Bactericidal activity of antibiotic combinations against clinical isolates of Enterococcus gallinarum and Enterococcus casseliflavus

Background Enterococcus gallinarum and Enterococcus casseliflavus are increasingly identified as causes of healthcare-associated infections. Although hepatobiliary infection and catheter-related bloodstream infection predominate, endovascular infection can also occur. The optimal treatment of these infections is unknown. Objectives To determine the in vitro activity of ampicillin, gentamicin and ceftriaxone against clinical isolates of E. gallinarum and E. casseliflavus. Methods Bloodstream isolates of E. gallinarum (n = 10) and E. casseliflavus (n = 8) were collected from unique patients between 2008 and 2022 at one institution. The activity of ampicillin was tested alone and combined with gentamicin and ceftriaxone by the time-kill method. Results All isolates were susceptible to ampicillin and none had gentamicin MIC > 128 mg/L. At concentrations of 0.25 × MIC, ampicillin plus gentamicin was synergistic against 5 of 10 E. gallinarum and all 8 E. casseliflavus, and was bactericidal against 12 of the 18 strains. In contrast, ampicillin plus ceftriaxone was synergistic and bactericidal against only 1 of 10 E. gallinarum and 0 of 8 E. casseliflavus. When the concentration of ampicillin was increased to 10 mg/L, above the MIC for all strains, the combination of ampicillin plus gentamicin was bactericidal against all isolates. Conclusions These data suggest that the ampicillin plus gentamicin combination is frequently synergistic and typically bactericidal and may be the preferred treatment option for endovascular infection caused by E. gallinarum or E. casseliflavus. In contrast to the findings for Enterococcus faecalis, ampicillin plus ceftriaxone is generally not synergistic or bactericidal and thus may not be as effective.