HACEK organisms

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The HACEK organisms are a group of fastidious Gram-negative bacteria that are an unusual cause of infective endocarditis, which is an inflammation of the heart due to bacterial infection. [1] HACEK is an abbreviation of the initials of the genera of this group of bacteria: Haemophilus , Aggregatibacter (previously Actinobacillus ), Cardiobacterium , Eikenella , Kingella . [1] The HACEK organisms are a normal part of the human microbiota, living in the oral-pharyngeal region. [2]

A fastidious organism is any organism that has complex or particular nutritional requirements. In other words, a fastidious organism will only grow when specific nutrients are included in its diet. The more restrictive term fastidious microorganism is used in microbiology to describe microorganisms that will grow only if special nutrients are present in their culture medium. Thus fastidiousness is often practically defined as being difficult to culture, by any method yet tried. An example of a fastidious bacterium is Neisseria gonorrhoeae, which requires blood or hemoglobin and several amino acids and vitamins to grow. Other examples include Campylobacter spp. and Helicobacter spp., which are capnophilic – require elevated CO2 – among other requirements. Fastidious organisms are not inherently "weak"—they can flourish and thrive in their particular ecological niche with its particular nutrients, temperature, and absence of competitors, and they can be quite difficult to kill off. But they are difficult to culture simply because it is difficult to accurately simulate their natural milieu in a culture medium. For example, Treponema pallidum is not easy to culture, and yet it is strong on its home turf, being rather difficult to eradicate 100% from all tissues of a person with syphilis.

Bacteria A domain of prokaryotes – single celled organisms without a nucleus

Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of the earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the bacterial phyla have species that can be grown in the laboratory . The study of bacteria is known as bacteriology, a branch of microbiology.

Infective endocarditis endocarditis that is characterized by inflammation of the endocardium caused by infectious agents.

Infective endocarditis is an infection of the inner surface of the heart, usually the valves. Symptoms may include fever, small areas of bleeding into the skin, heart murmur, feeling tired, and low red blood cell count. Complications may include valvular insufficiency, heart failure, stroke, and kidney failure.


The bacteria were originally grouped because they were thought to be a significant cause of infective endocarditis, but recent research has shown that they are rare and only responsible for 1.4–3.0% of all cases of this disease. [1]


HACEK originally referred to Haemophilus parainfluenzae , Haemophilus aphrophilus , Actinobacillus actinomycetemcomitans , Cardiobacterium hominis , Eikenella corrodens , and Kingella kingae . However, taxonomic rearrangements have changed the A to Aggregatibacter species and the H to Haemophilus species to reflect the recategorization and novel identification of many of the species in these genera. [1] Some reviews of medical literature on HACEK organisms use the older classification, [3] but recent papers are using the new classification. [4] [5] [6]

Haemophilus parainfluenzae is a species of Haemophilus.

Haemophilus aphrophilus was a species of Haemophilus, but the name has recently been changed to Aggregatibacter aphrophilus.

<i>Cardiobacterium hominis</i> species of Gammaproteobacteria

Cardiobacterium hominis is a Gram-negative bacillus (rod-shaped) bacterium commonly grouped with other bacteria into the HACEK group. It is one of several bacteria that is normally present in the mouth and upper part of the respiratory tract such as nose and throat. However, it may also rarely cause endocarditis, an infection of the heart valves.

A list of HACEK organisms:

Haemophilus haemolyticus is a species of gram-negative bacteria that is related to Haemophilus influenzae. H. haemolyticus is generally nonpathogenic, however there have been two cases of H.haemolyticus causing endocarditis

<i>Haemophilus influenzae</i> Species of bacterium

Haemophilus influenzae is a Gram-negative, coccobacillary, facultatively anaerobic pathogenic bacterium belonging to the Pasteurellaceae family. H. influenzae was first described in 1892 by Richard Pfeiffer during an influenza pandemic.

The Haemophilus influenzae type B vaccine, often called Hib vaccine, is a vaccine used to prevent Haemophilus influenzae type b (Hib) infection. In countries that include it as a routine vaccine, rates of severe Hib infections have decreased more than 90%. It has therefore resulted in a decrease in the rate of meningitis, pneumonia, and epiglottitis.

