A viability assay for individual Lactobacillus cells based on their electro-orientation between indium-tin-oxide electrodes

Lactic acid bacteria are widely used as probiotic products, making viability assessment of these bacteria an important parameter in the quality control procedures followed during the processing and preservation. In this study, we developed a rapid and easy single-cell viability assay for rod-shaped Lactobacillus casei that uses an alternating current (AC) electric field to induce electro-orientation (EO). Lactobacillus cells were sandwiched between two optically transparent indium-tin-oxide plate electrodes. An AC voltage of 2 V at 15 MHz was applied to a narrow electrode gap of 17 μm, corresponding to maximum cell length. We immediately identified opposite EO positions for living and dead cells using a light microscope; living cells stood upright, appeared as perfect dots, and oriented parallel to the electric field line, whereas dead cells lay flat, appeared as bars, and oriented perpendicular to the field. Besides saving time and labor, this method allowed easy viability estimation through automatic counting of dots or bars on the microscopic images using an image processing software. This simple and inexpensive apparatus provides a rapid and easy method for the assessment of the viability in Lactobacillus cells without chemical reagents.