The intrinsic steady-state fluorescence and fluorescence decay of <i>Staphylococcus epidermidis, Pseudomonas fluorescens, Enterobacter cloacae, Escherichia coli,</i> and <i>Bacillus subtilis</i> have been observed. Each organism exhibits a strong maximum in its emission spectrum at 330-340 nm when excited at 290 nm. Iodide quenching and denaturization experiments with 8 <i>M</i> urea provide strong evidence for the assignment of the 330-340-nm fluorescence to protein tryptophan. Most importantly, the decay of this bacterial protein-tryptophan fluorescence has been described by two exponential functions in all cases. The observation that characteristic protein-tryptophan fluorescence lifetimes have been obtained for each organism suggests that measurements of fluorescence lifetimes may be helpful in the rapid characterization of bacteria. Direct application will most likely be found in combination with the measurement of other luminescence parameters.

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