The HACEK organisms are a group of

fastidious A fastidious organism is any organism that has complex or particular nutritional requirements. In other words, a fastidious organism will only grow when specific nutrients are included in its medium. The more restrictive term fastidious microorga ...

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

that are an unusual cause of

infective endocarditis Infective endocarditis is an infection of the inner surface of the heart, usually the valves. Signs and symptoms may include fever, small areas of bleeding into the skin, heart murmur, feeling tired, and low red blood cell count. Complications ...

, 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 ''Haemophilus'' is a genus of Gram-negative, pleomorphic, coccobacilli bacteria belonging to the family Pasteurellaceae. While ''Haemophilus'' bacteria are typically small coccobacilli, they are categorized as pleomorphic bacteria because of t ...

'', ''

Aggregatibacter ''Aggregatibacter'' is a genus in the phylum Pseudomonadota (Bacteria), which contains three species, namely: * '' A. actinomycetemcomitans'' (Klinger 1912) Nørskov-Lauritsen and Kilian 2006, (type species of the genus); (from Greek noun (ἀ� ...

'' (previously '' Actinobacillus''), '' Cardiobacterium'', ''

Eikenella ''Eikenella corrodens'' is a Gram-negative facultative anaerobic bacillus that can cause severe invasive disease in humans. It was first identified by M. Eiken in 1958, who called it ''Bacteroides corrodens''. ''E. corrodens'' is a rare pericar ...

'', ''

Kingella ''Kingella'' is a genus of bacteria of the family Neisseriaceae. It belongs to the HACEK group of fastidious Gram-negative bacteria that tend to cause endocarditis. '' Kingella kingae'' is its type species In zoological nomenclature, a type s ...

''. The HACEK organisms are a normal part of the

human microbiota This article lists some of the species recognized as belonging to the human microbiome. Whole-body distributed *''Acinetobacter calcoaceticus'' *''Burkholderia cepacia'' *''Pseudomonas pseudoalcaligenes'' *''Peptostreptococcus spp'' Natural ...

, living in the

oral The word oral may refer to: Relating to the mouth * Relating to the mouth, the first portion of the alimentary canal that primarily receives food and liquid **Oral administration of medicines ** Oral examination (also known as an oral exam or or ...

- pharyngeal region. The bacteria were originally grouped because they were thought to be a significant cause of infective endocarditis, but recent research has shown that they are rare and only responsible for 1.4–3.0% of all cases of this disease.

Organisms

HACEK originally referred to '' Haemophilus parainfluenzae'', '' Haemophilus aphrophilus'', '' Actinobacillus actinomycetemcomitans'', '' Cardiobacterium hominis'', '' Eikenella corrodens'', and ''

Kingella kingae ''Kingella kingae'' is a species of Gram-negative facultative anaerobic β-hemolytic coccobacilli. First isolated in 1960 by Elizabeth O. King, it was not recognized as a significant cause of infection in young children until the 1990s, when ...

''. However, taxonomic rearrangements have changed the A to ''

'' species and the H to ''

'' 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: *''

''species **'' Haemophilus haemolyticus'' **''

Haemophilus influenzae ''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, non-motile, coccobacillary, facultatively anaerobic, capnophilic pathogenic bacterium of the family Pasteurellaceae. The bacter ...

'': The incidence of endocarditis due to ''H. influenzae'' declined after the introduction of the Hib vaccine. **'' Haemophilus parahaemolyticus'' **'' Haemophilus parainfluenzae'' *''

'' **'' Aggregatibacter actinomycetemcomitans'' (previously '' Actinobacillus actinomycetemcomitans'') **'' Aggregatibacter aphrophilus'' (previously '' Haemophilus aphrophilus'') **'' Aggregatibacter paraphrophilus'' (previously '' Haemophilus aphrophilus'') **'' Aggregatibacter segnis'' *'' Cardiobacterium'' **'' Cardiobacterium hominis'': This is the most common species in the genus ''Cardiobacterium''. **'' Cardiobacterium valvarum'' *''

'' **'' Eikenella corrodens'' *''

'' **''

Kingella denitrificans ''Kingella'' is a genus of bacteria of the family Neisseriaceae. It belongs to the HACEK group of fastidious Gram-negative bacteria that tend to cause endocarditis. ''Kingella kingae'' is its type species. Species As of 2021, five species be ...

'' **''

'': This is the most common species in the genus ''Kingella''.

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 Endocarditis is an inflammation of the inner layer of the heart, the endocardium. It usually involves the heart valves. Other structures that may be involved include the interventricular septum, the chordae tendineae, the mural endocardium, or the ...

involving native valves and are the most common Gram-negative cause of endocarditis among people who do not use drugs intravenously. They have been a frequent cause of culture-negative endocarditis. Culture-negative refers to an inability to produce a colony on regular agar plates because these bacteria are fastidious (require a specific nutrient). In addition to valvular infections in the heart, they can also produce other infections, such as bacteremia, abscess,

peritonitis Peritonitis is inflammation of the localized or generalized peritoneum, the lining of the inner wall of the abdomen and cover of the abdominal organs. Symptoms may include severe pain, swelling of the abdomen, fever, or weight loss. One part o ...

otitis media Otitis media is a group of inflammatory diseases of the middle ear. One of the two main types is acute otitis media (AOM), an infection of rapid onset that usually presents with ear pain. In young children this may result in pulling at the ear, ...

conjunctivitis Conjunctivitis, also known as pink eye, is inflammation of the outermost layer of the white part of the eye and the inner surface of the eyelid. It makes the eye appear pink or reddish. Pain, burning, scratchiness, or itchiness may occur. The ...

pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severi ...

, arthritis,

osteomyelitis Osteomyelitis (OM) is an infection of bone. Symptoms may include pain in a specific bone with overlying redness, fever, and weakness. The long bones of the arms and legs are most commonly involved in children e.g. the femur and humerus, while the ...

, and periodontal infections.

Treatment

The treatment of choice for HACEK organisms in endocarditis is the third-generation cephalosporin and

β-Lactam A beta-lactam (β-lactam) ring is a four-membered lactam. A ''lactam'' is a cyclic amide, and ''beta''-lactams are named so because the nitrogen atom is attached to the β-carbon atom relative to the carbonyl. The simplest β-lactam possible is ...

antibiotic

ceftriaxone Ceftriaxone, sold under the brand name Rocephin, is a third-generation cephalosporin antibiotic used for the treatment of a number of bacterial infections. These include middle ear infections, endocarditis, meningitis, pneumonia, bone and joint ...

Ampicillin Ampicillin is an antibiotic used to prevent and treat a number of bacterial infections, such as respiratory tract infections, urinary tract infections, meningitis, salmonellosis, and endocarditis. It may also be used to prevent group B stre ...

(a penicillin), combined with low-dose gentamicin (an

aminoglycoside Aminoglycoside is a medicinal and bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside (sugar). The term can also refer ...

) is another therapeutic option.
eMedicine, HACEK organism infection. June 2005.

References

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