Evaporation-induced stimulation of bacterial osmoregulation for electrical assessment of cell viability

Abstract

Bacteria cells use osmoregulatory proteins as emergency valves to respond to changes in the osmotic pressure of their external environment. The existence of these emergency valves has been known since the 1960s, but they have never been used as the basis of a viability assay to tell dead bacteria cells apart from live ones. In this paper, we show that osmoregulation provides a much faster, label-free assessment of cell viability compared with traditional approaches that rely on cell multiplication (growth) to reach a detectable threshold. The cells are confined in an evaporating droplet that serves as a dynamic microenvironment. Evaporation-induced increase in ionic concentration is reflected in a proportional increase of the droplet's osmotic pressure, which in turn, stimulates the osmoregulatory response from the cells. By monitoring the time-varying electrical conductance of evaporating droplets, bacterial cells are identified within a few minutes compared with several hours in growth-based methods. To show the versatility of the proposed method, we show detection of WT and genetically modified nonhalotolerant cells (Salmonella typhimurium) and dead vs. live differentiation of nonhalotolerant (such as Escherichia coli DH5α) and halotolerant cells (such as Staphylococcus epidermidis). Unlike the growth-based techniques, the assay time of the proposed method is independent of cell concentration or the bacteria type. The proposed label-free approach paves the road toward realization of a new class of real time, array-formatted electrical sensors compatible with droplet microfluidics for laboratory on a chip applications.bacteria | label free | osmoregulation | droplet | electrical microdevice

Differentiation of live and heat-killed E. coli by microwave impedance spectroscopy

“…Detection of bacteria and their viability in food, water and clinical samples is critically important in fields such as bioscience research, medical diagnosis, food screening and environment monitoring [1]. Conventional methods for bacteria detection, albeit sensitive and specific, are often time-consuming, infrastructure dependent, and require skilled technicians [2].…”

Analyzing Thermal Stability of Cell Membrane of Salmonella Using Time-Multiplexed Impedance Sensing

“…The model allows us to associate effective kinetic parameters of permeabilization in wildtype (WT) and heat-resistant (HR) cells and to demonstrate that the latter have a larger effective activation energy and consequently, a stronger cell envelope. The dropletbased impedance sensor is label-free (i.e., it does not use radioactive isotopes or fluorescent dyes) and can be miniaturized and integrated onto lab-on-a-chip platforms (20)(21)(22). In addition, supported by a physics-based quantitative model for the droplet impedance (23), we are able to tune the working frequency for optimum performance and characterize cells in the growth medium.…”

Detection of Antibiotics and Evaluation of Antibacterial Activity with Screen-Printed Electrodes

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