This patient should begin treatment with intravenous ampicillin, ceftriaxone, and vancomycin. She has bacterial meningitis, and although the definitive cause has not been determined, empiric treatment should be initiated to cover the most likely infecting organisms. The most common causes of bacterial meningitis are Streptococcus pneumoniae and Neisseria meningitides, which account for more than 80% of cases. Therefore, primary empiric antibiotic therapy must adequately cover these two organisms. Common empiric regimens include the third-generation cephalosporins ceftriaxone or cefotaxime, which are bactericidal β-lactams that penetrate the central nervous system (CNS) well with excellent coverage of these organisms. One of these agents is combined with vancomycin, which also penetrates the CNS adequately when it is inflamed and provides coverage of possible penicillin-resistant organisms until specific identification and sensitivities are known. Additional antibiotic coverage is needed in patients with risk factors for specific infections. Although Listeria monocytogenes makes up only a small percentage (<5%) of meningitis cases in immunocompetent persons, the incidence increases significantly with age. Therefore, in patients older than 50 years, such as this patient, or persons with impaired cell-mediated immunity, ampicillin is added to empiric therapy because Listeria is not adequately covered by the usual components of empiric antibiotic regimens. Therefore, in this patient, the combination of ampicillin, ceftriaxone, and vancomycin provides the most appropriate empiric coverage of the suspected pathogens while culture results are pending.

Ceftazidime and vancomycin without ampicillin would be inadequate for this patient because it lacks coverage for Listeria.

Meropenem is used primarily in patients with impaired cell-mediated immunity or nosocomial or neurosurgery-related meningitis in which more extensive coverage of gram-negative organisms, including Pseudomonas aeruginosa, is indicated.

Fluoroquinolones, such as moxifloxacin, have not been well studied for treatment of bacterial meningitis, and their use is typically limited to patients who cannot tolerate typical empiric therapies, such as those with a severe allergic response to β-lactam antibiotics.