A man with a prosthetic aortic valve and subacute calf pain.

Figure 1. Gram stain of the isolate from a blood culture (original magnification, 1000x) showing gram-negative rods with both filamentous (arrow) and dumbbell-shaped (arrowheads) morphologies characteristic of Cardiobacterium hominis. Diagnosis: Cardiobacterium hominis prosthetic valve endo-carditis. C. hominis is one of the fastidious gram-negative bacteria that belong to the HACEK group of organisms (Haemophilus parainfluenzae, Haemophilus aphrophilus, Haemophilus para-phrophilus, Actinobacillus actinomycetemcomitans, C. hominis, Ei-kenella corrodens, and Kingella kingae). Tucker et al. [1] first reported it as a cause of endocarditis in 1962. The HACEK group are part of the normal oropharyngeal flora. Poor dentition and/ or having undergone dental work within the past 6 months have been described as sources of infection [2]. HACEK organisms cause ∼3% of cases of infective endocarditis [3]. One study found that it takes, on average, 3.4 days (range, 1–10 days) to grow these organisms with automated blood culture systems [3]. On Gram stain, C. hominis appears as teardrop-and dumb-bell-shaped gram-negative rods. Figure 1 demonstrates the fil-amentous form of this organism. Classically, one can see a rosette shape on the Gram stain. The other gram-negative HA-CEK organisms have their own characteristics on Gram stain. Both A. actinomycetemcomitans and Haemophilus species are small coccobacillary rods, but, with the latter, one can occasionally see filamentous forms. E. corrodens typically appears as slender straight rods with round ends. K. kingae appears as short straight rods with square ends that form pairs and chains [4]. Biochemical testing of C. hominis usually reveals it to be oxidase positive, indole positive, and catalase negative; E. cor-rodens and K. kingae are oxidase positive, whereas Haemophilus species and A. actinomycetemcomitans are oxidase negative. A. actinomycetemcomitans is the only catalase-positive organism in the HACEK group [4]. The patient underwent bioprosthetic aortic valve replacement and completed a 6-week course of intravenous ceftriax-one. He remains healthy after 1 year of follow-up.