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Updated: Jan 19, 2026
Listeria monocytogenes Infection of the Brain
Published on: October 2, 2018
In vitro study of synergy of ampicillin with ceftriaxone against Listeria monocytogenes
J A Lepe1, A Rodríguez-Villodres, G Martín-Gutiérrez
1José A. Lepe, Hospital Universitario Virgen del Rocío. Avda. Manuel Siurot s/n. 41013 Seville. Spain. jalepe@cica.es.
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.
Insights
Combining ampicillin with ceftriaxone enhances ampicillin
Area of Science:
- Microbiology
- Infectious Diseases
- Pharmacology
Background:
- Listeria monocytogenes is a significant cause of invasive infections.
- Antibiotic resistance is a growing concern in treating bacterial infections.
- Understanding synergistic antibiotic combinations is crucial for effective treatment.
Purpose of the Study:
- To determine if combining ampicillin and ceftriaxone enhances ampicillin's in vitro activity against Listeria monocytogenes.
- To evaluate the synergistic effect of ampicillin and ceftriaxone against invasive clinical isolates of L. monocytogenes.
Main Methods:
- In vitro evaluation of ampicillin and ceftriaxone activity.
- Broth microdilution method to determine Minimum Inhibitory Concentrations (MICs).
- Checkerboard and time-kill curve assays to assess synergy.
Main Results:
- All tested Listeria monocytogenes strains were susceptible to ampicillin (MICs 0.25-0.5 mg/L).
- A bacteriostatic synergy was observed with a Fractional Inhibitory Concentration (FIC) index of 0.5.
- A significant reduction (2.5 log10 CFU) was noted with ampicillin combined with 16 mg/L ceftriaxone.
Conclusions:
- The combination of ceftriaxone and ampicillin demonstrates increased in vitro activity against Listeria monocytogenes.
- This combination may represent a valuable therapeutic option for invasive L. monocytogenes infections.
- Further clinical investigation is warranted to confirm efficacy in patients.
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