Question

A female presents with signs of meningitis. CSF shows gram-positive bacilli. The most likely organism is?

• A. Listeria
• B. Haemophilus influenzae
• C. Pneumococcus
• D. Pseudomonas

Hint:

Gram-positive rods in CSF of a vulnerable host means Listeria.

Answer 1

The Correct Option is A

Step 1: The key laboratory clue is gram-positive bacilli (rods) in the CSF of a patient with meningitis. Sort the options by Gram morphology.
Step 2: Listeria monocytogenes is a gram-positive bacillus and is the only option that matches the rod morphology. It is a well-known cause of meningitis in neonates, pregnant women, the elderly, and immunocompromised hosts.
Step 3: Exclude the others by morphology. Haemophilus influenzae is a gram-negative coccobacillus. Streptococcus pneumoniae is a gram-positive coccus (in pairs), not a bacillus. Pseudomonas aeruginosa is a gram-negative bacillus.
Step 4: Only Listeria is a gram-positive rod, so it is the most likely organism. Listeria also shows characteristic tumbling motility at 22 to 25 degrees Celsius and can grow at refrigeration temperature (cold enrichment).
Step 5: The correct answer is Listeria, option A.