The antibacterial activities of cefotaxime, gentamicin, tobramycin, and amikacin as well as the combinations of cefotaxime and aminoglycosides were tested against 50 strains of enterococci. Synergy studies were performed by the time-kill method, and with this technique the combination of cefotaxime plus gentamicin appeared to be more active than the combination of cefotaxime plus tobramycin or cefotaxime plus amikacin. Results of this in vitro study suggest cefotaxime and gentamicin may provide some activity against enterococci, although not sufficient to substitute for proven therapy such as penicillin-ampicillin and gentamicin.Enterococci have emerged as important bacterial pathogens that are frequently resistant to the new cephalosporin antimicrobial agents. Because these new agents are often utilized in combination with aminoglycosides, we thought it would be interesting to determine the in vitro activity of combinations of cefotaxime, the first third-generation agent available in the United States, and the three most commonly used aminoglycosides, gentamicin, tobramycin, and amikacin, to determine whether cefotaxime plus these agents might be reasonable candidates for use in patients with enterococcal disease.
MATERIALS AND METHODSFifty strains of enterococci were obtained from 24 urine cultures, 20 wound cultures, 2 sputum cultures, 2 bronchial washings, and 2 bile cultures performed in the Clinical Microbiology Laboratory at the University of Alabama Medical Center during November and December 1982.All strains grew in 6.5% NaCl heart infusion broth and as brownish-black colonies surrounded by a black zone on bile esculin agar. Precise determination of species was performed by the API 20S Streptococcus System (Analytab Products, Plainview, N.Y.). There were 46 strains of Streptococcus faecalis and 4 strains of Streptococcus faecium.Antimicrobial agents used in this study were kindly supplied as sterile standard reference powders by the following companies: cefotaxime by Hoechst-Roussell Pharmaceuticals, Inc., Somerville, N.J.; gentamicin by Schering Corp., Kenilworth, N.J.; tobramycin by Eli Lilly & Co., Indianapolis, Ind.; and amikacin by Bristol Laboratories, Syracuse, N.Y.The minimal inhibitory concentration (MIC) was determined for each isolate for cefotaxime, gentamicin, tobramycin, and amikacin by both agar dilution and broth dilution techniques. In the broth dilution technique, five colonies of each strain were picked to 2 ml of Mueller-Hinton broth (BBL Microbiology Systems, Cockeysville, Md.), which were incubated overnight at 37°C and then diluted to a final inoculum of 5 x 10' CFU/ml. Serial twofold dilutions were made in Mueller-Hinton broth, using borosilicate tubes with loose-fitting metal caps with a total volume of 1 ml. The MIC was defined as the lowest concentration of the antimicrobial agent causing no visible growth after incubation at 37°C for 18 h.From each clear tube and the cloudy tube with the least antimicrobial concentration, 0.01 ml was subcultured onto 5% sheep blood agar. The ...