What are the common organisms responsible for community-acquired pneumonia and what are the recommended antibiotic treatments?

Common Organisms in Community-Acquired Pneumonia

The most common bacterial pathogen causing community-acquired pneumonia is Streptococcus pneumoniae, followed by Haemophilus influenzae, Mycoplasma pneumoniae, Staphylococcus aureus, Legionella species, Chlamydia pneumoniae, and Moraxella catarrhalis. 1

Primary Bacterial Pathogens

The traditional bacterial pathogens responsible for CAP include:

• Streptococcus pneumoniae - remains the predominant pathogen, identified in approximately 48% of cases where an organism is detected 1, 2
• Haemophilus influenzae - second most common typical bacterial pathogen 1
• Mycoplasma pneumoniae - important atypical pathogen 1
• Staphylococcus aureus (including methicillin-susceptible strains) 1
• Legionella pneumophila - accounts for approximately 3% of cases 1, 2
• Chlamydia pneumoniae - identified in approximately 13% of hospitalized patients 1, 2
• Moraxella catarrhalis - less common, approximately 2% of cases 1, 2

Evolving Microbial Epidemiology

The microbial etiology of CAP is changing due to widespread pneumococcal conjugate vaccination, with increased recognition of viral pathogens 1. Up to 40% of patients with identified pathogens have viruses as the likely cause of CAP 3. Influenza A virus was identified in 19% of hospitalized CAP patients in one major UK study 2.

Resistant Pathogens Requiring Special Consideration

When specific risk factors are present, consider:

• Pseudomonas aeruginosa - in patients with prior respiratory isolation of this organism or recent hospitalization with parenteral antibiotics in the last 90 days 1
• Methicillin-resistant Staphylococcus aureus (MRSA) - in patients previously infected with MRSA 1
• Multi-drug resistant Streptococcus pneumoniae (MDRSP) - isolates resistant to two or more of: penicillin (MIC ≥2 mcg/mL), second-generation cephalosporins, macrolides, tetracyclines, and trimethoprim/sulfamethoxazole 4

Recommended Empirical Antibiotic Treatment

For Hospitalized Patients (Non-ICU)

First-line therapy consists of β-lactam plus macrolide combination OR respiratory fluoroquinolone monotherapy 1:

• β-lactam options: ampicillin-sulbactam 1.5-3g every 6 hours, ceftriaxone 1-2g daily, cefotaxime 1-2g every 8 hours, or ceftaroline 600mg every 12 hours 1
• Macrolide options: azithromycin 500mg daily or clarithromycin 500mg twice daily 1
• Fluoroquinolone monotherapy: levofloxacin 750mg daily or moxifloxacin 400mg daily 1

The combination of ceftriaxone plus azithromycin is specifically recommended for hospitalized patients without risk factors for resistant bacteria, with a minimum duration of 3 days 3.

For ICU Patients (Severe CAP)

Use β-lactam plus macrolide OR β-lactam plus fluoroquinolone 1. When Pseudomonas aeruginosa is documented or presumptive, combination therapy with an anti-pseudomonal β-lactam is required 4.

For Outpatient Management

Patients without comorbidities: amoxicillin, doxycycline, or macrolide (only if local pneumococcal resistance is <25%) 1

Patients with comorbidities (chronic heart, lung, liver, or renal disease; diabetes; alcoholism; malignancy; asplenia): combination therapy with amoxicillin-clavulanate or cephalosporin PLUS macrolide or doxycycline, OR respiratory fluoroquinolone monotherapy 1

Critical Diagnostic Considerations

Only 38% of hospitalized CAP patients have a pathogen identified 3. Routine sputum Gram stain and culture do not improve individual patient outcomes and have poor yield 1. However, blood and sputum cultures should be obtained when there is concern for multidrug-resistant pathogens such as Pseudomonas aeruginosa or MRSA 1.

Testing for Legionella urinary antigen and collecting lower respiratory tract secretions for Legionella testing is recommended in severe CAP 1.

Important Caveats

Patients with recent antibiotic exposure should receive treatment from a different antibiotic class due to increased risk of bacterial resistance 1. When expanded therapy for resistant pathogens is initiated but cultures are negative and the patient is improving, therapy should be narrowed within 48 hours 1.

Atypical pathogens are more common in younger patients (under 75 years) and should be covered in all patients with severe pneumonia 2.