Bactericidal effect of penicillin, ampicillin, and amoxicillin alone and in combination with tobramycin against Enterococcus faecalis as determined by kill-kinetic studies.
Fifteen strains of Enterococcus faecalis, all clinical blood culture isolates from patients with endocarditis, were studied by kill-kinetic experiments using penicillin G, ampicillin and amoxicillin alone and in combination with tobramycin. The median minimal inhibitory concentrations (MIC), were penicillin 4 mg/l, ampicillin 2 mg/l, amoxicillin 2 mg/l and tobramycin 32 mg/l. Equipotent doses of the antibiotics (1/2 x MIC, 1 x MIC and 4 x MIC) were used in the kill-kinetic studies. Synergism was studied using a combination of 1/2 x MIC of the beta-lactam antibiotic and 8 mg/l of tobramycin. The bactericidal activity did not exceed 10(+3) cfu/ml at 5 hours for any single compound. After 5 h all three beta-lactam antibiotics in combination with tobramycin resulted in synergism, i.e. more than one hundredfold reduction of colony forming units (cfu) as compared to the most active single agent. Amoxicillin had a significantly higher bactericidal potential than ampicillin or penicillin both alone and in combination with tobramycin. The clinical significance of these findings warrants further studies in vivo.
Westh
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Dept. of Clinical Microbiology, University of Copenhagen, Bispebjerg Hospital, Denmark.
Frimodt-Møller
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N
Gutschik
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D000658:Amoxicillin / Q000494:pharmacology; D000667:Ampicillin / Q000494:pharmacology; D001431:Bacteriological Techniques; D004357:Drug Synergism; D004359:Drug Therapy, Combination / Q000494:pharmacology; D004697:Endocarditis, Bacterial / Q000382:microbiology; D013293:Enterococcus faecalis / Q000187:drug effects*; D008826:Microbial Sensitivity Tests; D010400:Penicillin G / Q000494:pharmacology; D010406:Penicillins / Q000494:pharmacology*; D013290:Streptococcal Infections / Q000382:microbiology; D014031:Tobramycin / Q000494:pharmacology*
D010406:Penicillins; D000667:Ampicillin; D000658:Amoxicillin; D010400:Penicillin G; D014031:Tobramycin