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To investigate if Borrelia burgdorferi can persist in resident joint cells, an infection model using cell cultures of human synovial cells was established and compared to the interaction of Borrelia burgdorferi and human macrophages. Borrelia burgdorferi were found attached to the cell surface or folded into the cell membrane of synovial cells analysed by transmission electron and confocal laser scanning microscopy. In contrast to macrophages, morphologically intact Borrelia burgdorferi were found in the cytosol of synovial cells without engulfment by cell membrane folds or phagosomes. Borrelia burgdorferi were isolated from parallel cultures. Treatment with ceftriaxone eradicated extracellular Borrelia burgdorferi, but spirochetes were reisolated after lysis of the synovial cells. Borrelia burgdorferi persisted inside synovial cells for at least 8 weeks. These data suggested that Borrelia burgdorferi might be able to persist within resident joint cells in vivo.