Which Gram-positive diplococci are commonly associated with pneumonia?

• A. Bacillus anthracis
• B. Streptococcus pneumoniae
• C. Staphylococcus aureus
• D. Neisseria meningitidis

Answer: (B)

Pneumonia commonly involves a Gram-positive diplococcus known for its lancet-shaped cocci paired together. This organism, Streptococcus pneumoniae, is encapsulated, which helps it resist immune defenses and establish infection in the lungs. In the lab, it typically forms alpha-hemolytic colonies on blood agar and is optochin sensitive and bile soluble, features that help distinguish it from other bacteria. It is the leading cause of community-acquired pneumonia in adults, making it the best fit for this question. The other organisms listed do not present as Gram-positive diplococci associated with pneumonia: Bacillus anthracis is a rod, Staphylococcus aureus is cocci in clusters, and Neisseria meningitidis is a Gram-negative diplococcus.