Summary auto-generated
This study evaluated the in vitro activity of amoxycillin, a new semi-synthetic penicillin, against 55 clinical isolates of enterococci, including Streptococcus faecalis and related subspecies. Amoxycillin was more active than ampicillin or benzylpenicillin against all strains tested. While penicillins alone showed modest bactericidal activity against large inocula but failed to completely sterilize cultures, combinations of penicillins with aminoglycoside antibiotics (streptomycin, kanamycin, or gentamicin) produced strong synergistic bactericidal effects that sterilized cultures—but only when the enterococci were sensitive to the specific aminoglycoside used. Notably, 15 of 55 strains showed high-level streptomycin resistance, and two were also kanamycin-resistant, though all strains remained sensitive to gentamicin. The synergistic effect directly correlated with aminoglycoside sensitivity: resistant strains showed no synergy with penicillin-aminoglycoside combinations. The authors conclude that amoxycillin may offer advantages over ampicillin or benzylpenicillin for treating enterococcal infections, particularly given its superior oral absorption and bactericidal activity.
Key findings
- Amoxycillin demonstrated superior in vitro activity against enterococci compared to ampicillin or benzylpenicillin, though all three penicillins inhibited all 55 strains tested
- Penicillin-aminoglycoside combinations showed synergistic bactericidal effects that sterilized cultures, but only when enterococci were sensitive to the specific aminoglycoside used
- High-level streptomycin resistance occurred in 15 of 55 enterococcal strains (27%), with cross-resistance patterns to other aminoglycosides, while all strains remained sensitive to gentamicin
- Synergistic killing directly correlated with aminoglycoside susceptibility; no synergy was observed with penicillin-aminoglycoside combinations against resistant strains
- Amoxycillin's superior oral bioavailability and bactericidal activity suggest potential clinical advantages for treating enterococcal infections
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Abstract
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.