Causes of Bacterial Pneumonia
Streptococcus pneumoniae is the most common bacterial cause of community-acquired pneumonia (CAP), followed by Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus, and atypical pathogens such as Mycoplasma pneumoniae and Chlamydia pneumoniae. 1
Common Bacterial Pathogens in Community-Acquired Pneumonia
Primary Bacterial Pathogens
- Streptococcus pneumoniae is consistently the predominant pathogen in CAP, accounting for 10-30% of all cases 1, 2
- Haemophilus influenzae (typically non-typeable strains) is common, especially in patients with underlying bronchopulmonary disease 1, 3
- Moraxella catarrhalis is less common but still significant, particularly in patients with chronic lung disease 1, 4
- Staphylococcus aureus is an important cause, especially during influenza outbreaks 1, 5
- Atypical pathogens include:
Age-Related Patterns
- In children under 5 years, viral causes predominate, though bacterial co-infection is common 1
- In school-aged children and adolescents, S. pneumoniae remains most common, but M. pneumoniae increases in prevalence 1
- In adults, S. pneumoniae is the leading cause across all age groups, but risk for gram-negative pathogens increases with age and comorbidities 1, 2
Nosocomial (Hospital-Acquired) Pneumonia Pathogens
Hospital-acquired pneumonia involves a different spectrum of pathogens:
- Gram-negative bacilli are predominant, including:
- Gram-positive bacteria:
Risk Factors and Pathogenesis
Pathogenesis
- Most bacterial pneumonias occur through aspiration of bacteria colonizing the oropharynx or upper gastrointestinal tract 1
- Intubation and mechanical ventilation significantly increase the risk by altering first-line patient defenses 1
- Some pathogens like Legionella species are acquired through inhalation of contaminated aerosols 1
Risk Factors for Specific Pathogens
- Chronic obstructive pulmonary disease (COPD) increases risk for H. influenzae and M. catarrhalis 1, 4
- Recent hospitalization increases risk for gram-negative bacilli and S. aureus 1
- Alcoholism increases risk for Klebsiella pneumoniae and aspiration pneumonia 1
- HIV infection increases risk for S. pneumoniae, H. influenzae, and P. aeruginosa 1, 3
- Influenza infection predisposes to secondary bacterial pneumonia, particularly S. aureus and S. pneumoniae 1
Special Populations
Immunocompromised Hosts
- HIV-infected individuals have higher rates of bacterial pneumonia with S. pneumoniae and H. influenzae being most common 1, 3
- In advanced HIV disease, P. aeruginosa and S. aureus become more prevalent 1, 3
- Risk factors for bacterial pneumonia in HIV patients include low CD4+ count, injection drug use, and cigarette smoking 1
Children
- Viral causes predominate in younger children, with bacterial co-infection common 1
- S. pneumoniae remains the most common bacterial cause across all pediatric age groups 1
- Mixed viral-bacterial infections account for 8-40% of pediatric CAP cases 1
Diagnostic Considerations
- The pathogen is not identified in 20-60% of CAP cases despite comprehensive testing 1
- Blood or pleural fluid cultures of S. pneumoniae are positive in only 5-10% of cases 1
- Newer molecular techniques like PCR and host transcriptional signatures are improving pathogen identification 6
- Mixed infections (viral-bacterial or multiple bacterial pathogens) are common, occurring in 8-40% of cases 1
Clinical Implications
- Understanding the likely pathogens based on patient factors (age, comorbidities, setting) is crucial for appropriate empiric antibiotic selection 1
- The increasing prevalence of drug-resistant S. pneumoniae is a significant concern and is associated with increased mortality 1
- Prevention strategies include pneumococcal vaccination, particularly in high-risk groups 3
- Bacterial pneumonia may accelerate the progression of underlying diseases like HIV 3