Microtubules are associated with intracellular movement and spread of the periodontopathogen Actinobacillus actinomycetemcomitans.

Actinobacillus actinomycetemcomitans SUNY 465, the invasion prototype strain, enters epithelial cells by an actin-dependent mechanism, escapes from the host cell vacuole, and spreads intracellularly and to adjacent epithelial cells via intercellular protrusions. Internalized organisms also egress from host cells into the assay medium via protrusions that are associated with just a single epithelial cell. Here we demonstrate that agents which inhibit microtubule polymerization (e.g., colchicine) and those which stabilize polymerized microtubules (e.g., taxol) both increase markedly the number of intracellular A. actinomycetemcomitans organisms. Furthermore, both colchicine and taxol prevented the egression of A. actinomycetemcomitans from host cells into the assay medium. Immunofluorescence microscopy revealed that protrusions that mediate the bacterial spread contain microtubules. A. actinomycetemcomitans SUNY 465 and 652, strains that are both invasive and egressive, interacted specifically with the plus ends (growing ends) of the filaments of microtubule asters in a KB cell extract. By contrast, neither A. actinomycetemcomitans 523, a strain that is invasive but not egressive, nor Haemophilus aphrophilus, a noninvasive oral bacterium with characteristics similar to those of A. actinomycetemcomitans, bound to microtubules. Together these data suggest that microtubules function in the spread and movement of A. actinomycetemcomitans and provide the first evidence that host cell dispersion of an invasive bacterium may involve the usurption of host cell microtubules.