INFLUENCE OF INOCULUM, MEDIUM AND SERUM ON THE IN-VITRO SUSCEPTIBILITY OF COAGULASE-NEGATIVE STAPHYLOCOCCI TO TEICOPLANIN AND VANCOMYCIN

The susceptibility to teicoplanin of 100 coagulase negative staphyloco cci, predominantly isolated from intravenous catheter tips and exit si tes was determined by agar dilution on IsoSensitest media with and wit hout the addition of 20% inactivated horse serum using an inoculum of 10(4) cfu/spot. Incorporation of serum resulted in a three-fold increa se in the geometric mean MIC and a fourfold increase in the geometric mean MBC. Further tests, performed in DST and IsoSensitest medium supp lemented with 20% inactivated horse serum, resulted in a wide variatio n in teicoplanin MICs with differences in the geometric mean MICs bein g up to 8.8-fold in serum media compared to unsupplemented media, wher eas the MICs of vancomycin were unaffected. Under the various experime ntal conditions used, the susceptibility of the coagulase negative sta phylococci to teicoplanin varied from 31-100%, while all were consiste ntly susceptible to vancomycin. We therefore recommend that teicoplani n MICs be determined in the presence of serum as these are better rela ted to serum drug levels and reflect more accurately the conditions in vivo. Trials of teicoplanin as monotherapy are required to provide fu rther insight into the relationship between its in-vitro antibacterial activity and its clinical efficacy as the drug is easier to administe r, better tolerated and less toxic than vancomycin which might compens ate for its reduced activity in the presence of serum.