A patient admitted to an ICUs is on central venous line for the last one week; he is on ceftazidine and amikacin. After 7 days of antibiotics he develops a spike of fever and his blood culture is positive for gram positive cocci in chains, which are catalase negative. Following this, vancomycin was restarted but the culture remained positive for same organisms even after 2 weeks of therapy. The most likely organism causing infection is :