An anaerobic, gram-negative bacillus isolated from a cervical mucosal abscess that is bile-resistant, produces a black pigment, is indole negative, and ferments glucose, sucrose, and lactose. Which is it?

• A. Bacteroides fragilis group
• B. Bacteroides ureolyticus
• C. Porphyromonas gingivalis
• D. Prevotella melaninogenica

Answer: (D)

The clue pattern focuses on a black-pigmented, anaerobic, gram-negative rod from mucosal surfaces that can ferment multiple carbohydrates and is indole negative. That combination is characteristic of Prevotella melaninogenica. The dark pigment is a hallmark of pigmented Prevotella/ Bacteroides species, and among those, Prevotella melaninogenica is known to ferment sugars such as glucose, lactose, and sucrose, while giving an indole test that is negative. Other pigmented anaerobes like Porphyromonas gingivalis do not ferment these carbohydrates, and nonpigmented Bacteroides species (like the Bacteroides fragilis group) lack the black pigment and sugar fermentation profile described. Therefore, the organism described best matches Prevotella melaninogenica.