CSF Gram stain shows gram-positive cocci in pairs and chains. This finding most likely indicates infection with which organism?

• A. Streptococcus pneumoniae
• B. Neisseria meningitidis
• C. Staphylococcus aureus
• D. Listeria monocytogenes

Answer: (A)

The key idea is using the appearance of microorganisms on a CSF Gram stain to guide which organisms are most likely in meningitis. Gram-positive cocci arranged in pairs and in chains point toward a Streptococcus species. Among the common meningitis pathogens listed, Streptococcus pneumoniae is the classic streptococcal offender in adults and is typically described as Gram-positive diplococci in pairs (often Lancet-shaped), which can appear in short chains on stain preparations. This pattern helps distinguish it from Neisseria meningitidis, which are Gram-negative diplococci; Staphylococcus aureus, which are Gram-positive cocci in clusters; and Listeria monocytogenes, which are Gram-positive rods. Therefore, the Gram stain finding most strongly supports Streptococcus pneumoniae as the likely organism causing the infection. In practice, culture or PCR would confirm the diagnosis.