A 47 year old man with a history of sickle cell disease has had numerous hospitalizations requiring the placement of intravenous lines. The patient has poor peripheral venous access, and a catheter is placed in the right subclan vein. The patient subsequently develops right arm discomfo and swelling and a temperature of 40.1 degrees C with chills. Multiple blood cultures are taken, and gram-positive cocci are isolated. The organism is catalase positive and grows on mannitol salt agar, but does not turn the agar yellow; the colonies are gamma-hemolytic on a sheep blood agar plate. Which of the following organisms is the most likely cause of this patient's symptoms?