Modified assay medium for the turbidimetric assay of chlortetracycline in feeds

In previous experiments, the turbidimetric method for determining chlortetracycline-HCI (CTC-HCI) in feeds showed lower recovery than the AOAC plate method. Although the addition of vitamins to the turbidimetric medium improved results, values by the turbidimetric method remained about 10% lower than by the plate method. A modified (1.7X the weight recommended by the manufacturer) turbidimetric assay medium decreased assay sensitivity but did not significantly change the slope of S. aureus response to CTC-HCI. There was no evidence that vitamin fortification of the modified medium had any significant effect on the growth rate of test organism. Examination of about 100 samples of commercial feeds containing CTC-HCI showed excellent agreement in results between the turbidimetric and plate methods.