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PH-FD21

Purulent inflammation

How pus forms: why pyogenic bacteria draw in floods of neutrophils, how neutrophil enzymes liquefy tissue, and why pus walls off as an abscess, spreads as phlegmon or fills a cavity as empyema, and how it heals or spreads.

By the end, you can

  • Explain how pyogenic bacteria recruit neutrophils and how neutrophil enzymes liquefy tissue into pus.
  • Describe what pus contains and why its character varies with the organism.
  • Distinguish an abscess, phlegmon (cellulitis) and empyema, and relate each to the organism and the site.
  • Describe how an abscess is walled off and how it can point, drain or heal.
  • Explain how suppuration can spread locally and through the blood.
  • Recognise the classic appearances, sites and tests of suppuration, and explain why pus usually has to be drained and how antibiotics, fibrinolytics and source control act on it.

What it is

Purulent (suppurative) inflammation is acute inflammation in which neutrophils dominate the exudate and their enzymes liquefy the tissue into pus. It is usually caused by pyogenic (pus-forming) bacteria such as Staphylococcus aureus and Streptococcus pyogenes. Its main forms are an abscess (pus in a newly formed cavity), phlegmon (pus spreading diffusely through tissue) and empyema (pus in a cavity or hollow organ that already exists).

3 types to tell apart
Abscess
A localised collection of pus in a cavity carved out of the tissue and walled off; typical of Staphylococcus aureus. Boils, carbuncles, and abscesses of the lung, liver, brain, breast or around the anus.
Phlegmon (diffuse suppuration)
Purulent inflammation spreading between tissue planes without a wall, typical of streptococci. In English-language clinical use the spreading skin form is called cellulitis, and “phlegmon” often means an inflamed mass.
Empyema
Pus filling a cavity or hollow organ that already exists: the pleural space after pneumonia, an obstructed gallbladder or appendix, a joint (septic arthritis).

Normal function

Neutrophils: the first responders

Neutrophils are the most numerous white cells in the blood. Each has a of three to five lobes, which helps it squeeze between endothelial cells, and a cytoplasm packed with granules of enzymes (elastase, collagenase, myeloperoxidase) and antimicrobial proteins. The bone marrow makes enormous numbers every day, and each lives only a few days.

In the tissue, a neutrophil engulfs bacteria coated with antibody or complement and kills them inside a phagolysosome, with its enzymes and a burst of reactive oxygen species; myeloperoxidase converts these into hypochlorous acid, a bleach. These weapons are meant to work inside the cell, but some leak out, and dying neutrophils release the rest.

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