Which organism produces colonies that resemble a “molar tooth” on solid media? (Identify the classic appearance associated with cervicofacial infections.)

Introduction: Certain filamentous bacteria produce distinctive colony morphologies that are diagnostically useful. This question targets the hallmark “molar tooth” colony seen in actinomycosis.

Given Data / Assumptions:

• Cervicofacial infections with draining sinuses are characteristic of actinomycosis.
• Organisms grow slowly and show heaped, firm colonies on enriched media.

Concept / Approach: Actinomyces israelii, an anaerobic Gram-positive filamentous bacterium and part of normal oral flora, forms “molar tooth” colonies—dense, opaque, and heaped with a central depression—correlating with tissue granules known as sulfur granules.

Step-by-Step Solution: Link clinical setting (jaw, sinus tracts) with suspected organism (Actinomyces). Recall culture morphology: characteristic “molar tooth” appearance. Differentiate from aerobic actinomycetes (Nocardia) and Actinomadura used in mycetoma. Select Actinomyces israelii.

Verification / Alternative check: Standard lab atlases illustrate the “molar tooth” pattern for A. israelii on brain–heart infusion agar under anaerobic conditions.

Why Other Options Are Wrong:

• Nocardia brasiliensis / N. asteroides: Aerobic, weakly acid-fast; colony morphology differs.
• Actinomadura madurae: Agent of eumycetoma-like actinomycetoma; not known for “molar tooth” colonies.

Common Pitfalls: Confusing any filamentous bacterium with Actinomyces; colony morphology plus anaerobiosis are key clues.

Final Answer: Actinomyces israelii forms molar tooth–like colonies.