SummaryWe studied the sequence of cellular events leading to the dissemination of Listeria monocytogenes from the gut to draining mesenteric lymph nodes (MLNs) by confocal microscopy of immunostained tissue sections from a rat ligated ileal loop system. OX-62-positive cells beneath the epithelial lining of Peyer's patches (PPs) were the first Listeria targets identified after intestinal inoculation. These cells had other features typical of dendritic cells (DCs): they were large, pleiomorphic and major histocompatibility complex class II hi . Listeria were detected by microscopy in draining MLNs as early as 6 h after inoculation. Some 80±90% of bacteria were located in the deep paracortical regions, and 100% of the bacteria were present in OX-62-positive cells. Most infected cells contained more than five bacteria each, suggesting that they had arrived already loaded with bacteria. At later stages, the bacteria in these areas were mostly present in ED1-positive mononuclear phagocytes. These cells were also infected by an actA mutant defective in cell-to-cell spreading. This suggests that Listeria are transported by DCs from PPs to the deep paracortical regions of draining MLNs and are then transmitted to other cell populations by mechanisms independent of ActA. Another pathway of dissemination to MLNs was identified, probably involving free Listeria and leading to the infection of ED3-positive mononuclear phagocytes in the subcapsular sinus and adjacent paracortical areas. This study provides evidence that DCs are major cellular targets of L. monocytogenes in PPs and that DCs may be involved in the early dissemination of this pathogen. DCs were not sites of active bacterial replication, making these cells ideal vectors of infection.

A fatal untreated case of fulminant meningococcemia was examined to investigate the crossing of the blood-brain barrier (BBB) by Neisseria meningitidis. Microscopic examination showed bacteria in vivo adhering to the endothelium of both the choroid plexus and the meninges. Comparison of the isolates cultivated from the blood and the cerebrospinal fluid (CSF) revealed no antigenic variation of the pilin or the class 5 protein, whereas the expression of the PilC protein was greater in the CSF and the choroid plexus than in the blood. This was due to an increased activity of one of the pilC promotors. This higher expression of PilC correlated in vitro with greater adhesiveness to endothelial cells. A mutation in the single pilC locus of this strain abolished in vitro pilus-mediated adhesion to endothelial cells. These data suggest that PilC plays an important role in the crossing of the BBB, likely through pilus-mediated adhesion.

OPAT is practicable and effective and may be the best alternative treatment for patients suffering from osteomyelitis requiring intravenous therapy.

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