New carbon nanotube biosensor detects bacteria instantaneously

A research group from the Rovira i Virgili University (URV) in Tarragona has developed a biosensor that can immediately detect very low levels of Salmonella typhi, the bacteria that causes typhoid fever. The technique uses carbon nanotubes and synthetic DNA fragments that activate an electric signal when they link up with the pathogen.

An aptamer attached to an electrode coated with single-walled carbon nanotubes interacts selectively with bacteria.
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An aptamer attached to an electrode coated with single-walled carbon nanotubes interacts selectively with bacteria. The resulting electrochemical response is highly accurate and reproducible and starts at ultralow bacteria concentrations, providing a simple, selective method for pathogen detection.

“We have developed a new biosensor that can detect extremely low concentrations of bacteria immediately, easily and reliably”, F. Xavier Rius, lead author of the study and a professor in the Chemometrics, Qualimetrics and Nanosensors research group in the Analytical Chemistry and Organic Chemistry Department of the URV, tells SINC.

Rius’ team, jointly led by Jordi Riu, has come up with a technique that can detect extremely low levels of the bacteria Salmonella typhi, which causes typhoid fever. This new biosensor functions using a method, described this month in the scientific journal Angewandte Chemie International Edition (“Immediate Detection of Living Bacteria at Ultralow Concentrations Using a Carbon Nanotube Based Potentiometric Aptasensor”), which involves carbon nanotubes with inbuilt aptamers providing electrochemical readings.

The aptamers are small fragments of artificial DNA or RNA designed to attach themselves specifically to a particular molecule, cell or micro organism, in this case Salmonella. If the bacteria are not present, the aptamers remain on the walls of the carbon nanotubes. However, if they detect bacteria, they become activated and stick to it, and the carbon nanotubes generate an electric signal that is picked up by a simple potentiometer connected to the biosensor.

“The presence of the bacteria sparks a change in the interaction between the aptamers and the nanotubes, which takes place in a few seconds and creates an increase in the voltage of the electrode”, says Rius.
Traditional methods for identifying and measuring micro organisms require one or two days’ analysis. “This technique means small quantities of micro organisms can be detected simply and practically in real time, just the same as measuring the pH of water”, adds the researcher.

This study is part of the international research being carried out to find the most effective and fast ways of detecting all kinds of pathogens. The new biosensor makes it possible to identify a single cell of Salmonella in a five-millilitre sample and can successfully make quantitative measurements of up to 1,000 bacteria per millilitre.

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More efficient microbial fuel cells

Bacteria that generate significant amounts of electricity could be used in microbial fuel cells to provide power in remote environments or to convert waste to electricity. Professor Derek Lovley from the University of Massachusetts, USA isolated bacteria with large numbers of tiny projections called pili which were more efficient at transferring electrons to generate power in fuel cells than bacteria with a smooth surface. The team’s findings were reported at the Society for General Microbiology’s meeting at Heriot-Watt University, Edinburgh, today (7 September).

The researchers isolated a strain of Geobacter sulfurreducens which they called KN400 that grew prolifically on the graphite anodes of fuel cells. The bacteria formed a thick biofilm on the anode surface, which conducted electricity. The researchers found large quantities of pilin, a protein that makes the tiny fibres that conduct electricity through the sticky biofilm.

“The filaments form microscopic projections called pili that act as microbial nanowires,” said Professor Lovley, “using this bacterial strain in a fuel cell to generate electricity would greatly increase the cell’s power output.”
The pili on the bacteria’s surface seemed to be primarily for electrical conduction rather than to help them to attach to the anode; mutant forms without pili were still able to stay attached.

Microbial fuel cells can be used in monitoring devices in environments where it is difficult to replace batteries if they fail but to be successful they need to have an efficient and long-lasting source of power. Professor Lovley described how G. sulfurreducens strain KN400 might be used in sensors placed on the ocean floor to monitor migration of turtles.

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