An anaerobic, gram-negative rod can be presumptively identified by its Gram stain morphology and inhibition by bile and a 1-µg kanamycin disk. Which organism is this?

• A. Eubacterium lentum
• B. Bacteroides fragilis group
• C. Fusobacterium nucleatum
• D. Porphyromonas gingivalis

Answer: (C)

In this kind of test, you’re using a combination of shape (Gram stain) and selective growth-inhibition patterns to narrow down the organism among anaerobic Gram-negative rods. The key is a Gram-negative rod that is inhibited by both bile and a 1-µg kanamycin disk. Fusobacterium nucleatum fits this pattern: it is a slender, anaerobic Gram-negative rod, and in standard presumptive identification schemes it shows inhibition with bile-containing media and with a kanamycin disk, helping to confirm its identity.

Eubacterium lentum is a Gram-positive anaerobe, so it wouldn’t match the Gram-negative rod morphology. Bacteroides fragilis group are also Gram-negative anaerobes, but their kanamycin response and bile tolerance patterns differ from this dual-inhibition pattern seen with Fusobacterium nucleatum in common keys. Porphyromonas gingivalis, while a Gram-negative anaerobe, is typically identified with other characteristic tests and not primarily by this bile plus kanamycin inhibition combo.

So the combination of Gram-negative rod morphology plus inhibition by bile and by a 1-µg kanamycin disk most consistently points to Fusobacterium nucleatum.