Culture-negative infective endocarditis is commonly associated with which group of organisms (HACEK)?

• A. Staphylococcus aureus
• B. Streptococcus viridans
• C. HACEK organisms
• D. Enterococcus faecalis

Answer: (C)

Culture-negative infective endocarditis often results from fastidious organisms that don’t grow well on routine blood cultures. The HACEK group—Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, and Kingella—are classic culprits. These bacteria are part of the normal oropharyngeal flora and are notoriously slow-growing, requiring enriched media and longer incubation to yield growth in the laboratory. Because standard cultures can miss them, infections caused by HACEK organisms frequently appear culture-negative even though an endocarditis infection is present. In practice, recognizing this pattern prompts the use of prolonged incubation, specialized media, and adjunctive tests (serology or molecular methods) to confirm the infection. In contrast, many other common endocarditis pathogens, such as Staphylococcus aureus, Streptococcus viridans, and Enterococcus faecalis, typically produce positive blood cultures with routine testing, making culture negativity less typical for those organisms.