Which statement accurately describes Treponema pallidum in culture and visualization?

• A. It is a Gram-positive bacterium that stains well with Gram stain.
• B. It is a bacterium that can be readily cultured in standard media.
• C. It is a spirochete that cannot be cultured in vitro and is visualized by darkfield microscopy.
• D. It is a fungus that grows on Sabouraud agar.

Answer: (C)

Treponema pallidum is a spirochete with a slender, corkscrew shape. It does not grow in routine laboratory culture media, and there is no reliable in vitro culture system for it using standard bacterial media. Because it’s so thin and motile, it’s best seen with darkfield microscopy on material from lesions or exudates, which highlights its spiral form and motion. Gram staining is not reliable for this organism, and it is not a fungus, so it doesn’t grow on Sabouraud agar. This combination—inability to culture in standard media and visualization by darkfield microscopy—fits Treponema pallidum best.