As with other HACEK organisms, optimal growth of C hominis occurs in an environment containing supplemental carbon dioxide and high humidity. 14,15 C hominis is pleomorphic, appearing as pairs, chains, clusters, rosettes, or teardrops. In later years, however, it was further characterized and renamed C hominis. 5Ĭ hominis Bacillus: Described as having a Pasteurella-like appearance, C hominis was first called Group II D. 5,8,9 Growth of a small number of A actinomycetemcomitans colonies may be seen at 18 to 24 hours, but at least 48 hours of growth may be required. Like other HACEK organisms, A actinomycetemcomitans is an opportunistic pathogen that has been associated with meningitis, bacteremia, septic arthritis, osteomyelitis, central nervous system abscesses, thyroid abscesses, endophthalmitis, endocarditis, pericarditis, cellulitis, parotitis, UTIs, pneumonia, and periodontal infections. 9 A actinomycetemcomitans grows in broth and blood agar, often in granules that are attached to the side of the dish while the center of the dish remains clear. A actinomycetemcomitans may appear singularly, in pairs, or in clumps. 5,9 Most recently, it has been classified in the genus Aggregatibacter while maintaining the same species name. 5,9 Initially known as Bacterium actinomycetem comitans, it was reclassified as Haemophilus actinomycetemcomitans, but this classification is not preferred because of the organism’s lack of similarity to H influenzae. 13Ī Actinomycetemcomitans Bacillus: Aggregatibacter actinomycetemcomitans has been classified in various ways since its discovery. 13 H haemolyticus is often misidentified as Haemophilus influenzae, as some strains are not hemolytic, which is often the only characteristic used to differentiate the two types of bacteria. 9 H parainfluenzae is a pleomorphic coccobacillus that accounts for 75% of the flora of the upper respiratory tract. 9 H segnis, a small pleomorphic rod that often manifests in asymmetrical filamentous forms, is part of the normal flora of the mouth as well. 8,12Īphrophilus and paraphrophilus species, which are short rods, are part of the normal flora found in the mouth and pharynx of humans. Haemophilus species have been isolated from brain abscesses, meningitis, endocarditis, otitis media, sinusitis, epiglottitis, hepatic abscesses, intra-abdominal infections, pneumonia, necrotizing fasciitis, septic arthritis, osteomyelitis, postoperative surgical infections, and urinary tract infections (UTIs). 11 For the purposes of this review, all of the HACEK-causing species formally in the Haemophilus genus will be referred to by the genus name. H aphrophilus, H paraphrophilus, and H segnis recently were classified, along with A actinomycetemcomitans, into the genus Aggregatibacter. Haemophilus Species: Haemophilus species apart from the HACEK group include aphrophilus, haemolyticus, parahaemolyticus, parainfluenzae, paraphrophilus, and segnis. It is important to remember that there are other potential culture-negative species besides HACEK organisms, such as Legionella species, Bartonella species, Brucella species, and other fastidious pathogens. 1 With the advent of improved culture media and automated culture systems, prolonged incubation may no longer be necessary a 3- to 7-day incubation period may suffice. 8 Typically, HACEK organisms grow slowly and are subject to extended incubation periods of 7 to 21 days. 8,9 They grow well on nonspecific culture media rather than specialized agars like MacConkey’s agar, a growth medium for gram-negative bacteria. To grow, HACEK organisms need an atmosphere enriched with carbon dioxide. 4 This article will briefly review the HACEK pathogens, resulting infections, and common treatment approaches. 1,5-7 Rarely, HACEK organisms may cause severe systemic infections. 4 HACEK organisms are most often associated with infective endocarditis, accounting for up to 10% of cases. 2,3 These organisms commonly colonize the human oropharynx as normal, indigenous flora that could cause mouth infections. 1 The HACEK organisms include H aemophilus species, A ggregatibacter actinomycetemcomitans, C ardiobacterium hominis, E ikenella corrodens, and K ingella kingae. HACEK is an acronym for a group of organisms that are small, fastidious gram-negative bacilli.
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