Which organism forms Gram-positive branching filaments with molar-tooth colonies?

Prepare for the CAMLPR Microbiology Test. Utilize flashcards and engaging multiple-choice questions, each equipped with descriptions and explanations. Ready yourself for an excellent performance!

This question tests recognizing a bacterium by both its microscopic form and its distinctive culture appearance. Actinomyces species are Gram-positive, branching filamentous bacteria that grow as anaerobic or microaerophilic organisms. On culture, they form characteristic molar-tooth colonies—dense centers with radiating filaments that resemble a molar tooth. This combination of Gram-positive branching filaments with molar-tooth colony morphology is classic for Actinomyces israelii, often linked to cervicofacial actinomycosis and sulfur granules in draining sinus tracts.

Streptococcus viridans are Gram-positive cocci in chains, not branching filaments, and they don’t produce molar-tooth colonies. Clostridia tetani are Gram-positive rods with terminal spores and don’t form branching filaments that look like molar teeth. Pseudomonas aeruginosa is a Gram-negative rod with very different colony morphology and does not form filamentous colonies like Actinomyces.