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.
Also called: Aeromonas hydrophila, Aeromonas caviae, Aeromonas veronii, Plesiomonas shigelloides
Quick facts
- Type
- Bacterium · 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
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 →
- Wound infection and cellulitis after water exposure Cellulitis of the leg →
- Necrotizing soft-tissue infection (Aeromonas) Necrotizing soft tissue infection (necrotizing fasciitis, gas gangrene) →
- Septicaemia in people with cirrhosis, cancer or blood disorders Sepsis and septic shock →
- Neonatal meningitis and sepsis (Plesiomonas, rare) 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:
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
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.
Connections
← Back to the group: 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 (Frontiers in Microbiology, 2022); Plesiomonas shigelloides Revisited (Clinical Microbiology Reviews (ASM), 2016).
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Representative model, for educational purposes only.