Contactless Optical and Impedimetric Sensing for Droplet-Based Dose-Response Investigations of Microorganisms

The principle of droplet-based microfluidics was used for the characterization of dose/response functions of the soil bacteria Rhodococcus sp . and Chromobacterium vaccinii using a combination of optical and electrical sensors for the detection of bacterial growth and metabolic activity. For electrical characterization, a micro flow-through impedance module was developed which assessed the response of bacterial populations inside 500 nL fluid segments without direct galvanic contact between the electrodes and the electrolyte. It was found that the impedance sensor can detect an increase in cell density and is particularly suited for monitoring the metabolic response due to changes in the cultivation medium inside the separated fluid segments. Due to this sensitivity, the sensor is very useful for the investigation of highly resolved dose/response functions by micro fluid segment sequences. The impedimetric data agree well with the optical data concerning the characteristic response of bacteria populations in the different concentration regions of heavy metal ions, but the sensor supplies very valuable complementary data on metabolic activity in case of low or negligible cell division rates.