CSF concentration adequate for most susceptible organisms, except certain Gram negative bacilli, such as Acinetobacter, Flavobacterium, and Enterobacterales producing inducible (Enterobacter spp, Citrobacter freundii complex, Serratia spp, Morganella spp, Providencia spp, Proteus vulgaris, Proteus penneri, and some Hafnia spp) and extended-spectrum ß-lactamases. Ceftriaxone achieves sustained reliable bactericidal activity within the CSF despite its high protein binding. Agent of choice for penicillin-intermediate S. pneumoniae.