In infective endocarditis, the HACEK group of organisms (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella) have a characteristic feature that distinguishes them from Streptococcus viridans in terms of diagnosis. This feature is:
- A They produce large friable vegetations on the tricuspid valve
- B They are fastidious, slow-growing organisms that may require prolonged incubation (5-7 days), explaining culture-negative endocarditis on standard protocols ✓
- C They exclusively cause right-sided endocarditis in IV drug users
- D They are always associated with dental procedures
Explanation
HACEK organisms are part of the normal oropharyngeal flora and are a major cause of culture-negative endocarditis when standard 5-day incubation protocols are used, because they are slow-growing and fastidious. Extended incubation (14 days) or serological testing is required for detection. They cause subacute endocarditis on previously damaged or prosthetic valves. Modern automated blood culture systems (BACTEC) with continuous monitoring detect most HACEK organisms within the standard 5-day window, but clinicians must specify extended incubation if HACEK is suspected.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.