A 20 days old infant brought with the complaints of fever and irritability. His fontanelles are bulged and neck stiffness is present. Which of the following is the commonest cause of meningitis in postneonatal period?

Group B streptococcus, or S. agalactiae, was previously responsible for meningitis predominantly in neonates, but it has been repoed with increasing frequency in individuals >50 years of age, paicularly those with underlying diseases. Listeria monocytogenes is an increasingly impoant cause of meningitis in neonates (60 years, and immunocompromised individuals of all ages. Symptoms in infants are poor feeding, decreased interactiveness, increased crying, and lethargy. Other signs include fever, apnea, seizures, a bulging fontanel, and a rash. Ref: Roos K.L., Tyler K.L. (2012). Chapter 381. Meningitis, Encephalitis, Brain Abscess, and Empyema. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.