Dead or alive?

A new type of electronic sensor that can not only detect and classify bacteria, but can also tell if the bacteria are dead or alive is under development.

This approach, patented by Purdue University researchers, could ultimately allow food scientists to bypass traditional, time-consuming, growth culture-based microbial assays. The follow-on of this would mean that product could be released within hours of production without having to be quarantined until microbial clearance has been obtained by current methods.

The researchers, doctoral student Aida Ebrahimi and Muhammad Ashraful Alam, the Jai N. Gupta Professor of Electrical and Computer Engineering, believe that their approach might be used to create arrays of hundreds of sensors on an electronic chip, each sensor detecting a specific type of bacteria.

Using their ‘osmoregulation-based’ detection system, the researchers demonstrated differentiating wild-type and genetically modified non-halotolerant cells (Salmonella Typhimurium), dead vs live non-halotolerant (Escherichia coli DH5α) and halotolerant cells (such as Staphylococcus epidermidis).

The label-free approach paves the road towards realisation of a new class of real-time, array-formatted electrical sensors compatible with droplet microfluidics for lab-on-a-chip applications. The assay time of the proposed method is independent of cell concentration or the bacteria type.

The droplet sensor evolved from a device originally designed to detect small concentrations of negatively charged DNA molecules. The fact that the method could distinguish between live and dead bacteria was a serendipitous chance observation.

Bacteria cells maintain the proper internal pressure through osmoregulation, a process in which water, salts and other molecules move across the cell membrane. As a droplet begins to evaporate on the sensor, bacteria cells contained in the droplet detect the increasingly salty environment, triggering emergency valves called osmoregulatory transporters in the cell membrane. The cells then either take in or release water and charged molecules including salts, changing the electrical conductivity of the surrounding fluid in the droplet, which is measured by electrodes. This change in electrical conductivity varies according to whether a bacteria cell is dead or alive and also might be used to identify specific types of bacteria because they use fundamentally different osmoregulatory channels.

The hypothesis was challenged by using genetically mutated cells that do not have those osmoregulatory channels.