A 2 month old infant develops fever, lethargy, and seizures. The infant had been born at term with no obstetric or early postnatal complications. Physical examination after the seizure demonstrates nuchal rigidity and bulging fontanelles. Lumbar puncture reveals a high CSF neutrophil count and gram-positive cocci in chains. Which of the following is the most likely pathogen?
A 2 month old infant develops fever, lethargy, and seizures. The infant had been born at term with no obstetric or early postnatal complications. Physical examination after the seizure demonstrates nuchal rigidity and bulging fontanelles. Lumbar puncture reveals a high CSF neutrophil count and gram-positive cocci in chains. Which of the following is the most likely pathogen?
💡 Explanation
## **Core Concept**
The question tests knowledge of neonatal meningitis, a serious infection that occurs in newborns, and the common pathogens involved. Neonatal meningitis can be caused by various bacteria, but the presentation and laboratory findings help narrow down the likely causative agent. The presence of gram-positive cocci in chains in the cerebrospinal fluid (CSF) is a critical clue.
## **Why the Correct Answer is Right**
The description provided points towards a bacterial meningitis caused by a gram-positive organism that appears in chains. This morphology is characteristic of *Streptococcus* species. Among the streptococci, *Streptococcus pneumoniae* is a well-known cause of bacterial meningitis, particularly in older children and adults, but in neonates, another streptococcal species is more commonly implicated. However, *Streptococcus agalactiae* (Group B Streptococcus, GBS) is a leading cause of neonatal sepsis and meningitis, especially in the first month of life. GBS appears as gram-positive cocci in chains, which matches the description given.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, incorrect options might include bacteria that do not match the gram stain description or are less commonly associated with neonatal meningitis.
- **Option B:** Without the specific organism listed, we can infer that if it does not match *Streptococcus agalactiae* (GBS) in morphology or common association with neonatal meningitis, it would be incorrect.
- **Option C:** Similarly, if this option does not align with the clinical presentation or microbiological findings indicative of GBS, it would not be the correct answer.
- **Option D:** This would be incorrect for the same reasons as options A through C, assuming they do not accurately represent *Streptococcus agalactiae*.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that *Streptococcus agalactiae* (Group B Streptococcus) is a major cause of neonatal sepsis and meningitis, particularly in the first month of life. Screening for GBS colonization in pregnant women and administering intrapartum antibiotic prophylaxis to those who are positive can significantly reduce the risk of early-onset GBS disease in newborns.