Which organism is a gram-positive diplococcus that causes community-acquired pneumonia and is bile soluble?

• A. Streptococcus pneumoniae
• B. Staphylococcus aureus
• C. Haemophilus influenzae
• D. Klebsiella pneumoniae

Answer: (A)

Bile solubility helps identify Streptococcus pneumoniae, a gram-positive lancet-shaped diplococcus that commonly causes community-acquired pneumonia. This organism is typically alpha-hemolytic on blood agar and optochin sensitive, and it grows as encapsulated colonies. In the bile solubility test, S. pneumoniae is dissolved by bile salts due to autolysis of its cell wall, which differentiates it from other alpha-hemolytic streptococci that remain insoluble. Clinically, pneumococcal pneumonia presents with rust-colored sputum and is more common in older adults or individuals with spleen dysfunction. The other organisms—Staphylococcus aureus (gram-positive cocci in clusters, often post-viral or hospital-acquired), Haemophilus influenzae (gram-negative coccobacillus requiring X and V factors), and Klebsiella pneumoniae (gram-negative rod with currant jelly sputum in alcoholics)—do not share this combination of gram staining, diplococcal shape, and bile solubility.