Invasive growth of Actinobacillus actinomycetemcomitans on solid medium (TSBV)

Abstract

When grown on agar, most Actinobacillus actinomycetemcomitans form circular and convex colonies with an internal star-shaped morphology. Such colonies adhere firmly to the agar, and when removed, a star-shaped imprint similar to that of the intact colony remains. This study was undertaken to determine the nature of this in vitro growth. Stereo-microscopy and scanning and transmission electron microscopy showed that the star-shaped imprint of A. actinomycetemcomitans colonies reflected pseudopod-like extensions of the bacterial colony which penetrated deep into the TSBV agar. The center of the colonies consisted primarily of ghost-like cells, while the dense border of each colony, including the pseudopods, expressed characteristics of vital cells. The latter were embedded in vast amounts of extracellular vesicles, and the outer aspect of the pseudopods was lined with a border of such vesicles.