Characterization of Actinomyces israelii serotypes 1 and 2.

In a previous serological study, we compared 14 isolates of Actinomyces israelii serotype 2 with 13 serotype 1 cultures. The present study reports the morphological, physiological, and biochemical characteristics of these same 27 cultures. All of the isolates exhibited similar cellular morphology, and all but one produced the typical spider type microcolony on solid media. Twelve of 13 serotype 1 isolates produced the molar tooth type macrocolony, whereas only 2 of 14 serotype 2 cultures produced this type of rough colony. All of the serotype 1 isolates fermented arabinose with the production of acid; none of the serotype 2 cultures fermented this carbohydrate. All 27 cultures produced the greatest amount of growth when cultured under anaerobic conditions and grew poorly or not at all in air. Both groups of organisms produced similar reactions on other biochemical test media; these findings suggested that A. israelii serotype 2 should not be given a species designation.