**Question:** In a neonate with meningitis, Gram-positive bacilli were isolated on CSF examination. Which of the following is the likely causative organism?

A. Streptococcus pneumoniae
B. Staphylococcus aureus
C. Escherichia coli
D. Klebsiella pneumoniae

**Core Concept:**
Meningitis is an inflammation of the meninges, the protective coverings of the brain and spinal cord. In neonates, bacterial meningitis is a severe infection that requires prompt diagnosis and treatment. Gram-positive bacilli are rod-shaped bacteria that are typically found in the skin, respiratory tract, and gastrointestinal tract.

**Why the Correct Answer is Right:**
Gram-positive bacilli are the likely causative organisms in this scenario because the patient is a neonate, and these bacteria are commonly found in the skin, respiratory tract, and gastrointestinal tract. Among the options provided, Streptococcus pneumoniae (A) and Staphylococcus aureus (B) are the most common Gram-positive bacteria causing meningitis in neonates. Both are part of the normal flora in these regions and can cause invasive infections, including meningitis.

**Why Each Wrong Option is Incorrect:**
C. Escherichia coli (D) is a Gram-negative bacterium and is less likely to cause neonatal meningitis. However, if the neonate has a pre-existing condition or has been exposed to contaminated equipment, E. coli meningitis can occur.

D. Klebsiella pneumoniae (D) is another Gram-negative bacterium, which is less likely to cause neonatal meningitis than Gram-positive bacteria.

**Clinical Pearl:**
In neonates, it is crucial to consider Gram-positive bacteria as the causative agents in meningitis, as they are more likely to cause invasive infections in this population due to their ability to penetrate host tissues and cause severe infections. Gram-negative bacteria, like E. coli and K. pneumoniae, can also cause meningitis in neonates with specific risk factors or pre-existing conditions.

**Correct Answer:**
The correct answer is B. Staphylococcus aureus (Staphylococcus aureus), as it is a common Gram-positive coccus found in the skin and nares, and can cause meningitis in neonates, especially in those with pre-existing conditions or contaminated equipment.