Electrochemical detection and counting of Escherichia coli in the presence of a reducible coenzyme, lipoic acid.

Reduction of lipoic acid by bacteria coupled to oxygen consumption during glucose absorption can be followed potentiometrically with a pair of gold and reference electrodes in a minimal culture medium. The variations in potential as a function of time have the shape of a wave. A theoretical expression was derived relating the size of the original inoculum to the time preceding the appearance of the wave. The validity of that relation was experimentally verified with Escherichia coli, and the time needed for a drop of 100 mV was determined. Detection of small inocula, e.g., down to a range of 10 viable E. coli per liter, is possible in about 11 h by yeast extract stimulation. The method, technically simple and adequately sensitive, suggests the possibility of automated detectors of bacterial contaminations.