A 65-year-old male develops a fever and localized pain three weeks after undergoing total hip replacement surgery. Blood cultures grow Gram-positive cocci in clusters that are catalase-positive and coagulase-negative. Which of the following organisms is the most likely causative agent, and what is its key virulence factor in this context?

Explanation

The description of Gram-positive cocci in clusters, catalase-positive, and coagulase-negative points to a coagulase-negative *Staphylococcus* (CoNS). Among CoNS, *Staphylococcus epidermidis* is the most common cause of prosthetic device infections, such as prosthetic joint infections. Its key virulence factor in this context is its ability to form biofilms on foreign bodies, which protects the bacteria from host immune responses and antibiotics. *Staphylococcus aureus* is coagulase-positive and uses Protein A as a virulence factor. *Streptococcus pyogenes* is Gram-positive cocci in chains and uses M protein. *Enterococcus faecalis* is also Gram-positive cocci but typically in pairs or short chains, and while it can cause nosocomial infections, it's not the primary organism for prosthetic device infections with these specific lab findings.