Which combination of organisms is most likely in a polymicrobial brain abscess?

• A. Veillonella sp. and Clostridium sp.
• B. Eubacterium sp. and Veillonella sp.
• C. Peptostreptococcus sp. and Nocardia sp.
• D. Fusobacterium sp. and Peptostreptococcus sp.

Answer: (D)

Polymicrobial brain abscesses are typically driven by anaerobic bacteria from the oral cavity that form mixed infections in a low-oxygen environment. Fusobacterium and Peptostreptococcus are both common anaerobes found in oral and dental flora and are frequently involved together in such infections. Their co-existence reflects the dentogenic source and the ability of these anaerobes to thrive side by side in abscesses, producing a synergistic environment for growth and spread.

Fusobacterium is a gram-negative anaerobe, often from periodontal or oropharyngeal regions, while Peptostreptococcus is a gram-positive anaerobic cocci. Together, they typify the classic polymicrobial anaerobic pattern seen in brain abscesses arising from dental or sinus sources.

The other pairings either involve organisms less commonly seen together in brain abscesses or include pathogens like Nocardia that have different clinical patterns (often causing chronic or solitary abscesses and not the typical mixed anaerobic oral flora). Hence, the combination of Fusobacterium and Peptostreptococcus best fits the common polymicrobial, oral-origin scenario.