What are the most common bacterial organisms involved in community-acquired pneumonia (CAP)?

From the Guidelines

The most common bacterial organisms involved in community-acquired pneumonia (CAP) are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis, which should be covered by empiric antibiotic therapy. The choice of antibiotic should be based on effective treatment of these commonly reported bacterial pathogens 1. Other significant bacterial pathogens include Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila, and Staphylococcus aureus, with their prevalence varying by patient population 1. Gram-negative bacteria such as Klebsiella pneumoniae and Pseudomonas aeruginosa are less common in typical CAP but become more prevalent in patients with underlying lung disease, recent hospitalization, or immunosuppression 1. Understanding the likely causative organisms is crucial for empiric antibiotic selection, which typically includes coverage for both typical and atypical pathogens. Some key points to consider when selecting empiric antibiotic therapy for CAP include:

• The patient's age and underlying health status
• The presence of comorbidities or risk factors for drug-resistant pathogens
• The severity of illness and the need for hospitalization
• The potential for antibiotic resistance and the need for broad-spectrum coverage For outpatient treatment, a macrolide (like azithromycin) or doxycycline is often used for healthy patients without risk factors, while a respiratory fluoroquinolone or beta-lactam plus macrolide combination is preferred for those with comorbidities or risk factors for drug-resistant pathogens 1. It's also important to note 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 1. Therefore, empiric antibiotic therapy for CAP should be guided by the most recent and highest-quality evidence, taking into account the patient's individual risk factors and the local epidemiology of CAP.

From the FDA Drug Label

Table 13: Bacteriological Eradication Rates (Community-Acquired Pneumonia) S. pneumoniae 19/20 (95%) Haemophilus influenzae 12/12 (100%) Haemophilus parainfluenzae 10/10 (100%) Mycoplasma pneumoniae 26/27 (96%) Chlamydophila pneumoniae 13/15 (87%)

The most common bacterial organisms involved in community-acquired pneumonia (CAP) are:

• S. pneumoniae
• Haemophilus influenzae
• Haemophilus parainfluenzae
• Mycoplasma pneumoniae
• Chlamydophila pneumoniae These organisms are listed in the bacteriological eradication rates table for community-acquired pneumonia, with eradication rates ranging from 87% to 100% 2.

From the Research

Bacterial Organisms Involved in Community-Acquired Pneumonia (CAP)

The most common bacterial organisms involved in community-acquired pneumonia (CAP) include:

• Streptococcus pneumoniae, which remains the most common causative pathogen 3
• Haemophilus influenzae
• Moraxella catarrhalis
• Staphylococcus aureus
• Mycoplasma pneumoniae and Chlamydia pneumoniae, which are among the most common atypical pathogens isolated from patients with CAP 3

Resistance Patterns of Bacterial Organisms

Resistance patterns of these bacterial organisms vary, with:

• Streptococcus pneumoniae strains showing decreased susceptibility to penicillin and increased resistance to macrolides 3, 4
• Haemophilus influenzae strains exhibiting beta-lactamase production as the principal mechanism of resistance 4, 5
• Moraxella catarrhalis strains being extremely susceptible to most antimicrobial agents, except for clindamycin, azithromycin, and clarithromycin 4
• Fluoroquinolone resistance being rare among these bacterial organisms, with levofloxacin and moxifloxacin exhibiting good activity against them 6, 4, 5

Prevalence of Bacterial Organisms

The prevalence of these bacterial organisms in community-acquired pneumonia varies, with:

• Streptococcus pneumoniae being the most common causative pathogen, accounting for approximately 30-50% of cases 3
• Haemophilus influenzae and Moraxella catarrhalis being less common, accounting for approximately 10-20% of cases 3
• Mycoplasma pneumoniae and Chlamydia pneumoniae being common atypical pathogens, accounting for approximately 10-20% of cases 3