The gram-negative, anaerobic bacillus frequently implicated in serious infections such as brain and lung abscesses is which organism?

• A. Prevotella melaninogenica
• B. Eubacterium lentum
• C. Fusobacterium nucleatum
• D. Peptostreptococcus anaerobius

Answer: (C)

The main idea here is recognizing an anaerobic, gram-negative bacillus that is a common offender in deep-seated infections from the oral cavity, including brain and lung abscesses. Fusobacterium nucleatum fits this description precisely: it is a slender gram-negative, obligate anaerobic bacillus that is a frequent inhabitant of the oropharynx and a common cause of abscesses arising from aspiration or polymicrobial infections in the brain and lungs. Eubacterium lentum is a gram-positive anaerobic rod, and Peptostreptococcus anaerobius is a gram-positive anaerobic cocci, so they don’t match the gram-negative bacillus pattern. Prevotella melaninogenica is gram-negative and anaerobic, but the classic association with brain and lung abscesses in many exam contexts is Fusobacterium nucleatum due to its prominence in oropharyngeal–origin abscesses.