Activity of amoxycillin against enterococci and synergism with aminoglycoside antibiotics.

Amoxycillin was more active in vitro than ampicillin or benzylpenicillin against clinical isolates of enterococci. All 55 strains tested were sensitive to the three penicillins but 15 strains showed a high level of resistance to streptomycin and two of these were also insensitive to kanamycin. All strains were sensitive to gentamicin, which was the most active of the aminoglycoside antibiotics. The penicillins showed pronounced bactericidal activity against the enterococci but failed to sterilise cultures of these organisms. Combinations of penicillins and aminoglycosides invariably produced synergistic bactericidal effects which resulted in sterilisation of cultures of these organisms. Combinations of penicillins and aminoglycosides invariably produced synergistic bactericidal effects which resulted in sterilisation of cultures of enterococci provided that the strain was sensitive to the aminoglycoside moiety of the antibiotic combination. Synergism was not observed with a combination of a penicillin and an aminoglycoside when the enterococcus was resistant to the aminoglycoside. The data reported suggest that amoxycillin may have certain advantages, compared with ampicillin or benzylpenicillin, for the treatment of enterococcal infections.