Realization of Conductometry on a Digital Microfluidic Platform for Real-Time Monitoring of Bacillus Atrophaeus Endospore Germination

The advent of microelectromechanical system allows people to investigate microbial physiology in a microscale. This paper is to report the fabrication of a digital microfluidic chip coupled with platinum electrodes for conductometry and to monitor the in situ germination of Bacillus atrophaeus endospores. Germination was triggered by mixing B. atrophaeus endospore droplet with L-alanine droplet via electrowetting-on-dielectric process. The time course of ion release from germinating endospores was monitored by on-chip conductance. Results were compared with off-chip conductometry. We observed that both on-chip and off-chip measurements exhibited similar germination kinetics. However, compared with the off-chip measurement, the on-chip measurement took 63% of time to reach half of its final value. The discrepancy may be due to the difference on speeds of ion movement rather than germination ratios. This hypothesis was verified and supported by COMSOL Multiphysics simulation and phase contrast microscopy. The former showed that the microelectrodes (on-chip) generated an electric field that was 1.6 times stronger than did macroeletrodes (off-chip). The later delineated that the difference between germination ratios of on-chip and off-chip measurement at selected time points (10, 30, and 120 min) were not statistically significant (P > 0.05). This paper highlights conductometry on a digital microfluidic platform as a promising label-free technique to monitor in situ endospore germination.