HACEK Group: Understanding Fastidious Bacteria and Their Role in Infective Endocarditis

Contents

Introduction
General Characteristics of the HACEK Group
Members of the HACEK Group
Diagnosis
Conclusion

◉ Introduction

The HACEK group is a collection of Gram-negative bacteria often associated with systemic infections, particularly infective endocarditis. The acronym HACEK stands for the following genera: Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, and Kingella. Although rare, these bacteria account for approximately 1-3% of infective endocarditis cases, a serious condition requiring prompt and appropriate management.

◉ General Characteristics of the HACEK Group

HACEK group bacteria share several common features:

Gram-negative: They possess a thin cell wall and an outer membrane rich in lipopolysaccharides.
Fastidious: They grow slowly in culture and require specific conditions (e.g., CO₂-enriched media).
Oropharyngeal commensals: These bacteria naturally colonize the oral cavity and pharynx but can become pathogenic under certain conditions.

◉ Members of the HACEK Group

Haemophilus:

Main species: Haemophilus parainfluenzae, Haemophilus aphrophilus.
Clinical role: Associated with endocarditis, particularly in patients with prosthetic heart valves or underlying heart conditions.

Aggregatibacter:

Main species: Aggregatibacter actinomycetemcomitans.
Clinical role: Known to cause invasive infections, including endocarditis and brain abscesses.

Cardiobacterium:

Main species: Cardiobacterium hominis.
Clinical role: Rare but often linked to subacute endocarditis.

Eikenella:

Main species: Eikenella corrodens.
Clinical role: Frequently isolated in polymicrobial infections, particularly after human bites or oral trauma.

Kingella:

Main species: Kingella kingae.
Clinical role: More common in children, associated with osteoarticular infections and, rarely, endocarditis.

◉ Diagnosis

Diagnosis of HACEK infections relies on:

1. Blood Cultures: HACEK bacteria are fastidious and may require specific culture conditions for growth. For example:

Enriched media: Blood cultures should be incubated in nutrient-rich media (e.g., trypticase soy broth) with CO₂ (5-10%).
Extended incubation: Maintain cultures for at least 7-21 days, as some species (e.g., Cardiobacterium hominis) grow particularly slowly.
Special requirements: Certain species like Haemophilus need specific growth factors (X factor [hemin] and V factor [NAD]).

2. PCR (Polymerase Chain Reaction): A rapid and sensitive method to detect bacterial DNA, especially when blood cultures are negative or delayed.

3. Echocardiography: To assess valve damage and confirm endocarditis diagnosis.

◉ Conclusion

Though rare, the HACEK group represents a significant cause of infective endocarditis, particularly in at-risk patients (prosthetic valves, heart disease). Rapid management combining precise microbiological diagnosis (adapted blood cultures, targeted PCR) and appropriate antibiotic therapy is essential to improve patient outcomes. Future research should focus on pathogenic mechanisms and optimizing therapeutic strategies.