Bacterial pneumonia: lobar vs bronchopneumonia
- Lesion
Bacteria fill the alveoli with neutrophils. The consolidation is patchy (bronchopneumonia) or a whole lobe (lobar), and it usually resolves.
What it is
Bacterial pneumonia is acute inflammation of the lung parenchyma in which the alveoli fill with a neutrophil-rich exudate, making the lung solid (consolidation). Two anatomic patterns are recognised, lobar pneumonia and bronchopneumonia, though they overlap and the organism matters more than the pattern. Classic lobar pneumonia passes through four stages, and most cases resolve with restoration of normal lung.
Structure
- Congestion: heavy, red, boggy lung; vascular engorgement, protein-rich fluid with few neutrophils and many bacteria.
- Red hepatisation: red, firm, airless, liver-like lobe; alveoli full of neutrophils, red cells and fibrin; fibrinous pleuritis.
- Grey hepatisation: grey-brown, dry surface; red cells lysed, fibrin and neutrophils persist.
- Resolution: enzymes digest the exudate, macrophages clear it and the architecture returns; pleural fibrin may organise into adhesions.
- Bronchopneumonia: grey-red, slightly raised, poorly defined foci around bronchioles with neutrophils in bronchi and alveoli.
Step by step
- Bacteria reach the alveoli, usually by microaspiration of oropharyngeal flora, often after a viral infection damages defences.
- Macrophages detect them and release cytokines; capillaries dilate and leak.
- Neutrophils and fibrin fill the alveoli and spread through pores of Kohn: consolidation.
- Gas exchange fails in the solid lung: hypoxaemia, tachypnoea.
- Antibodies and neutrophils kill bacteria; enzymes liquefy the exudate.
- If the alveolar framework survives, the lung returns to normal; if not, abscess or fibrosis follows.
The four stages of lobar pneumonia
| Stage | Gross | Microscopy |
|---|---|---|
| Congestion | Heavy, red, boggy | Engorged capillaries, edema fluid, many bacteria, few neutrophils |
| Red hepatisation | Red, firm, airless, liver-like | Neutrophils, red cells and fibrin fill the alveoli |
| Grey hepatisation | Grey-brown, dry | Red cells broken down; fibrin and neutrophils remain |
| Resolution | Lung re-aerates | Exudate digested, macrophages clear debris |
Clinical notes
- Abrupt fever, chills, productive cough, pleuritic pain, bronchial breathing and dullness over the consolidated lobe.
- Gram stain of sputum: neutrophils with organisms (lancet-shaped Gram-positive diplococci in pneumococcal pneumonia); blood cultures are positive in a minority.
- Pneumonia is a leading infectious cause of death at the extremes of age; pneumococcal and Hib vaccines reduce it.
Test yourself
Think of the answer, then tap the question to see it.
What is the difference between lobar pneumonia and bronchopneumonia?
Lobar pneumonia consolidates a large part or all of a lobe; bronchopneumonia produces patchy consolidation centred on bronchioles, often in several lobes.
What is seen microscopically in red hepatisation?
Alveoli packed with neutrophils, extravasated red cells and fibrin.
Why are patients without a spleen at risk of severe pneumococcal pneumonia?
The spleen is key for clearing encapsulated bacteria opsonised by antibody and complement.
Connections
Microorganisms1
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).
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Representative model, for educational purposes only.