An identifying characteristic of Cardiobacterium hominis is

• A. Positive catalase
• B. Indole negative
• C. Positive oxidase
• D. Inability to grow on sheep blood agar

Answer: (C)

Oxidase activity is the identifying clue here. Cardiobacterium hominis consistently tests positive in the oxidase test, which detects the enzyme cytochrome c oxidase involved in the bacterial electron transport chain. This makes oxidase positivity a helpful and distinguishing feature when identifying this organism among fastidious gram-negative rods in the lab. The other traits aren’t as reliable for identification: growth on sheep blood agar is possible with prolonged incubation (so claiming an inability to grow there is incorrect), and catalase or indole results can vary and do not provide as consistent a identifying signal for this species. So, the positive oxidase result best identifies Cardiobacterium hominis.