Microscale electronic detection of bacterial metabolism

In this paper, we present a microscale impedance-based technique for detecting the metabolic activity of a few live bacterial cells. Impedance-based detection relies on measuring changes in the ac impedance of two electrodes in contact with a liquid where the bacteria are cultured, caused by the release of ionic species by metabolizing cells. Rapid detection of a few live cells (1-10) is, in theory, possible if the cells are confined into a volume on the order of nanoliters. A microfluidic biochip prototype has been fabricated to explore this technique, consisting of a network of channels and chambers etched in a crystalline silicon substrate. The complex impedance of bacterial suspensions is measured with interdigitated platinum electrodes in a 5.27 nl chamber in the biochip at frequencies between 100 Hz and 1 MHz. After 2 h of off-chip incubation, the minimum number of live cells suspended in a low conductivity buffer that could be easily distinguished from the same number of heat-killed cells was on the order of 100 Listeria innocua, 200 L. monocytogenes, and 40 Escherichia coli cells, confined into the 5.27 nl chamber. A number on the order of 100 live L. innocua cells suspended in Luria-Bertani (LB) broth produced a significantly higher signal than the same number of heat-killed cells, and a difference is evident even down to ∼5-20 cells. To the best of our knowledge, this is the first demonstration of microscale impedance-based detection of bacterial metabolism.