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

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

Answer: (C)

The main idea here is using specific growth-inhibition patterns with simple agents to presumptively identify an anaerobic Gram-negative rod. Fusobacterium nucleatum fits this pattern because it is a slender Gram-negative rod that is inhibited both by bile-containing media and by a 1-μg kanamycin disk. This combination of sensitivities helps distinguish it from other anaerobes in the list, particularly from Bacteroides fragilis, which is typically resistant to kanamycin and tolerates bile, and from the Gram-positive Eubacterium lentum. Porphyromonas gingivalis could share the Gram-negative rod morphology but does not match the described bile and kanamycin sensitivity pattern as cleanly. So the observed inhibition by bile and by a low-concentration kanamycin disk points to Fusobacterium nucleatum as the presumptive identification.