What are the common pathogens causing pneumonia in different patient populations, including community-acquired and hospital-acquired cases, and how do underlying conditions such as chronic obstructive pulmonary disease (COPD) or heart disease affect susceptibility?

Pathogens Causing Pneumonia

Community-Acquired Pneumonia (CAP)

Streptococcus pneumoniae remains the single most important pathogen across all CAP settings, accounting for 20-60% of identified cases in hospitalized patients and 9-20% in outpatients. 1

Outpatients Without Comorbidities

For otherwise healthy outpatients with no cardiopulmonary disease or modifying factors:

• S. pneumoniae is the most common bacterial pathogen identified by sputum culture (9-20% of episodes) 1
• Mycoplasma pneumoniae is the most frequently identified organism when serologic testing is performed (13-37% of episodes) 1
• Chlamydia pneumoniae accounts for up to 17% of outpatient CAP 1
• Respiratory viruses are identified in up to 36% of patients 1
• Haemophilus influenzae is particularly important in cigarette smokers 1
• Legionella species occur in 0.7-13% of outpatients, typically causing more severe illness 1
• An etiologic diagnosis remains unknown in 40-50% of outpatient cases 1

Outpatients With COPD or Heart Disease

The presence of chronic cardiopulmonary disease fundamentally shifts the pathogen spectrum toward drug-resistant organisms and gram-negative bacteria. 1

When COPD or congestive heart failure is present:

• S. pneumoniae (including drug-resistant strains) remains most common 1
• H. influenzae increases in frequency 1
• Enteric gram-negative bacilli (E. coli, Klebsiella spp.) become significant threats due to oropharyngeal colonization 1
• Moraxella catarrhalis emerges as a pathogen 1
• Atypical pathogens (M. pneumoniae, C. pneumoniae) remain important, often as mixed infections 1
• Aspiration with anaerobes must be considered if poor dentition, neurologic illness, or swallowing disorders are present 1

Hospitalized Patients (Non-ICU)

For patients requiring hospital admission but not ICU care:

• S. pneumoniae dominates at 20-60% of identified episodes 1
• H. influenzae accounts for 3-10% of episodes 1
• Atypical pathogens (M. pneumoniae, C. pneumoniae) have been reported in 40-60% of admitted patients, often as mixed infections, though this finding is not universally corroborated 1
• Staphylococcus aureus, enteric gram-negatives, Legionella, and viruses each cause up to 10% of episodes 1
• Aspiration pneumonia occurs in 3-6% of hospitalized patients 1
• No etiologic agent is identified in 20-70% of hospitalized patients 1

ICU-Admitted Patients (Severe CAP)

Severe CAP requiring ICU admission carries mortality rates up to 50% and demands immediate recognition of high-risk pathogens. 1

The most critical pathogens include:

• S. pneumoniae remains predominant even in severe disease 2
• Legionella pneumophila occurs in 26.3% of ICU-admitted CAP patients 2
• P. aeruginosa becomes important if structural lung disease (bronchiectasis), corticosteroid therapy (≥10 mg prednisone daily), broad-spectrum antibiotics in the past month, or malnutrition are present 1
• S. aureus and enteric gram-negatives increase in frequency 1

Healthcare-Associated Pneumonia (HCAP)

HCAP is now more common than traditional CAP among hospitalized patients, accounting for 67.4% versus 32.6% of culture-positive pneumonia cases requiring admission. 3

Defining HCAP Risk Factors

HCAP should be suspected when any of the following are present:

• Prior hospitalization within 90 days 4, 3
• Residence in a nursing home or extended care facility 1, 4
• Chronic dialysis within 30 days 4
• Home infusion therapy 4
• Immunosuppressive disease or therapy 4

HCAP Pathogen Spectrum

The microbiology of HCAP differs dramatically from CAP, with multidrug-resistant organisms predominating. 3

• Methicillin-resistant S. aureus (MRSA) is the most common pathogen at 24.6% of HCAP cases 3
• S. pneumoniae accounts for 20.3% 3
• P. aeruginosa causes 18.8% of HCAP 3
• Methicillin-sensitive S. aureus occurs in 13.8% 3
• H. influenzae accounts for 8.5% 3

In nursing home residents specifically:

