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# Pseudomembranous colitis: C. difficile toxins and the volcano lesion

- Lesion

After antibiotics disturb the normal gut bacteria, C. difficile overgrows and its toxins coat the colon with grey-yellow plaques.

Updated Nov 12, 2026

## Key points

- Almost any antibiotic can trigger it; age, hospitalization and immunosuppression raise risk.
- Pseudomembranes: adherent layer of neutrophils, fibrin, mucus and dead epithelial cells.
- Volcano lesion: mucopurulent exudate erupting from a damaged dilated crypt; pathognomonic in context.
- Pseudomembranes alone are not specific (ischaemia, necrotizing infection can make them).
- Diagnosis: detection of the toxin or its genes in stool.
- Complications: toxic megacolon, perforation, hypoalbuminaemia; recurrences are frequent; faecal microbiota transfer helps recurrent disease.

## What it is

Pseudomembranous colitis is generally caused by Clostridioides (formerly Clostridium) difficile and is also called antibiotic-associated colitis. Antibiotics disrupt the colonic microbiota, letting C. difficile overgrow. Its toxins A and B glucosylate small GTPases such as Rho, collapsing the cytoskeleton, opening tight junctions and killing epithelial cells; neutrophil-rich exudate erupts from injured crypts. Hospitals are the main reservoir.

## Structure

- Gross: grey-yellow plaques on a red mucosa, mostly in the colon.
- Micro: denuded surface, neutrophils in the superficial lamina propria, fibrin thrombi in capillaries.
- Mushroom- or volcano-shaped exudate from crypts spreading to form the pseudomembrane.

## Step by step

- Antibiotics reduce the diversity of the colonic microbiota.
- C. difficile spores germinate and the organism overgrows.
- Toxins A and B inactivate Rho GTPases in epithelial cells.
- The cytoskeleton and tight junctions fail; cells die and cytokines are released.
- Neutrophils and fibrin pour out of crypts and coalesce into pseudomembranes.

## Clinical notes

- Fever, leukocytosis, cramps and watery diarrhoea, usually not grossly bloody.
- Hypervirulent strains and up to two in five recurrence rates make prevention and hand hygiene essential.

## Memory aid

Antibiotics clear the field; C. diff plants volcanoes that rain ash over the colon.

## Test yourself

Think of the answer, then tap the question to see it.

## How do C. difficile toxins injure colonic epithelium?

They inactivate small GTPases such as Rho, disrupting the cytoskeleton and tight junctions and causing cell death and inflammation.

## What is the volcano lesion?

A plume of neutrophils, mucus and fibrin erupting from a damaged crypt, which coalesces into pseudomembranes.

## Are pseudomembranes specific to C. difficile?

No; ischaemia and necrotizing infections can also form them, but volcano lesions in the right setting are characteristic.

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## Connections

## Related 5

- [Infectious enterocolitis](https://durgical.com/en/general-pathology/infectious-enterocolitis/)
- [Ischaemic bowel disease](https://durgical.com/en/general-pathology/ischaemic-bowel-disease/)
- [Ulcerative colitis vs Crohn disease](https://durgical.com/en/general-pathology/ulcerative-colitis-vs-crohn-disease/)
- [Patterns of acute inflammation: serous, fibrinous, purulent, abscess, ulcer](https://durgical.com/en/general-pathology/patterns-of-acute-inflammation-serous-fibrinous-purulent-abscess-ulcer/)
- [Injury by therapeutic drugs: the classic examples](https://durgical.com/en/general-pathology/injury-by-therapeutic-drugs-the-classic-examples/)

## Related conditions 2

- [Clostridioides difficile infection](https://durgical.com/en/conditions/clostridioides-difficile-infection/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)

## Microorganisms 1

- [Clostridioides difficile](https://durgical.com/en/microbiology/clostridioides-difficile/)

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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: [About C. diff](https://www.cdc.gov/c-diff/about/index.html) (Centers for Disease Control and Prevention, 2024).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/pseudomembranous-colitis-c-difficile-toxins-and-the-volcano-lesion/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/pseudomembranous-colitis-c-difficile-toxins-and-the-volcano-lesion/