Colorimetric determination of peptide antibiotics: in-process assay of cyclic octapeptidic antibiotics in fermentation broths.

A spectrophotometric method is presented for monitoring the biosynthesis of a new complex of cyclic octapeptidic antibiotics in fermentation broths. The method is based on the extraction of antibiotic from alkaline broth with butanol. An ion-pair, formed between the octapeptides and bromthymol blue, is extracted into chloroform from a solution buffered to pH 7.5. The absorbance of the colored solution is measured at 420 nm. Results are in good agreement with those obtained by microbiological assay. The method is also applicable to other peptidic antibiotics such as polymyxin B and gramicidin.