|Cardiobacterium hominis on blood agar plate|
Cardiobacterium hominis is a Gram-negative rod commonly grouped with other bacteria into the HACEK group. It is one of several bacteria normally present in the respiratory tract. However, it may also rarely cause endocarditis, an infection of the heart valves.
C. hominis is a catalase-negative, oxidase-positive, indole-producing Gram-negative rod. Its morphology has classically been described as highly pleomorphic and irregularly staining, although homogeneous bacilli with uniform shapes may be seen with the addition of yeast extract.
Historically C. hominis has been sensitive to penicillin and penicillin derivatives such as ampicillin. However, penicillin-resistant strains, including those that produce beta-lactamases, have been described with increasing frequency. Clinical guidelines thus recommend that C. hominis and other HACEK organisms be presumed to harbor ampicillin resistance and therefore be treated with a third-generation cephalosporin. C. hominis and other HACEK organisms also exhibit in vitro susceptibility to trimethoprim-sulfamethoxazole, fluoroquinolones, and aztreonam. C. hominis is often resistant to erythromycin.