Aggregatibacter is a genus in the phylum Proteobacteria (Bacteria), which contains three species, namely:

Aggregatibacter actinomycetemcomitans is a Gram-negative, facultative anaerobe, nonmotile bacterium that is often found in association with localized aggressive periodontitis, a severe infection of the periodontium. It is also suspected to be involved in chronic periodontitis. Less frequently, A. actinomycetemcomitans is associated with nonoral infections such as endocarditis. Its role in aggressive periodontitis was first discovered by Danish-born periodontist Jørgen Slots, a professor of dentistry and microbiology at the University of Southern California School of Dentistry.


All of these organisms are part of the normal oropharyngeal flora, which grow slowly (up to 14 days), prefer a carbon dioxide–enriched atmosphere, and share an enhanced capacity to produce endocardial infections, especially in young children. Collectively, they account for 5–10% of cases of infective endocarditis involving native valves and are the most common Gram-negative cause of endocarditis among people who do not use drugs intravenously. They have been a frequent cause of culture-negative endocarditis. Culture-negative refers to an inability to produce a colony on regular agar plates because these bacteria are fastidious (require a specific nutrient).

In addition to valvular infections in the heart, they can also produce other infections, such as bacteremia, abscess, peritonitis, otitis media, conjunctivitis, pneumonia, arthritis, osteomyelitis, and periodontal infections.


The treatment of choice for HACEK organisms in endocarditis is the third-generation cephalosporin and β-Lactam antibiotic ceftriaxone. Ampicillin (a penicillin), combined with low-dose gentamicin (an aminoglycoside) is another therapeutic option. [7]

Related Research Articles

Gram-negative bacteria group of bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation

Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the gram-staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall sandwiched between an inner cytoplasmic cell membrane and a bacterial outer membrane.

<i>Actinomyces</i> genus of bacteria

Actinomyces is a genus of the Actinobacteria class of bacteria. They are all gram-positive. Actinomyces species are facultatively anaerobic, and they grow best under anaerobic conditions. Actinomyces species may form endospores, and, while individual bacteria are rod-shaped, Actinomyces colonies form fungus-like branched networks of hyphae. The aspect of these colonies initially led to the incorrect assumption that the organism was a fungus and to the name Actinomyces, "ray fungus".

Pasteurellaceae family of bacteria

The Pasteurellaceae comprise a large family of Gram-negative bacteria. Most members live as commensals on mucosal surfaces of birds and mammals, especially in the upper respiratory tract. Pasteurellaceae are typically rod-shaped, and are a notable group of facultative anaerobes. Their biochemical characteristics can be distinguished from the related Enterobacteriaceae by the presence of oxidase, and from most other similar bacteria by the absence of flagella.


A coccobacillus is a type of bacterium with a shape intermediate between cocci and bacilli. Coccobacilli, then, are very short rods which may be mistaken for cocci.

<i>Eikenella corrodens</i> species of bacterium

Eikenella corrodens is a fastidious Gram-negative facultative anaerobic bacillus. It was first identified by M. Eiken in 1958, who called it Bacteroides corrodens.

<i>Actinobacillus</i> genus of bacteria

Actinobacillus is a genus of Gram-negative, nonmotile and non-spore-forming, oval to rod-shaped bacteria occurring as parasites or pathogens in mammals, birds, and reptiles. It is a member of the Pasteurellaceae family. The bacteria are facultatively aerobic or anaerobic, capable of fermenting carbohydrates, and of reducing nitrates. The genomic DNA contains between 40 and 47 mol % guanine plus cytosine.

Kingella kingae is a species of Gram-negative facultative anaerobic β-hemolytic coccobacilli. First isolated in 1960 by Elizabeth O. King, it was not recognized as a significant cause of infection in young children until the 1990s, when culture techniques had improved enough for it to be recognized. It is best known as a cause of septic arthritis, osteomyelitis, spondylodiscitis, bacteraemia, and endocarditis, and less frequently lower respiratory tract infections and meningitis.

