A patient admitted to an ICU is on a 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 staed but the culture remained positive for the same organism even after 2 weeks of therapy. The most likely organism causing infection is
Staphylococcus aureus Streptococci viridans Enterococcus faecalis Coagulase negative Staphylococcus