Diagnostic ultrastructure of Listeria monocytogenes in human central nervous tissue

The pathogenesis of Listeria monocytogenes infection of the human central nervous system is poorly understood and ultrastructural aspects are not documented. A case of fatal human L. monocytogenes meningoencephalitis is described, in which diagnosis was confirmed by blood culture and in which special staining (Brown's) showed slender gram-positive bacilli in inflamed areas of the brainstem at autopsy. Electron microscopy of blocks rescued from formalin revealed rods, up to 2.5 microns long x 0.4 microns in diameter, with gram-positive type cell walls and distinctive conic ends, the latter being apparent in axial section only. The organism was either free within the cytoplasm or within endosomes or phagosomes of macrophages, smooth muscle cells, and endothelial cells. In one instance, one was seen adhering to the luminal aspect of the vascular endothelium. Central nervous system parenchymal cell infection was suspected but not confirmed. Intra-cytoplasmic bacteria were surrounded by an electron-transparent halo and, beyond that, a cloud of host-derived microfilaments, as previously described by others in vitro and in animals. The morphology of L. monocytogenes is compared with that of other intracellular pathogens and is found to be distinctive, suggesting the value of ultrastructural study in diagnosis. Such work could also help to solve outstanding pathogenetic questions of central nervous system invasion and spread.