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 the same organism even after 2 weeks of therapy. The most likely organism causing infection is-
This is case of VRE (Vancomycin resistant Enterococci)
Points in favor:

Gram+ve cocci in chain and Catalase –ve points towards Streptococcaceae family
Resistant to aminoglycoside, cephalosporins and vancomycin
S, aureus and CONS are catalase +ve, hence they are ruled out.
Staphylococcus aureus
Viridans streptococci
Enterococcus faecalis
Coagulase negative Staphylococcus