyzed the alpha5beta1 integrin expression by antibodies using flow cytometry. Fibronectin distribution as well as bacterial binding and uptake was analyzed by fluorescence microscopy. The number of internalized viable bacteria was determined using the lysostaphin protection assay. We used different protease inhibitors to analyze the role of proteases in bacterial uptake
Contrary to our expectations we detected high amounts of alpha5beta1 integrin on the surface of primary osteoblasts and fibroblast cell line cells, whereas endothelial and epithelial cell line cells had only a low integrin expression. Analysis of fibronectin distribution revealed low amounts of fibronectin on endothelial and epithelial cell line cells, whereas primary osteoblasts and fibroblasts were covered with thick fibronectin fibrils. S. aureus bound to these fibrils and proliferated there, but was not taken up into the host cells. Therefore, the fibronectin matrix fibrils around osteoblasts and fibroblasts form a mechanistic barrier against S. aureus. In contrast, the fibronectin on the analyzed endothelial and epithelial cells acts as the well investigated bridge builder between S. aureus and host cells. Microscopic analysis of bacterial uptake revealed low to high degradation of fibronectin fibrils, depending on the host cell type. This degradation as well as bacterial uptake could be blocked by serine protease inhibitors. Infection of host cells with a S. aureus protease knock out mutant did not lead to a reduction in fibronectin degradation; therefore, it can be assumed that serine proteases released by the host cell promote bacterial uptake.
Our results shed new light on the processes involved in the uptake of S. aureus into host cells.
Reference:
[1] Strobel, M., Pförtner, H., Tuchscherr, L., Völker, U., Schmidt, F., Kramko, N., Schnittler, H. J., Fraunholz, M. J., Löffler, B., Peters, G., and Niemann, S. (2016) Post-invasion events after infection with Staphylococcus aureus are strongly dependent on both the host cell type and the infecting S. aureus strain. Clin Microbiol Infect 22, 799-809
