Comparative activity of glycopeptide antibiotics against coagulase-negative staphylococci embedded in fibrin clots

The susceptibilities of 16 strains of coagulase-negative staphylococci to vancomycin and teicoplanin were determined by three microdilution methods run in parallel; (i) a reference method in which the medium was Iso-Sensitest broth, (ii) a method in which the organisms were suspended in plasma and (iii) one in which the bacteria were incorporated into a fibrin clot. In comparison with the reference method there was a less than two-fold change in the geometric mean MICs of both antibiotics in plasma, while in clot the increase was 12-fold for vancomycin and 28-fold for teicoplanin. MBCs showed a similar trend although the increase was greater for teicoplanin. These results suggest that protein binding may have a much greater impact on antimicrobial activity when the mobility of the protein is reduced within a clot.