**Question:** A patient admitted to an ICU is on central venous line for the last one week; they are on ceftazidine and amikacin. After 7 days of antibiotics, they develop a spike of fever and their blood culture is positive for gram positive cocci in chains, which are catalase negative. Following this, vancomycin was restarted but the culture remained positive for same organisms even after 2 weeks of therapy. The most likely organism causing infection is:

A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Enterococcus faecalis
D. Enterococcus faecium

**Correct Answer:**

**Core Concept:**

Gram-positive cocci in chains are a common feature of several bacterial species, but the clue to identifying the organism lies in the catalase test. Staphylococcus aureus and Staphylococcus epidermidis are positive catalase, while Streptococcus pneumoniae, Enterococcus faecalis, and Enterococcus faecium are negative for catalase.

**Why the Correct Answer is Right:**

The patient's blood culture results show gram-positive cocci in chains that are negative for catalase. Among the given options, only Staphylococcus species (A, B, and D) are catalase-positive, while Streptococcus pneumoniae (option B) is catalase-negative.

**Why Each Wrong Option is Incorrect:**

- Option A (Staphylococcus aureus) is a plausible choice but we are looking for a catalase-negative organism.
- Option B (Streptococcus pneumoniae) is catalase-negative, making it an incorrect choice.
- Option C (Enterococcus faecalis) is catalase-negative, making it an incorrect choice.
- Option D (Enterococcus faecium) is catalase-negative, making it an incorrect choice.

**Clinical Pearl:**

The clinical scenario highlights the importance of considering the catalase test in differentiating between Staphylococcus and Streptococcus species in blood culture results. This test aids in narrowing down the differential diagnosis and guides the choice of appropriate antibiotic therapy.