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

Objective: To evaluate if the in vitro activity of ampicillin increases when combined with ceftriaxone.
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 log10 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.
(©The Author 2019. Published by Sociedad Española de Quimioterapia. This article is distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)(https://creativecommons.org/licenses/by-nc/4.0/).)