**Question:** An infant had high grade fever and respiratory distress at the time of presentation to the emergency room. The sample collected for blood culture was subsequently positive showing growth of ααααα -hemolytic colonies. On Gram staining these were gram positive cocci. In the screening test for identification, the suspected pathogen is likely to be susceptible to the following agent:

A. Penicillin
B. Ceftriaxone
C. Vancomycin
D. Amoxicillin

**Core Concept:**

The accurate identification of bacteria is crucial for appropriate antimicrobial therapy in infections. Gram staining is a rapid screening test used to differentiate between Gram-positive and Gram-negative bacteria. In this case, the colonies are described as gram positive cocci, which indicates that the bacteria belongs to the Firmicutes phylum.

**Why the Correct Answer is Right:**

The correct answer is **C. Vancomycin**. Gram positive cocci are generally susceptible to vancomycin, as it targets the cell wall synthesis of bacteria, specifically inhibiting the formation of peptidoglycan, which is unique to Gram-positive bacteria.

**Why Each Wrong Option is Incorrect:**

**A. Penicillin:** Penicillin is a β-lactam antibiotic that is effective against Gram-positive bacteria as well as some Gram-negative bacteria. However, due to the presence of penicillinase, some bacteria can develop resistance to penicillin. In this scenario, penicillin may not be the optimal choice.

**B. Ceftriaxone:** Ceftriaxone is a broad-spectrum antibiotic that targets both Gram-positive and Gram-negative bacteria. However, as the bacteria belongs to the Firmicutes phylum (Gram-positive), ceftriaxone is not the correct choice.

**D. Amoxicillin:** Amoxicillin is a penicillin antibiotic that is effective against Gram-positive bacteria but may be less effective against some Gram-negative bacteria due to the presence of extended-spectrum β-lactamase enzymes. In this scenario, amoxicillin may not be the optimal choice.

**Clinical Pearl:**

In cases of suspected bacterial infections, particularly when the organism is known to be Gram-positive, using a drug like vancomycin is crucial for effective treatment. This is due to its selective inhibition of cell wall synthesis in Gram-positive bacteria, making it a reliable choice in treating infections caused by Gram-positive organisms.