Faster detection and classification of bacteria

DETECTING bacteria could now be a less daunting task, thanks to a new electronic sensor that can distinguish between living and dead bacteria cells.

The sensor can measure the changing electrical conductivity and differentiate between cells by harnessing their osmoregulation, the process responsible for the maintenance of the bacteria cell’s proper internal pressure. Bacteria cells in a droplet that begins to evaporate on the sensor detect the increasingly salty environment and trigger the osmoregulatory transporters, or emergency valves in the membrane. The cells either take in or release water and charged molecules including salts, which alters the electrical conductivity of the surrounding fluid in the droplet measured by electrodes. The change in electrical conductivity differs in a dead and alive bacteria cell, and therefore can be used to identify specific types of bacteria.

The new technology can be employed in food safety and medical diagnostics applications, allowing faster detection and classification of bacteria for as opposed to conventional laboratory technologies that require many hours of sample culturation to grow bacteria for analysis. The approach can also be used to create arrays of hundreds of sensors on an electronic chip, with each sensor capable of detecting a specific type of bacteria or identifying the effectiveness of particular antibiotics within minutes.  

The sensor evolved from a device designed to detect small concentrations of negatively charged DNA molecules in research that began about four years ago. The technology, tested with low concentrations of living and dead forms of E. coli, Salmonella and S. epidermidis bacteria, is considered label-free. It does not require that samples be treated with fluorescent dyes.

According to Muhammad Ashraful Alam, Purdue University's Jai N. Gupta professor of Electrical and Computer Engineering, "We have taken a step toward this long-term goal by showing how to distinguish between live and dead bacteria. This is important because you need to be able to not only detect and identify bacteria, but to determine which antibiotics are effective in killing them.”

"We did not anticipate that the sensor could be used to tell live and dead bacteria apart – it was a chance observation that eventually led us to this elegant way of measuring cell viability," Alam said.

She added that immediate physiological measurement is not only faster but also far superior. “To see if someone is alive, we can either count the grandchildren many generations later, which is analogous to the traditional growth-based techniques. Or, we can directly measure the person's pulse, analogous to the proposed 'osmoregulation-based' detection of bacteria.”

"Aida proved the hypothesis by using genetically mutated cells that do not have those osmoregulatory channels and therefore are less effective in regulating the pressure differential," Alam said.

She added: “In the end you want to provide a new tool for medicine and food safety, so you need to be able to quickly identify bacteria and the right antibiotics to treat infection. That requires an understanding of the dynamics of the cell membrane.”

The findings of the research, the bulk of which was conducted at the Birck Nanotechnology Center and Bindley Bioscience Center in Purdue's Discovery Park, are detailed in a paper by doctoral student Aida Ebrahimi and Alam and published in Proceedings of the National Academy of Sciences. The research is likewise documented in a Youtube video.

The technology has received a provisional patent through the Purdue Research Foundation's Office of Technology Commercialization.

Journal Reference: Aida Ebrahimi and Muhammad Ashraful Alam. Evaporation-induced stimulation of bacterial osmoregulation for electrical assessment of cell viability. Proceedings of the National Academy of Sciences, June 2016 DOI: 10.1073/pnas.1606097113

Purdue University. "Electronic bacteria sensor is potential future tool for medicine and food safety." ScienceDaily. www.sciencedaily.com/releases/2016/06/160614100347.htm (accessed August 17, 2016).