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 staed but the culture remained positive for the same organism even after 2 weeks of therapy. The most likely organism causing infection is

This is a case of vancomycin-resistant enterococci (VRE) infection. Gram positive cocci in chains and catalase negative suggest streptococcaceae family. Positive culture after 2 weeks of vancomycin therapy suggests resistance to vancomycin. Viridans streptococci are usually mouth commensals and are sensitive to vancomycin. Staphylococcus aureus and coagulase-negative staphylococci are gram-positive cocci in clusters, catalase +ve; hence, they are ruled out. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition