HACEK Organisms - A Mnemonic Nightmare
The HACEK groups of organisms are more than just a nightmare question on rounds. They play a significant role in the pathophysiology of culture negative infective endocarditis. Understanding the members of this group and their characteristic features will help you answer frequently asked questions in your exams and also keep a look out for them in your clinical practice.
So here is all you need to know about the HACEK group of organisms.
Name the members of the HACEK group.
Haemophillus sp
Aggregatibacter sp
Cardiobacterium hominis
Eikenella corrodens
Kingella sp
Why are they grouped together?
There is no taxonomical or microbiological basis for grouping these organisms together. The only reason that we see these organisms grouped together is because they are all organisms that have a propensity to cause culture negative infective endocarditis.
What are their common characteristics?
They are all fastidious gram negative organisms.
Just to review - a fastidious organism is any organism that has specific nutritional requirements to grow.
Where are they commonly found?
Every member of the HACEK group of organisms is actually a commensal organism. Most of the organisms are found in the oropharynx.
Gastrointestinal tract and genitourinary tracts also contain some HACEK organisms. (Eikenella sp. and Cardiobacterium sp.)
Apart from Infective Endocarditis, what else can the HACEK group cause?
wound infections
soft tissue abscesses
brain abscess
endophthalmitis
parotitis
empyema
osteomyelitis
Why are we so worried about them?
There are, of course, other organisms that cause endocarditis. Despite that, why are we worried about these organisms so much?
The HACEK group of organisms are slow growing and insidious and more often lead to subacute infective endocarditis.
Because they are slow growing, they don’t cause signs and symptoms that are severe. This leads to a delay in diagnosis. By the time a diagnosis is made, the vegetations formed by these HACEK groups of organims are large. The vegetations of the HACEK group, owing to their size, are at the much bigger risk of embolisation and disemination.
Prosthetic valves are especially at risk of being damaged by the HACEK group.
Specific Characteristics
Haemophillus Species
Named after - love for blood
two co factors are required for the growth of Haemophillus
Factor V - nicotinamide adenine dinucleotide
Factor X - hemin
Both of these factors are found in RBCs - hence the name Haemophillus.
There are three species in the Haemophillus genus that are pathogenic
H. aphophilus - most commonly causes infective endocarditis
H. parainfluenzae - causes infective endocarditis
H. influenzae - pneumonia and meningitis.
H. parainfluenzae vs H. influenzae - Whats the difference?
H. parainfluenzae requires factor V (NAD) but not factor X (hemin)
H. influenzae requires both.
Aggregatibacter actinomycetescomitans
Named after - propensity to form aggregates when grown in trypticase soy broth
This organism is notorious for causing gingival disease - especially early onset periodontitis. (EOP)
It produces a leucotoxin that can invade gingival cells
A little history
the genus was initially called Actinobacillus - due to internal star shapes formed by colonies on solid media
genus was changed to Aggregatibacter in 2006 - propensity to form clumps in broth media
Cardiobacterium hominis
Named after - the fact that this organism causes nothing other than infective endocarditis. Its literally the bacteria of the heart.
It is seen to cause infective endocarditis especially after intestinal procedures because it is a commensal of the large bowel.
Eikenella corrodens
Named after - its corrosive nature on agars resulting in pit formation. Named after microbiologist M Eiken.
It has a chlorine / bleach odour.
It is commonly a cause of cellulitis secondary to human bites, osteomyelitis and empyema.
In IV drug abusers it can lead to infective endocarditis.
This is believed to be because the drug abusers lick the needle before use for “good luck”
Kingella kingae
Named after - microbiologist Dr. Elizabeth King
It is rare - only 20 cases of Kingella infective endocarditis have been reported.
But Kingella infective endocarditis is fulminant and rapidly progressive
Species
Kingella genus has only 3 species - K. kingae, K dentrificans, K indologenes
only K kingae causes IE
Management
When should you suspect?
History of dental, urological or gastrointestinal procedures
Current periodontal disease
History of bite -human or canine (rarely)
History of IV drug abuse
Cardiac illness - congenital heart disease or valves
Diagnosis
Initially HACEK group of organisms was responsible for the so called “culture negative endocarditis”. As a result, longer incubation times of more than 5 days were recommended to allow the cultures to grow. This practice has now been shown to be ineffective in increasing the culture yield.
Current practices allow us to identify HACEK organisms reliably within 5 days, especially with the BacT / Alert systems.
HACEK organisms grow well on Chocolate agar compared to blood agar but do not grow well on media designed for enteric pathogens like MacConkey Agar.
Best way to grow them is by subculture in 5% sheep blood and chocolate agar under 5-10% CO2
The most recent advance in identifying causative organisms is MALDI TOF - matrix assisted laser desorption ionization-time of flight mass spectrometry
Treatment
Drug of choice - Ceftrioxone IV 2 gm
4 weeks if native valve disease
6 weeks if prosthetic valve disease
Ampicillin can also be used if the organism is shown to the sensitive to it on culture. In the past, a combination of Ampicillin and Gentamicin was used but this has been stopped due to the nephrotoxic nature of Gentamicin.
Prophylaxis
Prophylactic dose of antibiotics were given as a standard of care practice before dental procedures to prevent seeding of commensal flora into the blood stream. In 2007, updated guidelines stopped the use of antibiotics because no benefit was found.
In certain high risk cases however, prophylactic antibiotics are still recommended. They are
Presence of prosthetic valves
Prior history of IE
Uncorrected Congenital Heart Disease
Preexisting prosthetic heart valve disorders
Drug of choice in these cases is Amoxicillin or Ampicillin 2gm iv or oral as a single dose, 30-60 minutes before the procedure.
In case of penicillin allergy, Clindamycin 600 mg can be given as well.
Author: Narendran Sairam (Facebook)
Sources and citations
1.“Infective Endocarditis” Harrison's Principles of Internal Medicine, by J. Larry Jameson et al., 21th ed., vol. 1, McGraw-Hill Education, 2022.
2.Chambers, Stephen T., et al. “Hacek Infective Endocarditis: Characteristics and Outcomes from a Large, Multi-National Cohort.” PLoS ONE, vol. 8, no. 5, 2013,
3.Khan, Zartash Zafar. “Hacek Group Infections.” Background, Pathophysiology, Epidemiology, Medscape, 16 Oct. 2021, https://emedicine.medscape.com/article/218158-overview.
4.Smith, Kenneth. “Who Are the Hacek Organisms?” ASM.org, American Society of Microbiology, 11 Feb. 2019, https://asm.org/Articles/2019/February/Who-are-the-HACEK-organisms.
5.Khaledi, M., Sameni, F., Afkhami, H. et al. Infective endocarditis by HACEK: a review. J Cardiothorac Surg 17, 185 (2022). https://doi.org/10.1186/s13019-022-01932-5