Community-Acquired Pneumonia Guidelines

Primary Bacterial Pathogens

The American Thoracic Society recommends that Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae, Staphylococcus aureus, Legionella species, Chlamydia pneumoniae, and Moraxella catarrhalis are considered primary bacterial pathogens in community-acquired pneumonia, with Streptococcus pneumoniae being the most common, identified in approximately 48% of cases where an organism is detected 1
Haemophilus influenzae is the second most common typical bacterial pathogen, and Mycoplasma pneumoniae is an important atypical pathogen, according to the American College of Physicians 2, 3
Staphylococcus aureus, including methicillin-susceptible strains, is also a common pathogen, as stated by the American College of Physicians 1, 2, 3
Legionella pneumophila accounts for approximately 3% of cases, and Chlamydia pneumoniae is identified in approximately 13% of hospitalized patients, according to the American Thoracic Society 1, 2, 3
Moraxella catarrhalis is a less common pathogen, approximately 2% of cases, as reported by the American Thoracic Society 1

The American College of Physicians recommends considering Pseudomonas aeruginosa in patients with prior respiratory isolation of this organism or recent hospitalization with parenteral antibiotics in the last 90 days 2, 3
Methicillin-resistant Staphylococcus aureus (MRSA) should be considered in patients previously infected with MRSA, as stated by the American College of Physicians 2, 3

The American Thoracic Society recommends first-line therapy consisting of β-lactam plus macrolide combination OR respiratory fluoroquinolone monotherapy for hospitalized patients (non-ICU) 4, 5, 6
For ICU patients (severe CAP), the American College of Physicians recommends using β-lactam plus macrolide OR β-lactam plus fluoroquinolone 2, 3, 7
The American Thoracic Society recommends β-lactam options: ampicillin-sulbactam, ceftriaxone, cefotaxime, or ceftaroline, and macrolide options: azithromycin or clarithromycin, for hospitalized patients (non-ICU) 4, 5, 6
For outpatient management, the American Thoracic Society recommends amoxicillin, doxycycline, or macrolide for patients without comorbidities, and combination therapy with amoxicillin-clavulanate or cephalosporin PLUS macrolide or doxycycline, OR respiratory fluoroquinolone monotherapy for patients with comorbidities 8

Routine sputum Gram stain and culture do not improve individual patient outcomes and have poor yield, as reported by the American Thoracic Society 1
Blood and sputum cultures should be obtained when there is concern for multidrug-resistant pathogens such as Pseudomonas aeruginosa or MRSA, according to the American College of Physicians 2, 3
Testing for Legionella urinary antigen and collecting lower respiratory tract secretions for Legionella testing is recommended in severe CAP, as stated by the American Thoracic Society 8

Patients with recent antibiotic exposure should receive treatment from a different antibiotic class due to increased risk of bacterial resistance, as recommended by the American Thoracic Society 4, 5, 6
When expanded therapy for resistant pathogens is initiated but cultures are negative and the patient is improving, therapy should be narrowed within 48 hours, according to the American College of Physicians 2, 3

Streptococcus pneumoniae is the single most common cause of community‑acquired pneumonia, identified in about 41–48 % of cases where a pathogen is isolated, and this predominance persists in patients receiving methotrexate. 9, 10
Haemophilus influenzae ranks second, accounting for roughly 3–14 % of community‑acquired episodes and is especially important in methotrexate‑treated patients with underlying lung disease or immunosuppression. 9, 10
Moraxella catarrhalis contributes approximately 1–3 % of cases and should be considered in methotrexate‑treated individuals with chronic obstructive pulmonary disease or other structural lung abnormalities. 9, 10

Mycoplasma pneumoniae is an important atypical cause, representing 1–33 % of community‑acquired pneumonia depending on the studied population, and may affect methotrexate‑treated patients. 9, 10
Legionella pneumophila accounts for about 3 % of all cases; infection can be severe and warrants urinary antigen testing in methotrexate‑treated patients with severe pneumonia. 9, 10
Chlamydophila pneumoniae is detected in roughly 7–37 % of hospitalized pneumonia patients and frequently co‑exists with typical bacteria in polymicrobial infections among methotrexate users. 9, 10

Staphylococcus aureus (including methicillin‑susceptible strains) causes 1–10 % of community‑acquired pneumonia episodes, with methicillin‑resistant S. aureus comprising about 5 % of isolates in recent series; these rates apply to methotrexate‑treated patients as well. 9, 10

Influenza virus is responsible for approximately 4–30 % of community‑acquired lower respiratory tract infections and is a major cause of severe pneumonia in methotrexate‑treated patients, particularly during seasonal outbreaks. 9, 10
Overall, respiratory viruses are implicated in up to 60 % of community‑acquired lower respiratory tract infections and 30 % of pneumonia cases, with influenza being the most clinically significant. 9, 10

Polymicrobial pneumonia occurs in 6–26 % of hospitalized adults with community‑acquired pneumonia; the most frequent combinations are S. pneumoniae with atypical organisms or S. pneumoniae with H. influenzae. 10
Mixed bacterial‑viral infections are common, especially S. pneumoniae together with influenza virus, and should be anticipated in severe presentations of methotrexate‑treated patients. 10

Lobar consolidation on chest imaging suggests typical bacterial pathogens such as S. pneumoniae, H. influenzae, or K. pneumoniae and supports empiric therapy with a third‑generation cephalosporin plus a macrolide. 9, 10