HACEK organisms

The HACEK organisms are a group of fastidious Gram-negative bacteria that are an unusual cause of infective endocarditis (IE), which is an inflammation of the heart due to bacterial infection. HACEK is an abbreviation of the initials of the genera of this group of bacteria: Haemophilus, Aggregatibacter (previously Actinobacillus), Cardiobacterium, Eikenella, Kingella. The HACEK organisms are a normal part of the human microbiota, living in the oral-pharyngeal region.

The bacteria were originally grouped because they were thought to be a significant cause of infective endocarditis, but recent literature has shown that they are rare and only responsible for 1.4–3% of all cases of this disease.

Organisms

HACEK originally referred to Haemophilus parainfluenzae, Haemophilus aphrophilus, Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae. However, taxonomic rearrangements have changed the A to Aggregatibacter species and the H to Haemophilus species to reflect the recategorization and novel identification of many of the species in these genera. Some reviews of medical literature on HACEK organisms use the older classification, but recent papers are using the new classification.

A list of HACEK organisms:

• Haemophilus species
  • Haemophilus parainfluenzae
  • Haemophilus influenzae. The incidence of endocarditis due to H. influenzae declined after the introduction of the Hib vaccine.
  • Haemophilus haemolyticus
  • Haemophilus parahaemolyticus
• Aggregatibacter
  • Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans)
  • Aggregatibacter segnis
  • Aggregatibacter aphrophilus (previously Haemophilus aphrophilus)
  • Aggregatibacter paraphrophilus (previously Haemophilus aphrophilus)
• Cardiobacterium
  • Cardiobacterium hominis. Most common species in the Cardiobacterium genus.
  • Cardiobacterium valvarum
• Eikenella
  • Eikenella corrodens
• Kingella
  • Kingella kingae. Most common species in the Kingella genus.
  • Kingella denitrificans

Presentation

All of these organisms are part of the normal oropharyngeal flora, which grow slowly (up to 14 days), prefer a carbon dioxide–enriched atmosphere, and share an enhanced capacity to produce endocardial infections, especially in young children. Collectively they account for 5–10% of cases of infective endocarditis involving native valves and are the most common gram-negative cause of endocarditis among people who do not use IV drugs. They have been a frequent cause of culture-negative endocarditis. Culture negative refers to an inability to produce a colony on regular agar plates; this is because these bacteria are fastidious (requiring a specific nutrient).

In addition to valvular infections in the heart, they can also produce other infections such as bacteremia, abscess, peritonitis, otitis media, conjunctivitis, pneumonia, arthritis, osteomyelitis, and periodontal infections.

Treatment

The treatment of choice for HACEK organisms in endocarditis is the third generation cephalosporin and β-Lactam antibiotic ceftriaxone. Ampicillin (a penicillin), combined with low-dose gentamicin (an aminoglycoside) is another therapeutic option.