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 staphylococci, predominantly isolated from intravenous catheter tips and exit sites was determined by agar dilution on IsoSensitest media with and without the addition of 20% inactivated horse serum using an inoculum of 10(4) cfu/spot. Incorporation of serum resulted in a three-fold increase in the geometric mean MIC and a four-fold increase in the geometric mean MBC. Further tests, performed in DST and IsoSensitest medium supplemented with 20% inactivated horse serum, resulted in a wide variation in teicoplanin MICs with differences in the geometric mean MICs being up to 8.8-fold in serum media compared to unsupplemented media, whereas the MICs of vancomycin were unaffected. Under the various experimental conditions used, the susceptibility of the coagulase negative staphylococci to teicoplanin varied from 31-100%, while all were consistently susceptible to vancomycin. We therefore recommend that teicoplanin MICs be determined in the presence of serum as these are better related to serum drug levels and reflect more accurately the conditions in vivo. Trials of teicoplanin as monotherapy are required to provide further insight into the relationship between its in-vitro antibacterial activity and its clinical efficacy as the drug is easier to administer, better tolerated and less toxic than vancomycin which might compensate for its reduced activity in the presence of serum.