Community-acquired pneumonia: organisms
- Topic
Each bacterium has its usual patient and clue: pneumococcus for anyone, Klebsiella in alcohol use, Staphylococcus after flu, Legionella from water.
What it is
Community-acquired pneumonia arises outside hospital. The pneumococcus is the leading cause at all ages, but the host and setting point to other organisms. Atypical agents (Mycoplasma, Chlamydia, Legionella) produce interstitial or patchy disease and are not seen on Gram stain. Knowing the pairing of organism and patient is a core exam skill.
Structure
- Pneumococcus: classic lobar consolidation with fibrinous pleuritis.
- S. aureus and Klebsiella: necrotising pneumonia with abscesses and empyema.
- Legionella: bronchopneumonia with fibrin and macrophages, microabscesses; organisms seen only with silver stains or immunostains.
- Mycoplasma: interstitial mononuclear inflammation like viral pneumonia.
Step by step
- Most organisms colonise the nose or throat and reach the lung by microaspiration.
- Capsules (pneumococcus, Hib, Klebsiella) resist phagocytosis until antibody and complement opsonise them.
- Viral infection strips cilia and epithelium, opening the way for S. aureus and pneumococcus.
- Legionella is inhaled in aerosols and survives inside macrophages.
- Mycoplasma attaches to ciliated cells and impairs clearance without invading deeply.
Organism, patient and clue
| Organism | Typical patient | Clue |
|---|---|---|
| Streptococcus pneumoniae | Any age; elderly, asplenia, after influenza | Rusty sputum, lobar consolidation, Gram-positive diplococci |
| Haemophilus influenzae | COPD, children (Hib) | Small Gram-negative coccobacilli |
| Moraxella catarrhalis | COPD, elderly | Second commonest bacterial cause of COPD exacerbation |
| Staphylococcus aureus | After influenza or measles; injecting drug use | Abscesses, empyema, pneumatoceles |
| Klebsiella pneumoniae | Alcohol use, diabetes | Currant-jelly sputum, upper-lobe necrosis |
| Legionella pneumophila | Older, smokers, transplant | Water aerosols, urinary antigen, hyponatraemia |
Clinical notes
- Children under two and adults over 65 need pneumococcal vaccination; Colombia introduced the conjugate vaccine in its national programme.
- Pseudomonas is mainly a hospital organism but causes community pneumonia in cystic fibrosis, bronchiectasis and neutropenia.
- Atypical organisms do not respond to β-lactams alone.
Test yourself
Think of the answer, then tap the question to see it.
Which organism causes most community-acquired pneumonias?
Streptococcus pneumoniae (pneumococcus).
Which pneumonia gives thick, mucoid currant-jelly sputum in a person with alcohol use disorder?
Klebsiella pneumoniae, because of its abundant viscid capsule.
How is Legionella pneumonia usually acquired and diagnosed quickly?
By inhaling aerosols from contaminated water systems; rapid diagnosis is by urinary antigen or PCR.
Which organism causes secondary bacterial pneumonia after influenza with abscesses and empyema?
Staphylococcus aureus.
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Pneumonia (MedlinePlus, US National Library of Medicine, 2025); Legionnaires' Disease (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.