Which organism is most commonly responsible for community-acquired pneumonia in adults?

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

Answer: (A)

Identifying the most common bacterial cause of community-acquired pneumonia in adults. Streptococcus pneumoniae is the leading culprit in adult community-acquired pneumonia. It’s a Gram-positive lancet-shaped diplococcus that colonizes the upper respiratory tract and can be aspirated into the lungs, producing the typical pneumonia pattern with productive cough, fever, and often lobar consolidation on imaging. While other pathogens like Haemophilus influenzae and Moraxella species can cause CAP and Mycoplasma pneumoniae is a frequent cause of atypical pneumonia in younger adults, none are as consistently responsible for CAP in the adult population as pneumococcus. Staphylococcus aureus tends to cause pneumonia more readily in specific contexts, such as after influenza or presenting with more severe disease, so it’s less likely to be the most common cause of ordinary community-acquired pneumonia.