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[Tetracyclines: bacteriostatic or bactericidal drugs? In vitro studies with rolitetracycline, minocycline and doxycycline (author's transl)].

Minimal bacteriostatic and minimal bactericidal concentrations of rolitetracycline, minocycline and doxycycline on 20 different E. coli serotypes and 16 staphylococcus aureus strains have been compared in bouillon and serum. In addition growth curves of all strains in bouillon and serum without and with antibiotics in concentrations corresponding to the minimal inhibitory concentration of each strain have been followed for 24 hours. In E. coli minimal bactericidal concentrations of all 3 tetracyclines in bouillon on average were only twice as high as the minimal bacteriostatic concentrations of the drugs, tested. Minimal bacteriostatic and bactericidal concentrations in serum were significantly lower than in bouillon. 27 out of 20 E. coli strains were inhibited by less than 0.012 microgram/ml. In staphylococci minimal bactericidal concentrations were substantially higher than the minimal bacteriostatic concentrations (on average at least four times higher). In E. coli addition of serum increased the antimicrobial activity of all tetracyclines tested. Growth curves in bouillon tended to show some bactericidal shape at 3, 7 and 24 hours following addition of the tetracyclines at minimal bacteriostatic concentrations. Bactericidal activity of tetracyclines at the MIC was substantially improved by addition of serum. This was not true for staphylococci in which neither minimal bactericidal and bacteriostatic concentrations nor growth curves could be significantly improved by addition of serum.