This study describes fluorescence measurements made with a fiber-optic probe on model fermentation media samples containing natural media components, high densities of <i>Streptomyces aureofaciens</i> cell mass, and varying concentrations of tetracycline. Tetracycline, produced commercially from <i>S. aureofaciens</i>, is a highly fluorescent antibiotic, and these measurements were conducted to determine the correlation between the tetracycline content in the media and the observed fluorescence of the samples. Seven separate types of media samples were examined for this study. Each medium contained a nonfluorescent base of nutrients and salts along with one of seven different natural feedstocks at a concentration of 50 g/L. In addition, the samples contained <i>S. aureofaciens</i> cell mass densities of 50 g/L (based on dry cell mass). Excitation wavelengths of 290 nm, 390 nm, and 413 nm were used to probe the fluorescence of the samples, and emission spectra were obtained over the wave-length ranges of 320-360 nm, 420-600 nm, and 440-600 nm, respectively. Background fluorescence observed in the region of 320-360 nm upon 290-nm excitation in the samples was due to tryptophan fluorescence contained in the <i>S. aureofaciens</i> cell mass and was found to decrease dramatically as tetracycline levels increased from 0 to 8000 μg/mL. Plots of fluorescence intensity at a number of emission wavelengths versus tetracycline concentration in the samples clearly revealed strong correlations.

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