Hospital-acquired and aspiration pneumonia, and lung abscess
- Lesion
Pneumonia in hospital is caused by tougher bacteria. Aspiration of mouth or stomach contents causes necrotising pneumonia, which can hollow out into an abscess.
What it is
Hospital-acquired pneumonia develops after admission, often in ventilated patients, and involves Gram-negative rods and S. aureus, frequently resistant. Aspiration pneumonia follows inhalation of gastric contents or oral flora when protective reflexes fail; it is partly chemical and partly bacterial. A lung abscess is a localised area of suppurative necrosis that forms a cavity, most often after aspiration.
Structure
- Aspiration pneumonia: necrotising bronchopneumonia with acute inflammation and sometimes food particles and foreign-body giant cells.
- Abscess: cavity of a few millimetres to several centimetres filled with pus; if it drains into a bronchus it holds an air-fluid level.
- Microscopy: central suppurative destruction of lung, surrounded by fibrin, inflammation and, when chronic, a fibrous wall.
- Gangrene of the lung: large, poorly defined, fetid, multilocular cavities.
Step by step
- Reflexes that protect the airway fail and contents of the mouth or stomach enter the bronchi.
- Gastric acid causes chemical injury (pneumonitis) within hours.
- Oral bacteria, especially anaerobes, multiply in the injured tissue.
- Necrosis and liquefaction by neutrophil enzymes create a cavity.
- The cavity drains into a bronchus (foul sputum, air-fluid level) or is walled off by fibrosis.
- Spread to the pleura gives empyema; septic emboli can reach the brain.
Routes to a lung abscess
| Route | Typical organisms | Distribution |
|---|---|---|
| Aspiration | Oral anaerobes with aerobes | Single, right side, dependent segments |
| After necrotising pneumonia | S. aureus, Klebsiella, pneumococcus | Often multiple, basal |
| Septic embolism | S. aureus from endocarditis or thrombophlebitis | Multiple, any region |
| Obstruction | Mixed flora behind a tumour | Segment beyond the block |
Clinical notes
- Abscess: weeks of cough with copious foul-smelling purulent or bloody sputum, fever, weight loss, sometimes clubbing.
- Ventilator-associated pneumonia is prevented by head elevation, oral care and minimising sedation.
- Complications: empyema, hemorrhage, brain abscess or meningitis, and rarely secondary (AA) amyloidosis.
Test yourself
Think of the answer, then tap the question to see it.
Why are aspiration abscesses more common in the right lung?
The right main bronchus is wider and more vertical, in line with the trachea.
Which organisms are most often found in a lung abscess after aspiration?
Anaerobes of the oral flora such as Bacteroides, Prevotella, Fusobacterium and Peptostreptococcus, often mixed with aerobes.
What must be excluded when an older adult has a lung abscess?
An underlying bronchogenic carcinoma obstructing the airway.
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).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.