What are the common organisms responsible for Community-Acquired Pneumonia (CAP)?

Common Organisms Responsible for Community-Acquired Pneumonia

Streptococcus pneumoniae remains the single most common bacterial pathogen causing CAP across all age groups and severity levels, accounting for approximately 10-41.7% of identified cases. 1, 2

Primary Bacterial Pathogens

Most Common

• Streptococcus pneumoniae is consistently the predominant pathogen, representing the most frequent cause in outpatients, hospitalized patients, and ICU admissions, with rates of 10-41.7% depending on diagnostic methods used 1, 2
• Haemophilus influenzae (predominantly nontypeable strains) is the second most common bacterial cause, particularly in patients with underlying chronic bronchopulmonary disease, accounting for 4-14% of cases 1, 2

Atypical Pathogens

• Mycoplasma pneumoniae causes 4-39% of CAP cases overall, with higher rates (13-37%) in outpatient settings 1, 2, 3
• Chlamydophila pneumoniae accounts for 0-20% of cases, with higher prevalence in older children and adults 1, 2
• Legionella pneumophila represents 0-12.5% of cases, more common in severe CAP requiring ICU admission 1

Less Common Bacterial Causes

• Moraxella catarrhalis occurs in 1-3% of cases, primarily in patients with chronic lung disease 1, 2
• Staphylococcus aureus (including MRSA) causes severe CAP particularly during or following influenza outbreaks, with MRSA prevalence up to 3% in CAP patients 1
• Enteric gram-negative bacteria (Pseudomonas aeruginosa, Enterobacteriaceae) found in up to 2-6% of cases, typically in patients with structural lung disease, prior antibiotics, corticosteroid use, or septic shock on admission 1

Viral Pathogens

Viruses account for 14-35% of CAP cases in adults and are increasingly recognized as major contributors, often occurring as co-infections with bacteria. 1

• Influenza virus is the predominant viral cause, accounting for 4-30% of cases depending on season 1
• Respiratory syncytial virus (RSV) is particularly common in children but also affects adults 1
• Rhinovirus identified in up to 8% of severe CAP cases 1
• Other viruses include parainfluenza, adenovirus, metapneumovirus, coronavirus (including SARS-CoV-2), and herpes viruses 1

Age-Specific Patterns

Children

• Viruses predominate in children under 5 years, accounting for 14-35% of cases, with RSV being most common 1
• Streptococcus pneumoniae remains the leading bacterial cause, detected in 5-37% depending on diagnostic methods (blood culture 5-10%, serologic testing 16-37%) 1
• Mycoplasma pneumoniae increases with age: 15% in children under 5 years versus 42% in children over 5 years 1, 4
• Haemophilus influenzae accounts for approximately 5% of pediatric CAP 1

Adults and Elderly

• Streptococcus pneumoniae dominates across all adult age groups 1, 2
• Risk for gram-negative pathogens increases with advanced age, comorbidities, and prior healthcare exposure 1, 2

Severe CAP (ICU-Level Disease)

In patients requiring ICU admission, the pathogen distribution shifts:

• Streptococcus pneumoniae still most common at 41.7% overall incidence, representing over 80% of bacteremic cases 1
• Viral pathogens identified in 22% of severe CAP, often with bacterial co-infection 1
• Staphylococcus aureus, Legionella species, and gram-negative bacilli are encountered more frequently than in non-severe CAP 1

Critical Diagnostic Limitations

The causative pathogen remains unidentified in 20-60% of CAP cases despite comprehensive diagnostic testing. 1, 2, 3

• Blood or pleural fluid cultures for S. pneumoniae are positive in only 5-10% of cases 1
• Mixed infections (viral-bacterial or multiple bacterial pathogens) occur in 8-40% of cases 1, 2
• Post-influenza bacterial superinfection carries up to 10% mortality, most commonly with S. pneumoniae, S. aureus, or H. influenzae 1

Risk Factor-Based Pathogen Prediction

Specific Risk Factors for Resistant or Unusual Pathogens

• Pseudomonas aeruginosa risk factors: structural lung disease (bronchiectasis), corticosteroid use, recent broad-spectrum antibiotics (>7 days in past month), septic shock on presentation 1
• MRSA risk factors: prior MRSA infection/colonization, recurrent skin infections, severe pneumonia, recent influenza 1
• Aspiration pneumonia: anaerobic coverage only indicated with witnessed aspiration event, loss of consciousness, alcohol/drug overdose, seizures, or esophageal motility disorders 1

Common Pitfalls to Avoid

• Do not assume anaerobic coverage is routinely needed for CAP; it is only indicated in classic aspiration syndromes with documented risk factors 1
• Do not overlook viral etiologies, particularly influenza, which can present as primary viral pneumonia or predispose to severe bacterial superinfection 1
• Do not rely solely on blood cultures for S. pneumoniae diagnosis, as they are positive in only 5-10% of cases despite pneumococcus being the most common pathogen 1
• Consider drug-resistant S. pneumoniae in areas with high macrolide resistance (up to 33% in some regions) when selecting empiric therapy 5
• Recognize that normal respiratory flora on culture does not exclude bacterial pneumonia, as high-quality sputum samples are rarely obtained before antibiotics are started 6