A patient admitted to an ICU is on central venous line for the last one week. He is on Ceftazidime 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 started but the culture remained positive for same organism even after 2 weeks of therapy. The most likely organism causing infection is :