Cardiobacterium hominis has been recovered from endocarditis. Which identifying characteristic is true?

• A. Positive oxidase
• B. Requires a microaerophilic environment for growth
• C. Growth on MacConkey agar
• D. Inability to grow on sheep blood agar

Answer: (A)

Oxidase status is the main identifying feature here. Cardiobacterium hominis, a member of the HACEK group, are fastidious Gram-negative bacilli that are typically oxidase positive. The oxidase test detects cytochrome c oxidase in the bacterial respiratory chain, and a positive result helps distinguish these organisms from many other Gram-negative rods that are oxidase negative. These bacteria are slow growers and require enriched media with CO2, and they do not reliably grow on MacConkey agar. They can grow on sheep blood agar with extended incubation, but the most consistent identifying trait among the options is being oxidase positive.