ACTINOBACILLUS ACTINOMYCETEMCOMITANS: ADHERENCE MECHANISMS REQUIRED FOR INVASION

Colony phase variation

A. actinomycetemcomitans produces three distinct colonial morphologies on solid medium. A rough colony phenotype is typically generated by organisms upon isolation from the gingiva. These are small (~0.5–1 mm in diameter), translucent circular colonies with rough surfaces and irregular edges (Fig. 9.1). An internal star-shaped or crossed cigar morphology that embeds the agar is a distinguishing characteristic that gives A. actinomycetemcomitans its name (Zambon, 1985). In liquid culture, the rough colony phenotype cells form aggregates on the vessel walls, resulting in a clear medium (Fig. 9.1). Repeated subculture on agar of rough phenotypic isolates yields two distinct colonial variants; one is smooth surfaced and transparent, and the other is smooth surfaced and opaque (Slots, 1982; Scannapieco et al., 1987; Rosan et al., 1988; Inouye et al., 1990). The transparent smooth-surfaced variants appear to be an intermediate between the transparent rough-surfaced and opaque smooth-surfaced types (Inouye et al., 1990). In broth, the smooth-surfaced opaque type grows as a turbid homogeneous suspension, whereas the smooth-surfaced transparent type aggregates and adheres to the vessel walls (Inouye et al., 1990). In general, isolates undergo a rough-to-smooth variant transition soon after culture in vitro. In contrast, a smooth-to-rough variant transition that appears to be associated with nutritional requirements occurs only rarely during in vitro culture (Inouye et al., 1990; Meyer et al., 1991; Meyer, unpublished observation).