Aeromonas, Plesiomonas and HACEK Group
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Direct answer
Aeromonas, Plesiomonas and the HACEK group are three distinct groups of Gram-negative bacilli with different habitats. Aeromonas and Plesiomonas are oxidase-positive, motile organisms of fresh water and food that cause diarrhoea and wound infections after water exposure. The HACEK group are fastidious oral organisms that cause culture-negative endocarditis, which is why extended blood culture incubation matters.
What you must remember
- Aeromonas: oxidase-positive, motile, facultatively anaerobic Gram-negative rods that are widespread in fresh water, drinking water and food.
- Aeromonas syndromes: gastroenteritis, wound infection after water exposure or leech bites, and septicaemia in people with liver disease, malignancy or immunosuppression.
- Aeromonas resistance: usually resistant to ampicillin because of a chromosomal beta-lactamase.
- Plesiomonas shigelloides: oxidase-positive, lactose-negative Gram-negative rod linked to fresh water and shellfish, causing watery or dysentery-like diarrhoea.
- HACEK group: Haemophilus, Actinobacillus, Cardiobacterium, Eikenella and Kingella, all normal oral flora organisms.
- Culture-negative endocarditis: HACEK organisms are slow growing and fastidious, so they are missed unless blood cultures are held for an extended period.
- Treatment: diarrhoea is often self-limiting, fluoroquinolones or co-trimoxazole are used for serious Aeromonas infection, and ceftriaxone is the usual choice for HACEK endocarditis.
- Surgery: valve surgery is considered in HACEK endocarditis with heart failure, abscess or uncontrolled infection.
Telling a water-borne infection from endocarditis
These organisms rarely appear together in a clinical picture, but the exposure history separates them quickly. The steps below move from the history to the choice of treatment.
- Exposure history: freshwater swimming, seafood or leech bites suggest Aeromonas or Plesiomonas, while a recent dental procedure or oral infection suggests the HACEK group.
- Send the right specimen: stool is sent for culture with the organism named, because Aeromonas and Plesiomonas are not routine stool isolates, and blood cultures are held for longer.
- Identify: oxidase positivity and motility separate the water-borne Gram-negative rods, while the HACEK organisms need enriched media and CO2.
- Check susceptibility: Aeromonas is usually ampicillin-resistant, so empirical choices avoid ampicillin.
- Treat: mild diarrhoea is managed with fluids, and antibiotics are reserved for severe disease, while HACEK endocarditis is treated with ceftriaxone.
- Look for complications: echocardiography and surgical review are needed in endocarditis with heart failure or abscess formation.
Where the exam tries to catch you
These organisms are often grouped in questions on water-borne bacteria and endocarditis, so the lactose result and the negative culture are the usual traps.
- Trap: assuming Aeromonas is lactose-negative like Shigella Correct: many Aeromonas strains ferment lactose on MacConkey agar, whereas Plesiomonas does not.
- Trap: choosing ampicillin for Aeromonas infection Correct: Aeromonas is usually ampicillin-resistant, and fluoroquinolones or co-trimoxazole are preferred for serious infection.
- Trap: dismissing negative blood cultures in endocarditis as proof of no infection Correct: HACEK organisms are fastidious, so prolonged incubation and specialist identification are needed.
Frequently asked questions
Why are HACEK organisms a cause of culture-negative endocarditis?
HACEK organisms are fastidious and slow growing, so they often fail to appear in standard blood culture bottles incubated for only a few days. Extended incubation, specialist laboratory identification and molecular methods reduce the number of culture-negative cases of HACEK endocarditis.
Why is Aeromonas usually resistant to ampicillin?
Aeromonas species produce a chromosomal beta-lactamase that inactivates ampicillin, so the organism is usually resistant to this drug. Fluoroquinolones or co-trimoxazole are preferred for serious Aeromonas infection, per current guidance.
How does Plesiomonas shigelloides differ from Aeromonas?
Plesiomonas shigelloides is lactose-negative and produces indole, whereas many Aeromonas strains ferment lactose on MacConkey agar. Both are oxidase positive and motile, so biochemical tests are needed to separate them.
Which organisms make up the HACEK group?
The HACEK group comprises Haemophilus species, Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens and Kingella species. These are normal oral flora organisms that can enter the bloodstream after dental procedures or oral infection and cause endocarditis.
What causes diarrhoea with Plesiomonas shigelloides?
Plesiomonas shigelloides causes watery or dysentery-like diarrhoea after eating raw or undercooked shellfish or drinking contaminated fresh water. The illness is usually self-limiting, although antibiotics may be needed in severe cases per current guidance.