Which organism is gram-positive cocci, catalase-positive, coagulase-negative, normal skin flora, novobiocin-sensitive, and forms biofilm on prosthetic devices?

• A. Staphylococcus aureus
• B. Staphylococcus epidermidis
• C. Streptococcus agalactiae
• D. Enterococcus faecalis

Answer: (B)

Identifying Staphylococcus species by key tests and clinical context. Gram-positive cocci that are catalase-positive point to Staphylococcus. If they are coagulase-negative, you’re not dealing with Staphylococcus aureus (which is coagulase-positive) and you narrow to other coagulase-negative staphylococci. Among these, novobiocin sensitivity distinguishes Staphylococcus epidermidis (novobiocin-sensitive) from Staphylococcus saprophyticus (novobiocin-resistant). Staphylococcus epidermidis is a normal skin flora and is well known for forming biofilms on prosthetic devices, creating persistent infections because the slimy biofilm helps bacteria adhere to artificial surfaces and resist antibiotics. The other organisms listed do not fit this combination of catalase-positive, coagulase-negative, novobiocin-sensitive, skin-flora status, and prosthetic-device biofilm formation.