Final answer:
Staphylococcus epidermidis is the most common bacteria causing endocarditis 60-365 days after prosthetic valve surgery, followed by Staphylococcus aureus and gram-positive cocci such as viridans streptococci and Enterococcus faecalis.
Step-by-step explanation:
The most common bacteria responsible for endocarditis 60-365 days after prosthetic valve surgery include staphylococci, specifically Staphylococcus epidermidis, and to a lesser extent, Staphylococcus aureus. The gram-positive cocci, such as viridans streptococci and Enterococcus faecalis, along with the gram-negative HACEK group, also play a role in infection, though their incidence might be higher immediately after surgery or before 60 days. Endocarditis in prosthetic valve patients can result in the formation of bacterial vegetations, which may be treated with a combination of antibiotics like vancomycin, rifampin, and gentamicin, particularly because rifampin is effective against biofilms often associated with prosthetic devices.