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?

This baby has meningitis, which developed in the first 3 months of life but after the very early perinatal period. In this setting, group B Streptococcus is the most likely pathogen; it is confirmed by the presence of gram-positive cocci in chains in the CSF. Treatment of meningitis due to group B Streptococcus is still somewhat controversial because there are some penicillin-resistant strains. The combination of IV penicillin or IV ampicillin with IV gentamicin has been recommended. Coagulase-negative staphylococci are an impoant cause of pneumonia in babies on respirators. Escherichia coli is a gram-negative rod that can cause meningitis. Neisseria meningitidis is a gram-negative diplococcus that can cause meningitis. Ref: Ray C.G., Ryan K.J. (2010). Chapter 25. Streptococci and Enterococci. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.