A purulent joint aspirate Gram stain shows polymorphonuclear cells with intracellular and extracellular gram-negative diplococci. Which organism is most likely responsible?

A purulent joint aspirate Gram stain shows polymorphonuclear cells with intracellular and extracellular gram-negative diplococci. Which organism is most likely responsible?

• A. Neisseria gonorrhoeae
• B. Escherichia coli
• C. Staphylococcus aureus
• D. Streptococcus pneumoniae

Answer: (A)

The key idea is that seeing polymorphonuclear cells filled with intracellular gram-negative diplococci in a purulent joint fluid is classic for Neisseria gonorrhoeae causing septic arthritis. Neisseria gonorrhoeae is a gram-negative diplococcus, and it often appears inside neutrophils in purulent specimens. That intracellular presence in the context of joint infection strongly points to gonorrhea as the cause. Other organisms listed don’t fit this pattern: Staphylococcus aureus is a gram-positive cocci that typically appears in clusters; Streptococcus pneumoniae is a gram-positive diplococcus; Escherichia coli is a gram-negative rod. Their appearance in joint fluid and their staining morphology do not match the described finding.

The key idea is that seeing polymorphonuclear cells filled with intracellular gram-negative diplococci in a purulent joint fluid is classic for Neisseria gonorrhoeae causing septic arthritis. Neisseria gonorrhoeae is a gram-negative diplococcus, and it often appears inside neutrophils in purulent specimens. That intracellular presence in the context of joint infection strongly points to gonorrhea as the cause.

Other organisms listed don’t fit this pattern: Staphylococcus aureus is a gram-positive cocci that typically appears in clusters; Streptococcus pneumoniae is a gram-positive diplococcus; Escherichia coli is a gram-negative rod. Their appearance in joint fluid and their staining morphology do not match the described finding.