Which organisms are commonly associated with subacute endocarditis and belong to the HACEK group?

• A. Staphylococcus aureus
• B. Streptococcus mutans
• C. Haemophilus, Aggregatibacter, Cardiobacter, Eikenella, Kingella
• D. Pseudomonas aeruginosa

Answer: (C)

Subacute endocarditis is typically caused by organisms that grow slowly and often originate from the oropharyngeal flora. The HACEK group—Haemophilus, Aggregatibacter, Cardiobacter, Eikenella, and Kingella—are classic culprits in this setting. They are fastidious, small gram‑negative bacteria that require enriched media and longer incubation, which is why endocarditis from these organisms can present with a culture-negative result on standard tests. Among the options, this is the one that specifically identifies the organisms associated with subacute endocarditis and classified as HACEK. While other organisms can cause endocarditis, Staphylococcus aureus tends to cause acute, rapidly progressive disease; Pseudomonas aeruginosa is a non-HACEK organism often linked to different risk factors; Streptococcus mutans is a viridans streptococcus related to subacute endocarditis but not part of the HACEK group.