• S. aureus (29%), enteric gram-negative rods (15%), S. pneumoniae (9%), and Pseudomonas species (4%) are most frequent 4
• M. tuberculosis and certain viral agents (adenovirus, RSV, influenza) occur more commonly than in community-dwelling patients 1

Inappropriate initial antimicrobial therapy occurs in 28.3% of HCAP patients versus 13.0% of CAP patients and is an independent risk factor for hospital mortality. 3

Hospital-Acquired Pneumonia (HAP) and Ventilator-Associated Pneumonia (VAP)

HAP complicates 0.5-2.0% of hospitalizations, with six organisms causing approximately 80% of episodes. 5, 6

HAP/VAP Pathogen Distribution

The consistent pathogens across all studies:

• S. aureus (28.0% overall) is the most common HAP/VAP pathogen 6
• P. aeruginosa (21.8%) is the second most frequent 6
• Klebsiella species (9.8%) 6
• E. coli (6.9%) 6
• Acinetobacter species (6.8%) 6
• Enterobacter species (6.3%) 6
• Lower prevalences of Serratia species, Stenotrophomonas maltophilia, and community-acquired pathogens 6

Nearly half of HAP cases are polymicrobial. 5

VAP-Specific Considerations

In mechanically ventilated patients, VAP isolates demonstrate 5-10% less susceptibility to extended-spectrum antimicrobials compared to HAP isolates of the same species. 6

Critical VAP pathogens:

• P. aeruginosa, Acinetobacter, MRSA, and other antibiotic-resistant bacteria assume increasing importance 5
• Anaerobic organisms are rare in intubated patients with VAP, unlike nonintubated aspiration pneumonia 4

Risk Factors for HAP/VAP

• Mechanical ventilation for >48 hours 5
• ICU residence 5
• Duration of ICU or hospital stay 5
• Severity of underlying illness 5
• Presence of comorbidities 5

Special Population Considerations

Alcoholic Patients

S. pneumoniae (including drug-resistant strains) is the predominant pathogen in alcoholics with pneumonia. 7

Additional high-risk pathogens in alcoholics:

• Klebsiella pneumoniae is specifically associated with alcoholic patients 7
• Enterobacteriaceae and P. aeruginosa occur with increased frequency 7
• Anaerobic bacteria are important if aspiration with loss of consciousness occurred 7
• M. tuberculosis must always be considered, as alcoholism is a specific risk factor 7

Aspiration Pneumonia

The Infectious Diseases Society of America states that anaerobic coverage is not recommended for routine aspiration pneumonia in inpatient settings, except in cases of lung abscess, necrotizing pneumonia, or empyema. 4

Community-acquired aspiration pathogens:

• S. pneumoniae and H. influenzae are most common, particularly in patients with smoking history or COPD 4
• Enteric gram-negative bacilli (E. coli, Klebsiella) occur in patients with oropharyngeal colonization 4

Healthcare-associated aspiration pathogens:

• S. aureus (including MRSA) is increasingly common in nosocomial aspiration, particularly in ICU patients or those with diabetes or head trauma 4
• P. aeruginosa occurs with structural lung disease, prior antibiotics, or prolonged hospitalization 4

Emerging Resistance Patterns

Drug resistance among HAP and VAP pathogens has been increasing by 1% per year from 2004-2008. 6

Key resistance considerations:

• Age ≥65 years is an independent epidemiologic risk for drug-resistant S. pneumoniae 1
• Antimicrobial therapy within the preceding 90 days is the strongest predictor of multidrug-resistant pathogens 4
• Current hospitalization ≥5 days or hospitalization for ≥2 days in the preceding 90 days mandates coverage for resistant organisms 4

Common Pitfalls

• Do not assume CAP microbiology in patients with healthcare exposure—HCAP pathogens differ fundamentally and require broader empiric coverage 3
• Do not rely on β-lactam monotherapy for pseudomonal HAP—combination therapy with an antipseudomonal β-lactam plus an aminoglycoside or fluoroquinolone is required to prevent rapid resistance evolution 5
• Do not routinely add anaerobic coverage for aspiration pneumonia—reserve for lung abscess, necrotizing pneumonia, or empyema 4
• Do not overlook Legionella in cruise ship exposure or failure to respond to β-lactams—this intracellular pathogen requires macrolide or fluoroquinolone therapy 8