[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Other clinically important bacteria](https://durgical.com/en/microbiology/groups/other-clinically-important-bacteria/) › Aeromonas and Plesiomonas

# Aeromonas and Plesiomonas

- Bacterium

## Gram-negative?

Thin wall with an outer membrane: stains pink.

- For completeness

Freshwater bacteria that cause diarrhoea after eating seafood or drinking untreated water, and wound infections after water exposure.

Updated Nov 13, 2026

Also called: Aeromonas hydrophila, Aeromonas caviae, Aeromonas veronii, Plesiomonas shigelloides

## Quick facts

**Type**
Bacterium · [Other clinically important bacteria](https://durgical.com/en/microbiology/groups/other-clinically-important-bacteria/)

**Gram / shape**
Gram-negative · Straight rods

**Spread**
Untreated or poorly treated water

**Causes**
Acute gastroenteritis · Wound infection

**Treatment**
Most diarrhoea needs only rehydration

On this page

- The basics Key points
- At a glance
- Morphology
- Transmission
- Memory aid
- What the colours mean

- Clinical Virulence factors
- Diseases it causes
- Clinical picture
- Diagnosis

- Treatment Treatment
- Prevention

- Study Test yourself

- Connections Connections

## Key points

- Oxidase-positive Gram-negative rod in stool after fish or untreated water: Aeromonas or Plesiomonas.
- Wound in fresh water with fast cellulitis or necrosis: Aeromonas (seawater points to Vibrio vulnificus).
- Aeromonas is intrinsically resistant to ampicillin.
- Medicinal leeches can transmit Aeromonas.

## At a glance

**Gram**
Gram-negative

**Shape**
Straight rods

**Oxygen**
Facultative anaerobes

**Family**
Aeromonadaceae (Aeromonas); Enterobacterales (Plesiomonas)

## Morphology

- Motile Gram-negative rods
- Oxidase-positive, which separates them from most Enterobacterales
- Plesiomonas ferments inositol; Aeromonas is often beta-haemolytic on blood agar

## Transmission

- Untreated or poorly treated water
- Raw or undercooked fish and shellfish; travel to warm regions
- Wounds exposed to fresh or brackish water, mud or fish
- Medicinal leeches can carry Aeromonas

**Reservoir:** Fresh and brackish water, sewage, fish, shellfish and other animals

### Isolation precautions

What the care team adds to standard precautions.

- Standard

## Virulence factors

- Aeromonas: aerolysin (a pore-forming toxin), enterotoxins and proteases
- Beta-lactamases: Aeromonas is intrinsically resistant to ampicillin and some strains destroy carbapenems
- Plesiomonas: invasive strains cause dysentery-like colitis

## Diseases it causes

- Acute gastroenteritis, from watery diarrhoea to dysentery [Acute (viral) gastroenteritis →](https://durgical.com/en/conditions/acute-gastroenteritis/)
- Wound infection and cellulitis after water exposure [Cellulitis of the leg →](https://durgical.com/en/conditions/cellulitis-of-the-leg/)
- Necrotizing soft-tissue infection (Aeromonas) [Necrotizing soft tissue infection (necrotizing fasciitis, gas gangrene) →](https://durgical.com/en/conditions/necrotizing-soft-tissue-infection/)
- Septicaemia in people with cirrhosis, cancer or blood disorders [Sepsis and septic shock →](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- Neonatal meningitis and sepsis (Plesiomonas, rare) [Bacterial meningitis →](https://durgical.com/en/conditions/bacterial-meningitis/)

## Clinical picture

- Diarrhoea is usually self-limited; some patients have bloody stools or prolonged diarrhoea
- Rapidly spreading cellulitis or myonecrosis after an injury in a river, lake or fish tank
- Infection after leech therapy in plastic surgery

## Diagnosis

- Stool culture: ask the laboratory, as these organisms may not be sought routinely
- Oxidase test on colonies that look like Enterobacterales
- Multiplex stool PCR panels detect Plesiomonas
- Wound and blood cultures in invasive disease; MALDI-TOF identification

## Treatment

Most diarrhoea needs only rehydration. Invasive or severe disease is treated with fluoroquinolones, third-generation cephalosporins or trimethoprim-sulfamethoxazole (azithromycin for severe dysentery), guided by susceptibility testing; Aeromonas is intrinsically resistant to ampicillin, and some isolates resist carbapenems.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

- [Ciprofloxacin](https://durgical.com/en/pharmacology/ciprofloxacin/)
- [Ceftriaxone](https://durgical.com/en/pharmacology/ceftriaxone/)
- [Sulfamethoxazole + trimethoprim (co-trimoxazole)](https://durgical.com/en/pharmacology/sulfamethoxazole-trimethoprim/)
- [Azithromycin](https://durgical.com/en/pharmacology/azithromycin/)

## Prevention

- Safe drinking water and thorough cooking of fish and shellfish
- Keep open wounds out of fresh or brackish water, especially with cirrhosis
- Clean wounds promptly after water injuries

## Memory aid

Aeromonas and Plesiomonas are «river cousins» of Vibrio: same oxidase-positive family resemblance, but they prefer fresh water.

## Test yourself

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

## Which test separates Aeromonas and Plesiomonas from most Enterobacterales?

The oxidase test: both are oxidase-positive.

## Which medical therapy can transmit Aeromonas?

Medicinal leech therapy.

## Which antibiotic is Aeromonas intrinsically resistant to?

Ampicillin.

## In which patients does Aeromonas septicaemia occur most often?

People with cirrhosis, cancer or other serious underlying disease.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/aeromonas-and-plesiomonas/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

## Connections

## Related 4

- [Vibrio vulnificus and Vibrio parahaemolyticus](https://durgical.com/en/microbiology/vibrio-vulnificus-and-vibrio-parahaemolyticus/)
- [Vibrio cholerae](https://durgical.com/en/microbiology/vibrio-cholerae/)
- [Shigella](https://durgical.com/en/microbiology/shigella/)
- [Escherichia coli](https://durgical.com/en/microbiology/escherichia-coli/)

## Related conditions 5

- [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/)
- [Cellulitis of the leg](https://durgical.com/en/conditions/cellulitis-of-the-leg/)
- [Necrotizing soft tissue infection (necrotizing fasciitis, gas gangrene)](https://durgical.com/en/conditions/necrotizing-soft-tissue-infection/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [Bacterial meningitis](https://durgical.com/en/conditions/bacterial-meningitis/)

[← Back to the group: Other clinically important bacteria](https://durgical.com/en/microbiology/groups/other-clinically-important-bacteria/)

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Aeromonas and Human Health Disorders: Clinical Approaches](https://pmc.ncbi.nlm.nih.gov/articles/PMC9195132/) (Frontiers in Microbiology, 2022); [Plesiomonas shigelloides Revisited](https://pmc.ncbi.nlm.nih.gov/articles/PMC4786884/) (Clinical Microbiology Reviews (ASM), 2016).

## **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.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/aeromonas-and-plesiomonas/