HACEK organisms are a group of fastidious, slow-growing, gram-negative bacteria the growth of which requires an atmosphere of carbon dioxide. Species belonging to this group include several Haemophilus species, Aggregatibacter (formerly Actinobacillus) actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae. HACEK bacteria normally reside in the oral cavity and have been associated with local infections in the mouth. They are also known to cause severe systemic infections—most often bacterial endocarditis, which can develop on either native or prosthetic valves (Chap. 124).
In large series, up to 3% of cases of infective endocarditis are attributable to HACEK organisms, most often A. actinomycetemcomitans, Haemophilus species, and C. hominis. Invasive infection typically occurs in patients with a history of cardiac valvular disease, often in the setting of a recent dental procedure, nasopharyngeal infection, or tongue piercing or scraping. The aortic and mitral valves are most commonly affected. The clinical course of HACEK endocarditis tends to be subacute; however, embolization is common. The overall prevalence of major emboli associated with HACEK endocarditis ranges from 28% to 71% in different series. On echocardiography, valvular vegetations are seen in up to 85% of patients. The vegetations are frequently large, although vegetation size has not been directly correlated with the risk of embolization. Cultures of blood from patients with suspected HACEK endocarditis may require up to 30 days to become positive, and the microbiology laboratory should be alerted when a HACEK organism is being considered. However, most cultures that ultimately yield a HACEK organism become positive within the first week, especially with improved culture systems such as BACTEC. In addition, polymerase chain reaction techniques (e.g., of cardiac valves) are facilitating the diagnosis of HACEK infections. Because of the organisms' slow growth, antimicrobial testing may be difficult, and β-lactamase production may not be detected. E-test methodology may increase the accuracy of susceptibility testing.
Haemophilus species are differentiated by their in vitro growth requirements for X factor (hemin) and V factor (nicotinamide adenine dinucleotide). H. aphrophilus requires only X factor for growth, while species designated para- require only V factor. H. aphrophilus and H. parainfluenzae are the Haemophilus species most commonly isolated from cases of HACEK endocarditis; H. paraphrophilus is less common. Of patients with HACEK endocarditis due to Haemophilus species, 60% have been ill for <2 months before presentation, and 19–50% develop congestive heart failure. Mortality rates as high as 30–50% were reported in older series; however, more recent studies have documented mortality rates of <5%. H. aphrophilus also causes invasive bone and joint infections, and H. parainfluenzae has been isolated from other infections, such as meningitis; brain, dental, and liver abscess; pneumonia; and septicemia.
A. actinomycetemcomitans can be isolated from soft tissue infections and abscesses in association with Actinomyces israelii. Typically, patients who develop endocarditis with A. actinomycetemcomitans have ...