Which organism is coagulase negative and catalase positive?

• A. Streptococcus pneumoniae
• B. Staphylococcus aureus
• C. Staphylococcus epidermidis
• D. Streptococcus pyogenes

Answer: (C)

Distinguishing organisms by how they react to catalase and coagulase tests is the idea here. Catalase distinguishes staphylococci (catalase positive) from streptococci (catalase negative). Within the staphylococci, the coagulase test separates Staphylococcus aureus (coagulase positive) from coagulase-negative staphylococci. So the organism that is catalase positive and coagulase negative is Staphylococcus epidermidis. The other options don’t fit: Streptococcus species are catalase negative, and Staphylococcus aureus is coagulase positive. Staphylococcus epidermidis is a common skin inhabitant and a frequent opportunistic pathogen, especially with implanted devices due to its ability to form biofilms.