Comparison of metronidazole assay by microbiological and chemical methods

Chemical (thin layer chromatography/fluorescence quenching in situ) and microbiological (agar well, diffusion technique with Clostridium perfringens as indicator strain) methods of assaying metronidazole have been compared. On dummy samples made with pure metronidazole in pooled human serum, both methods had a coefficient of variation ranging from 5.5 to 9.6 per cent of the mean. The microbiological method slightly underestimated the real amounts, and also had lower values than the chemical procedure. Comparison of serum and urine samples taken during the early, middle and late periods after medication to volunteers showed that biotransformation to antibacterially active metabolites contributes significantly to the antibacterial activity, particularly in urine. Biotransformation explains why microbiologically determined concentrations were higher than those determined chemically in samples taken at least 16-20 hours after intake of tablets or suppositories. It is important to be aware of the circumstance that the results of microbiological assay are sensu strictu limited to the particular indicator strain used, since other bacteria may exhibit other patterns of sensitivity to metronidazole and its metabolites.