What are the most common organisms causing community-acquired pneumonia (CAP)?

From the Guidelines

The most common organisms causing community-acquired pneumonia (CAP) are Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae, Chlamydia pneumoniae, and respiratory viruses, with Streptococcus pneumoniae being the single most common bacterial cause, as stated in the 2019 American Thoracic Society (ATS)/Infectious Diseases Society of America (IDSA) guideline 1.

Common Organisms

The guideline highlights that the microbial etiology of CAP is changing, particularly with the widespread introduction of the pneumococcal conjugate vaccine, and there is increased recognition of the role of viral pathogens. Atypical pathogens like Mycoplasma pneumoniae and Chlamydia pneumoniae are particularly common in younger patients and those with milder disease. Haemophilus influenzae is more prevalent in patients with underlying lung disease. Respiratory viruses, including influenza, respiratory syncytial virus (RSV), and more recently SARS-CoV-2, are significant causes of CAP, especially during seasonal outbreaks.

Less Common Pathogens

Less common but important pathogens include Legionella pneumophila, which is associated with exposure to contaminated water sources, and Staphylococcus aureus, which often follows influenza infection. The prevalence of these organisms varies by geographic region, patient age, comorbidities, and vaccination status. Understanding the likely causative organisms helps guide empiric antibiotic therapy, which typically includes coverage for both typical and atypical pathogens until culture results are available, as recommended by the ATS/IDSA guideline 1.

Key Considerations

It's essential to note that the need for specific anaerobic coverage for CAP is generally overestimated, and anaerobic coverage is clearly indicated only in the classic aspiration pleuropulmonary syndrome in patients with a history of loss of consciousness as a result of alcohol/drug overdose or after seizures in patients with concomitant gingival disease or esophageal motility disorders, as mentioned in the 2007 Infectious Diseases Society of America (IDSA)/American Thoracic Society (ATS) consensus guidelines 1. However, the most recent and highest quality study, the 2019 ATS/IDSA guideline 1, takes precedence in guiding clinical practice.

From the Research

Common Organisms of Community-Acquired Pneumonia

The most common organisms causing community-acquired pneumonia (CAP) include:

• Streptococcus pneumoniae, which is considered the leading cause of CAP 2, 3, 4
• Haemophilus influenzae, another common bacterial cause of CAP 2, 4
• Atypical bacteria such as Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella species 2, 3
• Respiratory viruses, which are now considered to be the most common pathogens detected in CAP, including rhinovirus, human metapneumovirus, respiratory syncytial virus, human parainfluenza virus, and human adenoviruses 5

Bacterial Causes of CAP

Bacterial causes of CAP also include:

• Staphylococcus aureus 4
• Moraxella catarrhalis 4, 6
• Enterobacteriaceae 4
• Pseudomonas 4

Antimicrobial Resistance

There is increasing resistance to antimicrobial agents among CAP pathogens, including:

• Streptococcus pneumoniae, with high rates of resistance to penicillin and macrolides 2, 6
• Haemophilus influenzae, with resistance to ampicillin and trimethoprim/sulfamethoxazole 6
• Moraxella catarrhalis, with high rates of beta-lactamase production 6