Determination of teicoplanin concentration in serum using a bioassay technique

An accurate and sensitive bioassay for determining concentrations of teicoplanin in serum has been developed using modifications of procedures described for assaying vancomycin. A linear relationship (r = 0.9983) was obtained between the diameter of the zone of inhibition and log10 teicoplanin concentration over the range 0.25-32 micrograms/ml. The medium used was Antibiotic Medium No. 1 (Oxoid, UK) adjusted to pH 5.5-5.7 by the addition of hydrochloric acid and containing sodium chloride to a final concentration of 3%. The indicator organism used was a Bacillus subtilis ATCC 6633 (NCTC 10400). Teicoplanin was assayed: (i) in the presence of beta-lactams and cephalosporins, including ceftazidime, by prior treatment of serum with broad-spectrum beta-lactamase mixtures (Genzyme Laboratories, UK); (ii) in the presence of aminoglycosides by prior treatment of serum with cellu-ion phosphate (100 mg/ml) followed by centrifugation; (iii) in the presence of sulphamethoxazole and/or trimethoprim by the addition of p-aminobenzoic acid and/or thymidine to the assay medium. Teicoplanin was assayed in the presence of either rifampicin or erythromycin by using a rifampicin-resistant, erythromycin-resistant clinical isolate of Staphylococcus aureus as indicator organism. The lower limit of sensitivity of this assay was 1 microgram/ml. The presence of undeclared, broad-spectrum antimicrobials was detected by screening serum samples for antimicrobial activity using an assay plate seeded with Escherichia coli NCTC 10418.