Which organism is NOT classified as part of the HACEK group?

• A. Streptococcus pneumoniae
• B. Haemophilus species
• C. Aggregatibacter actinomycetemcomitans
• D. Eikenella corrodens

Answer: (C)

HACEK organisms are a small, distinct group of fastidious bacteria historically linked to subacute endocarditis. The group is defined by the genera Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, and Kingella. They are typically part of the oral flora and tend to grow slowly on routine culture media, requiring special conditions to grow.

In this list, Haemophilus species and Eikenella corrodens clearly belong to the HACEK group, and Aggregatibacter actinomycetemcomitans is also a member (historically known as Actinobacillus actinomycetemcomitans). The organism that does not fit into this group is Streptococcus pneumoniae, a Gram-positive diplococcus not classified among HACEK organisms.