We describe an optical system that detects the presence of E. coli bacteria, making use of the bacteria's natural fluorescence properties. The system provides an excitation signal at 365 nm and detects the emission signal, from the bacteria, at approximately 445 nm. The system also allows the intensity of the emitted signal to be compared with a user-programmable threshold. This allows rapid testing of many samples in a laboratory setting. Complete setup and performance details are provided, enabling the experimentalist to tailor the system parameters to other species of microorganisms, which may have fluorescence properties at other wavelengths.