Which organism is catalase positive, helping differentiate it from Actinomyces?

• A. Nocardia sp.
• B. Actinomyces sp.
• C. Corynebacterium sp.
• D. Streptococcus sp.

Answer: (A)

Catalase testing hinges on the enzyme that breaks down hydrogen peroxide into water and oxygen. Actinomyces species are catalase negative, reflecting their usually anaerobic/microaerophilic nature. Nocardia species are catalase positive, so when hydrogen peroxide is added you’ll see bubbling from the rapid release of oxygen. This difference helps separate them in the lab because they can look similar under the microscope as branching Gram-positive filaments, but the presence of catalase activity points to Nocardia rather than Actinomyces. While some other choices may also be catalase positive, the classic pairing used to differentiate from Actinomyces in these contexts is Nocardia.