Most Common Cause of Community-Acquired Pneumonia
Streptococcus pneumoniae (pneumococcus) is definitively the most common cause of community-acquired pneumonia in adults, accounting for approximately two-thirds of all bacteremic pneumonia cases and consistently identified as the leading bacterial pathogen across all severity levels from outpatient to ICU settings. 1, 2
Evidence Hierarchy
The most authoritative evidence comes from multiple IDSA/ATS guidelines spanning two decades, all consistently identifying pneumococcus as the predominant pathogen:
- S. pneumoniae is identified in 20-60% of CAP cases where a pathogen is determined, making it the single most frequently isolated organism 1
- In outpatient settings, pneumococcus accounts for 9-20% of all episodes when using sputum culture, and up to 36% when using more sensitive detection methods 1, 2
- Among hospitalized patients, pneumococcus represents 39% of identified pathogens (95% CI: 36.1-41.8%) 1
- In severe CAP requiring ICU admission, pneumococcus accounts for 21.6% of identified cases (95% CI: 15.9-28.3%) and represents over 80% of all bacteremia cases 1, 3
Secondary Pathogens by Frequency
After pneumococcus, the pathogen hierarchy differs by clinical setting:
Outpatient CAP:
- Mycoplasma pneumoniae: 13-37% of cases, particularly during epidemic years 1, 2, 4
- Respiratory viruses: up to 36% of cases, with influenza being predominant 1, 2
- Chlamydophila pneumoniae: up to 17% of outpatients 1
Hospitalized non-ICU patients:
- Haemophilus influenzae (nontypeable): 3-10% of cases, especially in patients with underlying bronchopulmonary disease 1, 2
- Mycoplasma pneumoniae: 10.8% of hospitalized patients 1
- Respiratory viruses: 12.8% of hospitalized cases 1
ICU/Severe CAP:
- Legionella species: 17.8% of ICU admissions (significantly higher than non-ICU settings at 3.6%) 1, 2
- Staphylococcus aureus: 8.7% of ICU cases, particularly during influenza outbreaks 1
Critical Diagnostic Caveat
No pathogen is identified in 40-70% of CAP cases despite comprehensive diagnostic testing 1, 4, 3. This does not diminish the importance of pneumococcus—studies suggest many culture-negative cases are actually undetected pneumococcal infections, as outcomes in pathogen-negative patients mirror those with confirmed pneumococcal disease 1.
Mixed Infections
Mixed infections occur in 8-40% of cases where pathogens are identified, most commonly pneumococcus co-infected with a respiratory virus 2, 3. This reinforces that pneumococcus remains the dominant bacterial threat even when viral pathogens are present 2.
Clinical Implications
The overwhelming predominance of pneumococcus across all CAP severity levels explains why empiric antibiotic regimens universally prioritize antipneumococcal coverage, with beta-lactams forming the backbone of therapy for hospitalized patients 1. The 2007 IDSA/ATS guidelines explicitly state that empiric therapy must cover pneumococcus regardless of clinical presentation, as no clinical features reliably distinguish pneumococcal from atypical pathogen infection 1, 5.