Interactions between Borrelia burgdorferi and eukaryote cells: comparative ultrastructural aspects.

In the present study, mammalian VERO cells, human polymorphonuclear leukocytes (PMN) and mouse peritoneal macrophages (MPM) were coincubated with B31 Borrelia burgdorferi strain (Bb) and examined by transmission electron microscopy. The results indicate that the spirochete adheres to the mammalian cells mainly by the apical pole and less frequently by the lateral wall. In VERO cells and MPM cell penetration is accomplished especially by cytoplasmic membrane destruction, the spirochete appearing free in the cytoplasm, but also by phagocytosis. In this latter case the pathogen is seen in phagosome. In PMN the internalization takes place exclusively by phagocytosis, the spirochete undergoing destruction in phagosomes. Sometimes an alteration of the phagosomal membrane is observable, probably produced by the pathogen as a mechanism to avoid fusion with the lysosome. Other aspects indicate the contribution of the parasitized cell cytoskeleton to the spreading of Borrelia to adjacent cells. MPM does not represent a very efficient barrier against Bb, as can be seen from the cellular behaviour of the spirochetes in this study, PMN seems to a more efficient barrier, by its unique capacity to destroy the microorganism after phagocytosis.