Bacterial cellular morphologies

Bacteria and Archaea are classified by direct examination with the light microscope according to their morphology and arrangement.

Pharynx part of the throat that is behind the mouth and nasal cavity

The pharynx is the part of the throat behind the mouth and nasal cavity, and above the esophagus and larynx – the tubes going down to the stomach and the lungs. It is found in vertebrates and invertebrates, though its structure varies across species.

Well studied Periodontal pathogens are bacteria that have been shown to significantly contribute to periodontitis.

Suttonella indologenes, formerly Kingella indologenes, is a Gram-negative rod-shaped bacterium of the family Cardiobacteriaceae. Like other members of its family, it is a bacterium that is assumed to be normally present in the respiratory tract. It has been found to rarely cause endocarditis, an infection of the heart valves. It also been found in the eye. It may cause eye infections. Little is known about it as a bacterium other than its structure and biochemical composition. Like other members in its family, it has a characteristic 16S ribosomal RNA which consists of 1474 base pairs.

N-glycosyltransferase is an enzyme in prokaryotes which transfers individual hexoses onto asparagine sidechains in substrate proteins, using a nucleotide-bound intermediary, within the cytoplasm. They are distinct from regular N-glycosylating enzymes, which are oligosaccharyltransferases that transfer pre-assembled oligosaccharides. Both enzyme families however target a shared amino acid sequence asparagine—-any amino acid except proline—serine or threonine (N–x–S/T), with some variations.


  1. 1 2 3 4 5 Nørskov-Lauritsen, N (Apr 2014). "Classification, identification, and clinical significance of haemophilus and aggregatibacter species with host specificity for humans". Clinical Microbiology Reviews. 27 (2): 214–40. doi:10.1128/CMR.00103-13. PMC   3993099 . PMID   24696434.
  2. Feder HM, Jr; Roberts, JC; Salazar, J; Leopold, HB; Toro-Salazar, O (Jun 2003). "HACEK endocarditis in infants and children: two cases and a literature review". The Pediatric Infectious Disease Journal. 22 (6): 557–62. doi:10.1097/01.inf.0000069795.12338.cf. PMID   12799515.
  3. Raza, SS; Sultan, OW; Sohail, MR (Aug 2010). "Gram-negative bacterial endocarditis in adults: state-of-the-heart". Expert Review of Anti-infective Therapy. 8 (8): 879–85. doi:10.1586/eri.10.76. PMID   20695743.
  4. Chambers, ST; Murdoch, D; Morris, A; Holland, D; Pappas, P; Almela, M; Fernández-Hidalgo, N; Almirante, B; Bouza, E; Forno, D; del Rio, A; Hannan, MM; Harkness, J; Kanafani, ZA; Lalani, T; Lang, S; Raymond, N; Read, K; Vinogradova, T; Woods, CW; Wray, D; Corey, GR; Chu, VH; International Collaboration on Endocarditis Prospective Cohort Study, Investigators (2013). "HACEK infective endocarditis: characteristics and outcomes from a large, multi-national cohort". PLoS ONE. 8 (5): e63181. Bibcode:2013PLoSO...863181C. doi:10.1371/journal.pone.0063181. PMC   3656887 . PMID   23690995.
  5. 1 2 3 4 Sen Yew, H; Chambers, ST; Roberts, SA; Holland, DJ; Julian, KA; Raymond, NJ; Beardsley, J; Read, KM; Murdoch, DR (Jun 2014). "Association between HACEK bacteraemia and endocarditis". Journal of Medical Microbiology. 63 (Pt 6): 892–5. doi:10.1099/jmm.0.070060-0. PMID   24681996.
  6. 1 2 3 Wassef, N; Rizkalla, E; Shaukat, N; Sluka, M (May 15, 2013). "HACEK-induced endocarditis". BMJ Case Reports. 2013: bcr2012007359. doi:10.1136/bcr-2012-007359. PMC   3670033 . PMID   23682079.
  7. , eMedicine, HACEK organism infection. June 2005.