Question

A patient has a brain abscess. The aspirated material is foul smelling and shows red fluorescence under ultraviolet light. Which organism could be implicated?

• A. Staphylococcus aureus
• B. Peptostreptococcus
• C. Bacteroides
• D. Acanthamoeba

Hint:

Red fluorescence under UV light is a classic clue for pigmented Bacteroides species.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The vignette gives two strong clues: a foul smelling abscess, which points to anaerobic infection, and red fluorescence of the pus under UV light, which is a specific laboratory sign.

Step 2: Key Concept:
Foul smelling pus is typical of mixed anaerobic infection, since anaerobes produce short chain fatty acids and volatile compounds that give the pus its bad odour. Among anaerobes, pigmented species of Bacteroides and Prevotella (older name Bacteroides melaninogenicus) produce a black pigment that glows brick red under ultraviolet light, which is used as a quick identifying test in the lab.

Step 3: Detailed Explanation:
Bacteroides fits both clues directly: it is an obligate anaerobe, so its infections are classically foul smelling, and the pigmented group shows the characteristic red fluorescence under UV light. Brain abscess is also a recognised site for anaerobic infection, often from spread of dental or sinus infection.
Staphylococcus aureus is a facultative anaerobe and its pus does not have a foul odour or red fluorescence, so it is inconsistent with both clues.
Peptostreptococcus is also anaerobic and can cause foul smelling pus, but it does not produce the pigment responsible for the red UV fluorescence, so it does not match the second clue.
Acanthamoeba is a free living amoeba that causes granulomatous encephalitis or keratitis, not a foul smelling pyogenic abscess, and it has no UV fluorescence property.

Step 4: Final Answer:
The combination of foul odour and red fluorescence under UV light points to pigmented Bacteroides.