| Haemophilus aphrophilus, Haemophilus haemolyticus, Haemophilus parahaemolyticus, Haemophilus parainfluenzae, Haemophilus paraphrophilus, Haemophilus segnis | Brain abscess, endocarditis, endophthalmitis, epiglottitis, hepatic abscess, intra-abdominal infection, meningitis, neonatal sepsis, necrotizing fasciitis, otitis media, pneumonia, sinusitis, septic arthritis, urinary tract infection |
| Aggregatibacter actinomycetemcomitans | Brain abscess, cellulitis, empyema, endocarditis, endophthalmitis, osteomyelitis, periodontal infection, parotitis, pericarditis, pneumonia, synovitis, thyroid abscess, urinary tract infection |
| Cardiobacterium hominis | Endocarditis, meningitis |
| Eikenella corrodens | Abscessed tooth, Bartholin’s gland abscess, brain abscess, cellulitis, conjunctivitis, dacryocystitis, empyema, endocarditis, endometritis, gingivitis, intra-abdominal abscess, intravascular space infections, keratitis, liver abscess, mediastinitis, meningitis, mycotic aneurysm, otitis externa, parotitis, pericarditis, pneumonia, septic pulmonary emboli, subdural empyema, thyroid abscess, thyroiditis |
| Kingella dentrificens, Kingella indologenes, Kingella kingae | Abscess, endocarditis, epiglottitis, intervertebral diskitis, meningitis, oropharyngeal infections, osteomyelitis, septic arthritis |
Members of the HACEK group are normal indigenous flora of the oral cavity. Hematogenous seeding of the bloodstream may occur after dental manipulation, but is more likely to occur in the setting of ordinary daily activities, particularly among individuals with periodontal disease. Transient or sustained bacteremia puts individuals with underlying valvular heart disease at risk for developing infective endocarditis (IE). Antibiotic prophylaxis (AP) prior to dental manipulation has not been shown to protect against IE, therefore the guidelines for AP have been modified to include only individuals in the highest risk group. While millions of patients undergo dental procedures annually, cases of IE caused by HACEK group organisms are rare.
Diagnosis
Bacteria in the HACEK group are commonly but often erroneously considered in the differential diagnosis for culture-negative endocarditis. In the past the traditional method of increasing the recovery of HACEK bacteria was to extend the incubation of blood culture bottles from 5 to 7 days to 2 to 3 weeks. However, with improvements in blood culture methods, this practice is no longer recommended. A recent multicenter study showed that the mean and median times to detection of HACEK isolates using current laboratory methods and media were 3 and 3.4 days, respectively. In addition, none of the cultures that were held for prolonged incubation and terminal subculturing yielded additional growth. Several other studies have demonstrated that isolation of HACEK organisms usually occurs within 5 days, suggesting that prolonged incubation is no longer needed to detect HACEK bacteria.
HACEK organisms typically grow on 5% sheep blood and chocolate agar but not on MacConkey’s agar. Because growth is often poor or absent in an unenhanced atmosphere, incubation in 5% to 10% carbon dioxide (CO2) is recommended. After growth is observed, standard biochemical tests will identify individual HACEK species. The use of 16S ribosomal RNA (rRNA) gene analysis may be useful for HACEK organisms that are not readily identified with standard biochemical tests.
Clinical features
Endocarditis caused by members of the HACEK group typically occurs in individuals with pre-existing valvular abnormalities and/or prosthetic valves. The results of a recent large, multicenter cohort showed that individuals with HACEK IE tend to be younger than those with non-HACEK IE with a median age of 47 years vs. 60 years, respectively. According to the afore-mentioned study, the most common members of the HACEK group identified were Haemophilus spp. (40%) followed by Aggregatibacter spp. (34%), Cardiobacterium spp. (14%), Eikenella corrodens (5%), and Kingella spp. (5%).
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