[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Lung](https://durgical.com/en/general-pathology/areas/lung/) › Hospital-acquired and aspiration pneumonia, and lung abscess

# 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.

Updated Nov 12, 2026

## Key points

- Hospital-acquired: Pseudomonas, Enterobacterales (Klebsiella, E. coli), S. aureus including MRSA, Acinetobacter.
- Aspiration risk: reduced consciousness (alcohol, drugs, anaesthesia, seizures, stroke), dysphagia, vomiting, poor dental hygiene.
- Aspiration lesions favour the right lung (straighter right main bronchus): posterior upper lobe and superior lower lobe segments when supine.
- Lung abscess flora: oral anaerobes (Bacteroides, Prevotella, Fusobacterium, Peptostreptococcus), often mixed with aerobes.
- Other abscess routes: necrotising pneumonia (S. aureus, Klebsiella), septic emboli (right-sided endocarditis), obstruction by carcinoma.
- Always exclude an underlying carcinoma in an older adult with 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](https://durgical.com/en/conditions/aspiration-pneumonia-and-pneumonitis/) | Oral anaerobes with aerobes | Single, right side, dependent segments |
| [After necrotising pneumonia](https://durgical.com/en/conditions/lung-abscess/) | S. aureus, Klebsiella, pneumococcus | Often multiple, basal |
| [Septic embolism](https://durgical.com/en/conditions/infective-endocarditis/) | S. aureus from endocarditis or thrombophlebitis | Multiple, any region |
| [Obstruction](https://durgical.com/en/conditions/lung-cancer/) | 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.

## Memory aid

Asleep, drunk or seizing: the mouth's anaerobes slide down the right bronchus.

## 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.

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## Connections

## Related 6

- [Patterns of acute inflammation: serous, fibrinous, purulent, abscess, ulcer](https://durgical.com/en/general-pathology/patterns-of-acute-inflammation-serous-fibrinous-purulent-abscess-ulcer/)
- [Patterns of necrosis: coagulative, liquefactive, caseous, fat, fibrinoid, gangrenous](https://durgical.com/en/general-pathology/patterns-of-necrosis-coagulative-liquefactive-caseous-fat-fibrinoid-gangrenous/)
- [Bacterial pneumonia: lobar vs bronchopneumonia](https://durgical.com/en/general-pathology/bacterial-pneumonia-lobar-vs-bronchopneumonia/)
- [Bronchiectasis](https://durgical.com/en/general-pathology/bronchiectasis/)
- [Lung carcinoma: histologic types](https://durgical.com/en/general-pathology/lung-carcinoma-histologic-types/)
- [Caries, gingivitis and periodontitis: the lesion view](https://durgical.com/en/general-pathology/caries-gingivitis-and-periodontitis-the-lesion-view/)

## Related conditions 4

- [Hospital-acquired and ventilator-associated pneumonia](https://durgical.com/en/conditions/hospital-acquired-and-ventilator-associated-pneumonia/)
- [Aspiration pneumonia and pneumonitis](https://durgical.com/en/conditions/aspiration-pneumonia-and-pneumonitis/)
- [Lung abscess](https://durgical.com/en/conditions/lung-abscess/)
- [Healthcare-associated infections (CLABSI, CAUTI, VAP)](https://durgical.com/en/conditions/healthcare-associated-infections/)

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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](https://medlineplus.gov/pneumonia.html) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/hospital-acquired-and-aspiration-pneumonia-and-lung-abscess/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/hospital-acquired-and-aspiration-pneumonia-and-lung-abscess/