[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Other Gram-negative rods](https://durgical.com/en/microbiology/groups/other-gram-negative-rods/) › HACEK organisms

# 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.

Updated Nov 13, 2026

Also called: HACEK group, Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella

## Quick facts

**Type**
Bacterium · [Other Gram-negative rods](https://durgical.com/en/microbiology/groups/other-gram-negative-rods/)

**Gram / shape**
Gram-negative · Small

**Spread**
Endogenous · Dental disease

**Causes**
Infective endocarditis · Septic arthritis

**Treatment**
Third-generation cephalosporins

On this page

- The basics Key points
- At a glance
- Morphology
- Transmission
- In Colombia
- Memory aid
- What the colours mean

- Clinical Virulence factors
- Diseases it causes
- Clinical picture
- Diagnosis

- Treatment Treatment
- Prevention

- Study Test yourself

- Connections Connections

## Key points

- HACEK = Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella: oral flora that cause endocarditis.
- The most common Gram-negative cause of infective endocarditis and a classic culture-negative cause.
- Human bite or fist-fight wound: Eikenella corrodens (corrodes the agar, resists clindamycin).
- Child under four with septic arthritis and mild fever: Kingella kingae; ask for PCR.
- Usual treatment: ceftriaxone or ampicillin-sulbactam.

## 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 →](https://durgical.com/en/conditions/infective-endocarditis/)
- Septic arthritis in children under four years (Kingella kingae) [Septic Arthritis of the Knee →](https://durgical.com/en/conditions/septic-arthritis-of-the-knee/)
- Osteomyelitis and spondylodiscitis in young children (Kingella kingae) [Osteomyelitis →](https://durgical.com/en/conditions/osteomyelitis/)
- Human-bite and clenched-fist wound infections (Eikenella corrodens) [Mammalian bites (dog, cat, human) →](https://durgical.com/en/conditions/mammalian-bites/)
- Periodontitis (Aggregatibacter actinomycetemcomitans) [Periodontitis →](https://durgical.com/en/conditions/periodontitis/)
- Brain and other visceral abscesses [Brain abscess →](https://durgical.com/en/conditions/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:

- [Ceftriaxone](https://durgical.com/en/pharmacology/ceftriaxone/)
- [Ampicillin + sulbactam](https://durgical.com/en/pharmacology/ampicillin-sulbactam/)
- [Ciprofloxacin](https://durgical.com/en/pharmacology/ciprofloxacin/)

## 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

## In Colombia

HACEK infections are not an individual notifiable event in Colombia; they appear in hospital series of infective endocarditis and childhood osteoarticular infection.

## Memory aid

Picture five shy passengers from the mouth who ride the blood to the heart valves: they hide from routine cultures, and ceftriaxone brings all five home.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/hacek-organisms/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

## Connections

## Related 4

- [Haemophilus influenzae](https://durgical.com/en/microbiology/haemophilus-influenzae/)
- [Viridans streptococci](https://durgical.com/en/microbiology/viridans-streptococci/)
- [Capnocytophaga canimorsus](https://durgical.com/en/microbiology/capnocytophaga-canimorsus/)
- [Actinomyces israelii](https://durgical.com/en/microbiology/actinomyces-israelii/)

## Related conditions 6

- [Infective endocarditis](https://durgical.com/en/conditions/infective-endocarditis/)
- [Septic Arthritis of the Knee](https://durgical.com/en/conditions/septic-arthritis-of-the-knee/)
- [Osteomyelitis](https://durgical.com/en/conditions/osteomyelitis/)
- [Mammalian bites (dog, cat, human)](https://durgical.com/en/conditions/mammalian-bites/)
- [Periodontitis](https://durgical.com/en/conditions/periodontitis/)
- [Brain abscess](https://durgical.com/en/conditions/brain-abscess/)

[← Back to the group: Other Gram-negative rods](https://durgical.com/en/microbiology/groups/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](https://www.msdmanuals.com/professional/infectious-diseases/gram-negative-bacilli/hacek-infections) (MSD Manual Professional Edition, 2025); [Kingella kingae Osteoarticular Infections Approached through the Prism of the Pediatric Orthopedist](https://pmc.ncbi.nlm.nih.gov/articles/PMC8778174/) (Microorganisms (MDPI), 2021).

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Representative model, for educational purposes only.

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