A gram-positive coccus that is catalase positive, lysostaphin resistant, and modified oxidase positive is best identified as a member of which genus?

• A. Micrococcus
• B. Lactococcus
• C. Pediococcus
• D. Staphylococcus

Answer: (A)

When identifying gram-positive cocci, catalase activity narrows the field to two main groups: Staphylococcus and Micrococcus. The key differentiators here are lysostaphin sensitivity and the oxidase reaction. Staphylococcus is lysostaphin sensitive because its cell wall cross-bridges are targeted by lysostaphin, and it typically tests negative on the oxidase test. Micrococcus, on the other hand, is resistant to lysostaphin and tests oxidase positive. So the combination of catalase positive, lysostaphin resistant, and oxidase positive points to Micrococcus. This pattern helps distinguish it from Staphylococcus, as well as from other gram-positive cocci like Lactococcus or Pediococcus, which are generally catalase negative.