The characteristic colony morphology of Actinomyces israelii on solid agar resembles which of the following?

• A. Medusa head
• B. A molar tooth
• C. A fried egg
• D. Ground glass

Answer: (B)

Actinomyces israelii grows as branching, filamentous Gram-positive bacteria that form dense, radiating colonies on solid media. This combination creates a central, compact core with outward spiky filaments, producing a silhouette that resembles a molar tooth. That distinctive tooth-like appearance is the classic colony morphology taught for Actinomyces israelii and serves as a handy visual cue in culture identification.

Other descriptors you might hear for different organisms reflect their own colony patterns—medusa head is associated with organisms whose filaments radiate like a halo, fried egg describes certain Mycoplasma colonies with a dense center and pale halo, and ground glass isn’t the characteristic look for Actinomyces. So the molar tooth description specifically fits Actinomyces israelii.