A 50-year-old man on ICU develops acute bacterial meningitis confirmed on lumbar puncture (CSF WCC 2500/µL, predominantly neutrophils, protein 3.2 g/L, glucose 0.5 mmol/L with serum glucose 7.0 mmol/L). Gram stain shows Gram-positive diplococci. He is commenced on IV ceftriaxone 2 g BD. According to current guidelines, which adjunctive therapy should be administered?