An appropriate single antibiotic for the empiric treatment of presumed bacterial meningitis in a six month old child would be:

Both cefotaxime and ceftriaxone are effective single agents for the empiric treatment of bacterial meningitis beyond two or three months of life, when Listeria monocytogenes and gram-negative enteric organisms are no longer a concern. Although cefuroxime does enter the CSF and was once thought to be acceptable treatment for bacterial meningitis, subsequent studies showed consistently poorer results than with other regimens. 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