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Chapter: Microbiology and Immunology: Bacteriology: Haemophilus, Pasteurella, and Actinobacillus

HACEK Group of Bacteria

The acronym HACEK refers to a group of fastidious slow-growing Gram-negative bacilli—Haemophilus species (H. parainfluenzae, H. aphrophilus, and H. paraphrophilus), A. actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, and Kingella species.

HACEK Group of Bacteria

The acronym HACEK refers to a group of fastidious slow-growing Gram-negative bacilli—Haemophilus species (H. parainfluenzae,H. aphrophilus, and H. paraphrophilus), A. actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, and Kingella species. 

These organisms are normallypresent in the oropharynx, which under certain conditions can cause severe infections, such as endocarditis. Since they are fastidious organisms to grow, they are usually incriminated as a frequent cause of culture-negative endocarditis. These organisms also cause a wide variety of diseases including oti-tis media, conjunctivitis, pneumonia, septic arthritis, osteo-myelitis, urinary tract infections, wound infections, brain abscess, periodontal infections, etc. Infective endocarditis (IE) is a major clinical condition caused by HACEK group of organisms:

Haemophilus species is responsible for 0.5–1% of all cases ofIE. Of these, majority (nearly 40%) are caused by H. aphroph-ilus, followed by H. parainfluenzae. H. influenzae rarely causesIE despite causing bacteremias. Nearly one-third of cases of H. aphrophilus IE are due to dental disease and one-fifth aredue to sinusitis or otitis media.


A. actinomycetemcomitans is the causative agent of localizedjuvenile periodontitis, a presentation of early-onset peri-odontitis. The condition is characterized by gingivitis and severe periodontal attachment loss. The bacteria also cause IE, especially in patients with underlying heart disease and with infection of prosthetic valve, usually aortic.


C. hominis are Gram-negative or Gram-variable, pleomor-phic bacilli with round swelling of both ends. They are characteristically arranged in chains, clusters, or rosettes. These organisms are present as part of the normal flora of the mouth and upper respiratory tract. These bacteria usually cause infections of the blood stream preceded by oral pathology. C. hominis causes endocarditis in majority (75%) of patients with underlying heart diseases. Arterial embolization is also observed in many patients.


E. corrodens are Gram-negative, pleomorphic, often cocco-bacillary. They are facultatively anaerobic. They are found as part of the normal oral flora and many other mucosal surfaces. E. corrodens is so named due to its ability to cor-rode the agar during growth in the culture.


E. corrodens is a well-known cause of cellulitis followinghuman bites and clenched-fist injuries, soft-tissue infec-tions, and endocarditis in drug addicts. This organism is also associated with a variety of respiratory tract infec-tions including pneumonia, empyema, etc. They cause endocarditis in most of the patients with underlying heart valve lesions.

      Kingella species are small Gram-negative bacteria and varyin shapes from cocci to coccobacilli. The genus Kingella consists of three species: Kingella kingaeKingella denitrifi-cans, and Kingella indologenes. K. kingae usually causes endo-carditis. The endocarditis caused by K. kingae, although rare, progresses very rapidly.

Laboratory diagnosis of IE caused by HACEK organisms depends on blood culture, followed by subculture on blood agar and chocolate agar supplemented with vitamins and other nutrients. The media are incubated in a humid atmosphere of 5% carbon dioxide or anaerobically for 24–72 hours. Blood cul-ture bottles are incubated for a minimum of 14 days, followed by terminal subculture before declaring it negative.

Antibiotics are the mainstay of treatment. Due to delay in blood culture, antibiotics are usually started empirically. Ceftriaxone is the drug of choice. Ampicillin combined with gentamicin is also equally useful. However, of late, resistance to beta-lactams has been reported among HACEK group of organisms.

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