Pharmacodynamics of trimethoprim-sulfamethoxazole in Listeria meningitis: a case report.

Infections in the cerebrospinal fluid (CSF) occur in an area of impaired host defenses; therefore, bactericidal antibiotics that reach adequate concentrations in the CSF are necessary for treatment. Measurements of antibiotic penetration into the CSF include CSF inhibitory and bactericidal titers, the absolute antibiotic concentration in the CSF, and the CSF: serum concentration ratio. We present the case of a patient with Listeria monocytogenes meningitis who failed to respond clinically to standard therapy, and whose organism demonstrated tolerance to Ampicillin (MBC: MIC = 258:1) that successfully responded to trimethoprim-sulfamethoxazole (TMP-SMX). The CSF peak bactericidal titer to TMP-SMX was 1:8, corresponding to that reported as necessary for successful outcome in patients with meningitis. The CSF peak: MBC ratios for TMP and SMX were less than 3:1 and equal to 3:1, respectively. These individual ratios are lower than those suggested for successful treatment of meningitis; however, the recommended ratios were established using single agents and did not account for synergistic activity with a drug combination such as TMP-SMX. The failure of standard therapy in this patient underscores the importance of MIC/MBC testing when tolerance is suspected or when CSF penetration of antibiotics is relatively poor. In addition, measurements of CSF inhibitory and bactericidal titers, which incorporate the antibiotic concentration in the CSF, susceptibility of the infecting microorganism, and host defense factors, may be useful in monitoring patients with meningitis.