HACEK organisms is a group of fastidious, slow growing, pleomorphic Gram negative bacilli which form part of the oropharynx commensals and require enhanced carbon dioxide tension for recovery in culture.

HACEK refers to the initials of organisms including:

• Haemophilus influenzae, Haemophilus parainfluenzae, Aggregatibacter aphrophilus (formerly Haemophilus aphrophilus and Haemophilus paraphrophilus)^[1]

• Aggregatibacter actinomycetemcomitans (formerly Actinobacillus actinomycetemcomitans)^[1]

• Cardiobacterium hominis, Cardiobacterium valvarum

• Eikenella corrodens

• Kingella kingae, Kingella denitrificans

The pathogenesis of HACEK endocarditis is presumed to be related to colonization of the oropharynx with bacteria that gain access to the vascular space following either trauma or local infection. In addition to valvular infections in the heart, these organisms can also produce other infections such as bacteremia, abscesses, peritonitis, otitis media, conjunctivitis, pneumonia, peritonitis, arthritis and osteomyelitis, and periodontal infections.

HACEK organisms account for about 3% of cases of endocarditis,^[2] with an insidious onset and subacute course. They are the most common Gram negative casue of endocarditis among patients who do not use intravenous drugs. Most patients with HACEK endocarditis have preexisting cardiac defects or prior dental manipulations. The duration of symptoms prior to diagnosis is reported to be longer than 2 weeks but shorter than 6 months.^[3]

Given the fastidious growth requirements, HACEK bacteria are a recognized cause of culture-negative endocarditis. The organisms can be identified after subculture on 5 to 8% sheep blood and chocolate agar at 35 to 37°C for 48 to 72 hours in an aerobic atmosphere containing 5 to 10% CO₂.^[4] Although the mean duration for incubation of blood cultures until detection of growth is 3 to 5 days, up to 30 days may be required. Biochemical properties are used to differentiate members of the HACEK group. However, biopatterns may be varied by inoculum volume and growth conditions, which prompts the utilization of molecular techniques such as 16S rRNA sequencing.^[5][6][7]

The treatment of endocarditis caused by HACEK organisms should be based on antimicrobial susceptibility tests, including tests for β-lactamase ctivity. Since many of these organisms are slow growing, such tests can be problematic. Although there are limited published clinical data demonstrating the efficacy of ceftriaxone or ampicillin–sulbactam therapy, these drugs should be considered the regimens of choice for the treatment of patients with HACEK endocarditis.^[8] Fluoroquinolones should be considered as an alternative agent for patients unable to tolerate β-lactam therapy. The American Heart Association recommends that native-valve and prosthetic-valve endocarditis be treated for 4 weeks and 6 weeks, respectively.^[9] HACEK endocarditis is associated with a favorable prognosis, with a cure rate of 82 to 87% of patients with medical treatment alone or associated with surgery.^[10]

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