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Listeria monocytogens: synergistic effects of ampicillin and gentamicin
Abstract:
Listeria monocytogenes infections are most common in newborn infants and persons with impaired defense mechanisms. There are reports of successful treatment with ampicillin alone: however, there is uncertainty as to what regimen constitutes the most effective therapy. The purpose of this study was to illustrate the in-vitro synergism between ampicillin and gentamicin against L. monocytogenes. Seven strains of L. monocytogenes isolated from bloods or cerebrospinal fluids of infants and three control strains obtained from the Center for Disease Control were tested. Minimal inhibitory concentrations of ampicillin and gentamicin were determined in Todd-Hewitt broth with an inoculum of 10(5) organisms/ml. Killing curves were determined for ampicillin 6 microgram/ml, gentamicin, 0.5 microgram/ml, and the combination of ampicillin, 6 microgram/ml, plus gentamicin, 0.5 microgram/ml. Incubation of approximately 10(7) organisms/ml with these concentrations of ampicillin and gentamicin caused no significant reduction in the viable bacterial population in 24 hours. The combination, on the other hand, was bactericidal in all seven strains isolated from patients and one control strain. The authors believe the ultimate test of the superiority of this combination to ampicillin alone must come from clinical studies. However, the synergistic and bactericidal effects of ampicillin with gentamicin may be very desirable in treatment of newborns and patients with underlying disease.
Insights
This study found that combining ampicillin and gentamicin shows synergistic bactericidal effects against Listeria monocytogenes. This combination therapy may be beneficial for treating infections in newborns and immunocompromised patients.
Area of Science:
- Infectious Diseases
- Microbiology
- Pharmacology
Background:
- Listeria monocytogenes infections pose significant risks, particularly to newborns and immunocompromised individuals.
- Current treatment guidelines for Listeria monocytogenes infections, often involving ampicillin, lack definitive consensus on optimal therapeutic regimens.
- Ampicillin monotherapy has shown success, but its limitations necessitate exploring alternative or combination therapies.
Purpose of the Study:
- To investigate the in vitro synergistic activity of ampicillin and gentamicin against clinical isolates of Listeria monocytogenes.
- To evaluate the bactericidal efficacy of the ampicillin-gentamicin combination compared to monotherapy.
Main Methods:
- Minimal inhibitory concentrations (MICs) of ampicillin and gentamicin were determined for seven clinical Listeria monocytogenes strains and three control strains.
- Killing curves were generated using specific concentrations of ampicillin (6 µg/ml), gentamicin (0.5 µg/ml), and their combination.
- Bacterial viability was assessed after 24-hour incubation at a high inoculum (10^7 organisms/ml).
Main Results:
- Neither ampicillin nor gentamicin alone at the tested concentrations significantly reduced the viable bacterial population of Listeria monocytogenes within 24 hours.
- The combination of ampicillin and gentamicin demonstrated bactericidal activity against all seven patient-derived strains and one control strain.
- Synergistic effects were observed, indicating enhanced killing compared to individual agents.
Conclusions:
- The combination of ampicillin and gentamicin exhibits synergistic and bactericidal properties against Listeria monocytogenes in vitro.
- This combination therapy holds promise for treating Listeria monocytogenes infections in vulnerable populations, including newborns and immunocompromised patients.
- Further clinical studies are warranted to confirm the superiority of this combination regimen over ampicillin monotherapy in patient populations.