Dubbed “sensing of phage-triggered ion cascade”, or SEPTIC, the technique employs bacteriophages - viruses that attack bacteria. Each type of bacteriophage attacks a specific bacterial species or range of species.
“Rapid and sensitive identification of bacteria is extremely important in clinical, veterinary and agricultural practice, as well as for microbiological threat detection and reduction,” said Mosong Cheng of Texas A&M. “Given its fast response, high specificity and relatively low cost, SEPTIC could be invaluable in clinical, veterinary and agricultural practice, as well as in the current fight against bioterrorism.”
When a phage injects its DNA into a live bacterium it causes the cell to temporarily leak about 100 million ions into its surroundings. “The ion leakage brings a strong disturbance to the electric field,” said Laszlo Kish. “These microscopic electrical field fluctuations are picked up by a small antenna: the so-called nanowell.”
The nanowell device contains two 20 nm-thick titanium electrodes on a lithium niobate substrate. A 150 nm-wide, 4 micrometre-long gap between the two electrodes forms the nanowell. Two probes connect the device to an external amplifier.
The researchers tested the technique on three strains of E. coli bacteria, using three different phages. They had a 100% success rate in detecting and identifying the bacteria. What’s more, they were able to detect live bacteria within minutes.
According to the scientists, the electrical field fluctuations caused by ion leakage have very different dynamics to background noise. “Due to this reason and to the selectivity of phages - phages never infect a wrong bacterium - the method is virtually free of false alarm,” said Kish.
The technique also has the advantage of speed. Other methods for detecting bacteria often take hours or days to complete, as well as requiring complicated equipment, which makes them unsuitable for use in the field. And techniques such as polymerase chain reaction culturing to investigate DNA cannot distinguish between living and dead bacteria.
The researchers reckon they could improve the performance of their device by connecting it to a chip containing a junction field-effect transistor to detect and amplify the signal. This could potentially produce a sensitivity of one bacterium per microlitre of solution.
“Our ultimate aim is to have a biochip where hundreds of nanowells and their preamplifiers are integrated,” said Kish. “Each nanowell covers a different phage and, if a relevant bacterium is present, the corresponding nanowell will signal and identify the bacterium. This would be a pen-size biolab that would be able to identify hundreds of bacteria in just five minutes.”
Now the scientists are working on implanting the nanowell device in plants so that it could warn if the organism was coming under viral attack.
The researchers reported their work in the Journal of Biological Physics and Chemistry.