HACEK organisms
- Bacterium
Gram-negative
Thin wall with an outer membrane: stains pink.- Exam priority: high
Five slow-growing mouth bacteria that can settle on heart valves, infect human-bite wounds and, in toddlers, joints and bones.
Also called: HACEK group, Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella
Quick facts
- Type
- Bacterium · Other Gram-negative rods
- Gram / shape
- Gram-negative · Small
- Spread
- Endogenous · Dental disease
- Causes
- Infective endocarditis · Septic arthritis
- Treatment
- Third-generation cephalosporins
At a glance
- Gram
- Gram-negative
- Shape
- Small, often pleomorphic rods and coccobacilli
- Oxygen
- Facultative anaerobes; most grow better with extra CO2
- Family
- Pasteurellaceae (Haemophilus, Aggregatibacter), Cardiobacteriaceae, Neisseriaceae (Eikenella, Kingella)
- Size
- Small; fastidious and slow to grow
Morphology
- Fastidious Gram-negative rods or coccobacilli that need a CO2-enriched atmosphere
- Eikenella corrodens colonies pit (corrode) the agar and smell of bleach
- Kingella kingae gives small beta-haemolytic colonies of short rods in pairs or chains
- Cardiobacterium hominis is pleomorphic and may form rosettes
Transmission
- Endogenous: they live in the mouth and upper airway and enter the blood through gums, teeth or mucosa
- Dental disease and dental procedures can release them into the bloodstream
- Human bites and clenched-fist injuries inoculate Eikenella into the hand
- Kingella spreads from child to child in day care through oropharyngeal carriage
Reservoir: Human mouth, dental plaque and upper respiratory tract
Isolation precautions
What the care team adds to standard precautions.
- Standard
Virulence factors
- Adhere to damaged or prosthetic valves and build large, friable vegetations that embolise
- Kingella kingae makes an RTX cytotoxin that damages joint and bone tissue
- Aggregatibacter actinomycetemcomitans makes a leukotoxin that drives aggressive periodontitis
- Some strains produce beta-lactamase, so ampicillin alone is not reliable
Diseases it causes
- Infective endocarditis, native or prosthetic valve: the most common Gram-negative cause of endocarditis Infective endocarditis →
- Septic arthritis in children under four years (Kingella kingae) Septic Arthritis of the Knee →
- Osteomyelitis and spondylodiscitis in young children (Kingella kingae) Osteomyelitis →
- Human-bite and clenched-fist wound infections (Eikenella corrodens) Mammalian bites (dog, cat, human) →
- Periodontitis (Aggregatibacter actinomycetemcomitans) Periodontitis →
- Brain and other visceral abscesses Brain abscess →
Clinical picture
- Subacute endocarditis with large vegetations and embolic events, often after weeks of fever
- A classic cause of endocarditis with negative blood cultures
- A toddler who limps or refuses to bear weight, with little fever and modest inflammatory markers
Diagnosis
- Several blood cultures before antibiotics; tell the laboratory that HACEK is suspected
- Inoculate joint fluid or bone samples into blood-culture bottles to improve Kingella recovery
- Specific real-time PCR for Kingella on joint fluid or a throat swab
- Broad-range 16S rRNA PCR of valve tissue when cultures stay negative
- Echocardiography to look for vegetations
Treatment
Third-generation cephalosporins (ceftriaxone) or ampicillin-sulbactam are the usual choice, because beta-lactamase producers make ampicillin unreliable; fluoroquinolones are an alternative. Susceptibility differs between species, and Eikenella resists clindamycin.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Good oral hygiene and dental care, especially in people with valve disease
- Antibiotic prophylaxis before dental procedures for the highest-risk cardiac patients
- Early cleaning and medical care of human bites and fist injuries
Test yourself
Think of the answer, then tap the question to see it.
Which group of organisms is the most common Gram-negative cause of infective endocarditis?
The HACEK organisms: Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella and Kingella.
Which HACEK organism is linked to human bites and clenched-fist injuries?
Eikenella corrodens, which pits the agar and is resistant to clindamycin.
How can the laboratory improve recovery of Kingella kingae from joint fluid?
By inoculating the fluid into blood-culture bottles and using specific PCR on joint fluid or a throat swab.
Why is ampicillin alone not a reliable choice for HACEK endocarditis?
Some strains produce beta-lactamase; ceftriaxone or ampicillin-sulbactam is used instead.
Connections
← Back to the group: Other Gram-negative rods
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: HACEK Infections - Infectious Disease - MSD Manual Professional Edition (MSD Manual Professional Edition, 2025); Kingella kingae Osteoarticular Infections Approached through the Prism of the Pediatric Orthopedist (Microorganisms (MDPI), 2021).
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Representative model, for educational purposes only.