In vitro study of synergy of ampicillin with ceftriaxone against Listeria monocytogenes

OBJECTIVES: To evaluate if the in vitro activity of ampicillin increases when combined with ceftriaxone. MATERIAL AND METHODS: The activity of ampicillin and ceftriaxone was evaluated against six Listeria monocytogenes invasive clinical isolates. Ampicillin and ceftriaxone MICs were determined by the broth microdilution method. Synergy was evaluated by checkerboard and time-kill curves methods. RESULTS: . All six L. monocytogenes strains were susceptible to ampicillin (MICs 0.25-0.5 mg/L). A bacteriostatic synergy was demonstrated by the FIC index of 0.5 and a 2.5 log(10) CFU reduction on the six strains studied for MIC ampicillin plus 16 mg/L ceftriaxone concentrations. CONCLUSIONS: The association of ceftriaxone with ampicillin increases the in vitro activity of ampicillin, and therefore could be a valuable option in the treatment of invasive infection by L. monocytogenes.