Causes of pneumonia
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Causes of Pneumonia
Pneumonia is an infection of the lungs causing consolidation of normally air-filled alveoli. It can be caused by a wide range of agents, broadly grouped below.
1. Bacteria
Typical Bacteria
- Streptococcus pneumoniae (pneumococcus) - the single most common bacterial cause across all age groups; classically causes lobar consolidation
- Staphylococcus aureus - aggressive course, can follow influenza or varicella; associated with lung abscesses, pneumatoceles, and empyema; MRSA strains are increasingly recognized
- Klebsiella pneumoniae - typically in alcoholics and diabetics; causes upper lobe cavitating pneumonia
- Haemophilus influenzae - common in COPD patients and young unimmunized children
- Group A Streptococcus (Streptococcus pyogenes)
- Gram-negative enteric bacilli - Escherichia coli, Serratia, Pseudomonas spp. (especially in hospital-acquired and nursing-home-acquired pneumonia)
- Bordetella pertussis (whooping cough) - in children, including partially immunized ones
- Anaerobes (e.g., Peptostreptococcus, Bacteroides spp.) - particularly in aspiration pneumonia, alcoholism, and poor dentition
Atypical Bacteria
These cause a more insidious, "walking pneumonia" pattern with gradual onset, low-grade fever, and dry cough:
- Mycoplasma pneumoniae - most common atypical agent in school-age children and young adults; accounts for ~17% of CAP in ages 5-13
- Chlamydia pneumoniae - similar presentation to Mycoplasma; seen in all ages
- Legionella pneumophila - spread via contaminated water/cooling systems; can cause severe disease with multiorgan involvement (Legionnaires' disease)
- Chlamydia trachomatis - in neonates (afebrile pneumonitis at 6-8 weeks); largely eliminated where prenatal screening is practiced
Neonatal-specific Bacteria
- Group B Streptococcus - early-onset (within 48 hrs) or late-onset (7-10 days)
- Listeria monocytogenes
- Gram-negative enteric organisms (Klebsiella, E. coli)
2. Viruses
Viruses are the most common overall cause of pneumonia in children, accounting for over 70% of hospitalized pediatric cases in the US:
- Respiratory syncytial virus (RSV) - leading cause in infants and toddlers
- Influenza A and B - also predisposes to secondary bacterial pneumonia (especially S. aureus)
- Human rhinovirus
- Parainfluenza virus
- Human metapneumovirus (hMPV)
- Adenovirus
- Measles and Varicella (chickenpox) - can cause viral pneumonia and predispose to bacterial superinfection
- Cytomegalovirus (CMV) - in immunocompromised patients and infants 1-3 months
- Coronaviruses - including SARS-CoV, MERS-CoV, and SARS-CoV-2 (COVID-19); also avian influenza (H5N1) and swine flu (H1N1)
3. Mycobacteria
- Mycobacterium tuberculosis - in endemic areas, immigrants, and immunocompromised; causes upper lobe cavitating disease
- Non-tuberculous mycobacteria (NTM) - in immunocompromised hosts
4. Fungi (mostly in immunocompromised patients)
- Pneumocystis jirovecii (PCP) - classic AIDS-defining illness; bilateral interstitial infiltrates
- Cryptococcus neoformans
- Coccidioides immitis - endemic in the American Southwest
- Aspergillus species - in neutropenic/transplant patients
- Histoplasma capsulatum - endemic in the Ohio/Mississippi River valleys
5. Other Organisms
- Ureaplasma urealyticum - in infants 1-3 months
- Parasitic/protozoan causes - rare; seen in specific geographic or immunocompromised settings
Classification by Setting (Epidemiological Categories)
| Type | Common Pathogens |
|---|---|
| Community-Acquired (CAP) | S. pneumoniae, M. pneumoniae, C. pneumoniae, viruses, H. influenzae |
| Hospital-Acquired (HAP) | Gram-negative bacilli (Pseudomonas, Klebsiella), S. aureus (MRSA) |
| Ventilator-Associated (VAP) | MDR gram-negatives, MRSA |
| Aspiration Pneumonia | Oral anaerobes, gram-negatives |
| Immunocompromised Host | PCP, Cryptococcus, CMV, Aspergillus, atypical mycobacteria |
Radiographic Pattern - Pathogen Correlation
| Pattern | Common Causes |
|---|---|
| Lobar consolidation | S. pneumoniae, Klebsiella |
| Bronchopneumonia | S. aureus, gram-negatives, anaerobes |
| Interstitial pneumonia | Viruses, Mycoplasma |
| Cavitation | M. tuberculosis, S. aureus, gram-negatives |
| Round pneumonia | S. pneumoniae |
Key Risk Factors That Determine Causative Agent
- Age - neonates, infants, children, and adults have markedly different pathogen distributions
- Immunocompromise (HIV/AIDS, transplant, malignancy) - opens the door to opportunistic organisms
- Alcoholism - predisposes to Klebsiella, Haemophilus, anaerobes
- Cystic fibrosis - early S. aureus, later Pseudomonas
- Sickle cell disease - encapsulated bacteria (pneumococcus, Salmonella, Klebsiella)
- Hospitalization/ventilation - MDR organisms
- Vaccination status - unimmunized children at risk of H. influenzae type b and pneumococcus
Sources: Textbook of Family Medicine 9e; Tintinalli's Emergency Medicine; Grainger & Allison's Diagnostic Radiology; Murray & Nadel's Respiratory Medicine
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