Community-acquired pneumonia in adults is most commonly caused by which organism?

The most common cause of community-acquired pneumonia in adults is Streptococcus pneumoniae. This organism is a type of bacteria that frequently colonizes the upper respiratory tract without causing disease in healthy individuals. However, under certain conditions, such as when the immune system is compromised or due to other underlying health issues, it can invade the lungs and lead to pneumonia.

S. pneumoniae is known for causing lobar pneumonia, characterized by a distinctly acute onset and classical symptoms such as fever, cough, and pleuritic chest pain. The significant incidence of pneumonia resulting from this pathogen is supported by epidemiological studies and makes it a key organism to be aware of when diagnosing and treating pneumonia in the community setting.

While Mycoplasma pneumoniae, Haemophilus influenzae, and Chlamydia pneumoniae are also associated with pneumonia, they are less common causes compared to Streptococcus pneumoniae. Mycoplasma pneumoniae tends to cause atypical pneumonia, often seen in younger populations. Haemophilus influenzae is more associated with patients with underlying lung diseases or in elderly populations. Chlamydia pneumoniae can be involved, but it does not have the same prevalence in community-acquired pneumonia as S. pneumoniae.