Question

A case of brain abscess reveals Gram-positive, branching, filamentous organisms on microscopy. The organism is weakly acid-fast and grows on paraffin bait media. Which is the most likely etiologic agent?

• A. Actinomyces israelii
• B. Mycobacterium tuberculosis
• C. Nocardia asteroides
• D. Cryptococcus neoformans

Hint:

Gram-positive branching filamentous organism with weak acid-fastness = Nocardia. Gram-positive branching filamentous organism without acid-fastness = Actinomyces.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question describes a specific morphological and biochemical profile for an organism causing a central nervous system infection (brain abscess).
Key features include "Gram-positive," "branching filamentous," "weakly acid-fast," and "paraffin bait" growth.
Step 2: Detailed Explanation:

Nocardia species: These are aerobic, Gram-positive, catalase-positive bacteria that form branching filaments similar to fungal hyphae.

Weakly Acid-Fast: Unlike Mycobacterium, which is strongly acid-fast, Nocardia is "weakly acid-fast," meaning it retains carbol-fuchsin stain only when decolorized with a weak acid (e.g., 1% sulfuric acid) instead of the standard acid-alcohol.

Paraffin Baiting: Nocardia has the unique ability to utilize paraffin as a sole carbon source. The "paraffin bait technique" is used in the lab to selectively isolate Nocardia from clinical samples.

Pathogenesis: Nocardia asteroides most commonly causes opportunistic infections in immunocompromised hosts, often starting in the lungs and disseminating to the brain, forming multiloculated abscesses.

Differentiation from Actinomyces (A): Actinomyces also forms branching filaments but is strictly anaerobic and is not acid-fast. Actinomyces typically forms "sulfur granules" in tissues.

Step 3: Final Answer:
The combination of aerobic growth, filamentous branching, and weak acid-fastness is pathognomonic for Nocardia asteroides.