A 57-year-old man arrives at the emergency room complaining of weakness, fatigue, and intermittent fever that has recurred for several weeks. The patient had a cardiac valvular prosthesis implanted 5 years ago. Blood cultures grew ceain gram-positive cocci on which catalase and coagulase test were done. The gram-positive organisms failed to ferment mannitol, and their growth was inhibited by novobiocin. CATALASE TEST SLIDE COAGULASE TEST Lesions seen on skin What is the most likely infectious agent?

Physical examination reveals petechiae (pinpoint, nonraised, purplish red spots caused by intradermal hemorrhage) on the chest and stomach. Catalase-positive and coagulase negative The patient is probably suffering from bacterial endocarditis caused by S. epidermidis infection of the prosthetic hea valve. S. epidermidis Coagulase-negative organism Unable to ferment mannitol Sensitive to novobiocin Resistant to penicillin. Patients with congenital hea malformations, acquired valvular defects (for example, rheumatic hea disease), prosthetic valves, and previous bacterial endocarditis show an increased incidence of bacterial endocarditis. Intravenous drug users also have a high risk for infection. S. viridans can be ruled out, because streptococci are catalase negative, which is a feature that distinguishes them from catalase-positive staphylococci. S. aureus is coagulase positive so it is ruled out. S. saprophyticus is coagulase negative but resistant to novobiocin, hence also ruled out here