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

From the case scenario, the most likely organism involved is Enterococcus faecalis. The organism is described as gram positive cocci in chains, which are catalase negative. This helps rule out Staphylocococcus aureus and coagulase-negative Staphylococcus. Streptococcus viridians is usually sensitive to penicillin and vancomycin hence it is ruled out. Here, even after treatment with aminoglycosides, cephalosporin, and vancomycin, the culture remains positive. So, this is a case of vancomycin-resistant enterococci. The mechanism is the alteration of the D-alanyl-D-alanine chain in cell wall. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition