Purulent material from a cerebral abscess showed Gram-positive cocci in chains and Gram-negative bacilli with pointed ends; aerobically incubated plates showed no growth at 24 hours. Most likely presumptive identification of the anaerobic agents?

• 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)

The key idea is recognizing a mixed anaerobic infection from two distinct morphologies and the lack of growth in aerobic culture. Purulent brain abscess material showing Gram-positive cocci in chains points to anaerobic cocci, most commonly Peptostreptococcus. The Gram-negative bacilli with pointed ends are Fusobacterium, a slender, fusiform rod. The fact that aerobic plates show no growth at 24 hours confirms these are obligate anaerobes, which fits both organisms.

Thus, the best fit is Fusobacterium species plus Peptostreptococcus species. The other pairings don’t align with the observed Gram stain patterns or the anaerobic growth characteristics.