Invasion of epithelial cells by Actinobacillus actinomycetemcomitans: a dynamic, multistep process

The invasion process of Actinobacillus actinomycetemcomitans, a periodontopathogen, was studied with microscopy and viable quantitative assays using both KB and Madin-Darby canine kidney (MDCK) epithelial cells. Microscopy revealed that the events associated with the A. actinomycetemcomitans invasion process occurred rapidly. Scanning electron micrographs revealed A. actinomycetemcomitans associated with craters on the KB cell surface and others entering the KB cells through apertures with lip-like rims within 30 min of infection. Both transmission electron and immunofluorescence micrographs demonstrated that by this time some bacteria had, in fact, already entered, replicated, and exited host cells. Scanning electron micrographs revealed that infected KB cells exhibited fibrillar protrusions which contained bulges with the conformation of bacteria. Some protrusions formed intercellular connections between KB cells. Immunofluorescence micrographs revealed protrusions which harbored A. actinomycetemcomitans. The spread of internalized A. actinomycetemcomitans from one MDCK epithelial cell monolayer to another was demonstrated using a sandwich assay developed in our laboratory. Transcytosis of A. actinomycetemcomitans through polarized MDCK cells was also demonstrated. This study indicates that soon after entry of A. actinomycetemcomitans bacteria into epithelial cells, they undergo rapid multiplication and may subsequently be found in protrusions which sometimes extend between neighboring epithelial cells. The protrusions are thought to mediate the cell-to-cell spread of A. actinomycetemcomitans. Cell-to-cell spread may also occur by the endocytosis of A. actinomycetemcomitans bacteria which have been released into the medium via rudimentary protrusions which do not interconnect epithelial cells. The finding that the A. actinomycetemcomitans invasion process is so dynamic sheds significant new light on the interaction of this periodontopathogen with mammalian cells.