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In vitro activity of ampicillin and ceftriaxone against ampicillin-susceptible Enterococcus faecium

Michael P Lorenzo1, James M Kidd2, Stephen G Jenkins3,4

  • 1Department of Pharmacy, Rhode Island Hospital, Providence, RI, USA.

The Journal of Antimicrobial Chemotherapy
|May 4, 2019
PubMed
Abstract

Insights

The combination of ceftriaxone and ampicillin showed synergy in some ampicillin-susceptible Enterococcus faecium isolates, but not uniformly. Clinical use requires caution for E. faecium infections.

Area of Science:

  • Microbiology
  • Infectious Diseases
  • Pharmacology

Background:

  • Enterococcus faecium and Enterococcus faecalis are common causes of healthcare-associated infections.
  • Ampicillin is a standard treatment for susceptible enterococcal infections.
  • Ceftriaxone is a third-generation cephalosporin with activity against some Gram-positive bacteria.

Purpose of the Study:

  • To evaluate the in vitro activity of the combination of ceftriaxone and ampicillin against clinical isolates of ampicillin-susceptible Enterococcus faecium.
  • To compare the synergistic activity of this combination against E. faecium and Enterococcus faecalis.

Main Methods:

  • Clinical isolates of ampicillin-susceptible E. faecium (n=29) and E. faecalis (n=10) were tested.
  • Susceptibility testing was performed using gradient diffusion strip (GDS) and broth microdilution (BMD).
  • Synergy was assessed using GDS crossing, double disc diffusion, and time-kill studies.

Main Results:

  • Ampicillin MICs for E. faecium ranged from 0.25-4 mg/L (BMD) and 1-8 mg/L (GDS).
  • Synergy between ceftriaxone and ampicillin was observed in 82.8% of E. faecium isolates by GDS and 75.9% by double disc diffusion.
  • Time-kill studies showed synergy in 33.3% of E. faecium and 100% of E. faecalis isolates.

Conclusions:

  • The combination of ceftriaxone and ampicillin demonstrated variable synergy against ampicillin-susceptible E. faecium.
  • While synergy was consistent in E. faecalis, it was not uniform in E. faecium.
  • Clinical correlation is needed, and caution is advised when using this combination for E. faecium infections.

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