A team of physicists and chemists in the US has used a tiny rotating sphere to detect individual bacterium. The researchers claim that the technique could be used for a range of applications including defending against biological attacks and reducing the time it takes to develop new antibiotics.

The creation of biological sensors for the detection of bacteria and other dangerous micro-organisms is making it easier for doctors to diagnose some diseases. While micro-electromechanical systems (MEMS) have been developed to detect bacteria, they rely on bacteria sticking to tiny vibrating cantilevers and changing the frequency of oscillation. However, these techniques are very difficult to implement in liquids - a more natural environment for most bacteria - because any viscous fluid will dampen the vibrations, which greatly reduces the sensitivity of these techniques.

Now physicists Brandon McNaughton, Raoul Kopelman and colleagues at the University of Michigan have develeoped a new detection technique that can detect bacteria in a liquid. The device uses a 2-µm diameter magnetic sphere that is rotated in a liquid by an external magnetic field. The sphere is coated with antibodies that grab hold of certain bacteria.

Asynchronous rotation

If the sphere is spun fast enough, it is no longer synchronized with the rotation of the external field. This "asynchronous rotation rate" is highly sensitive to small changes in the drag of the surrounding fluid. When a bacteria attaches to the sphere, the rotation slows down significantly – something that can be observed using standard optical microscopy techniques (Appl. Phys. Lett. 91 224105).

According to McNaughton the sensor can also determine when subsequent bacteria stick to the sphere by measuring step shifts in the sensor's rotational frequency. It can even tell if an attached bacterium changes in size. The team used its technique to detect the common bacterium E. Coli but according to McNaughton, the method could be adapted for other bacteria.

"This detection aspect can be used for bio-defence applications, where rapid and sensitive techniques are needed," said McNaughton. "There are also potential applications for detecting bacteria in water and food. However, our main focus has been to identify bacterial strains (like "superbugs") and determine the strain's susceptibility to antibiotics." The researchers say they are also working on monitoring bacterial growth.

The team are building a stand-alone prototype device that can both detect bacteria and measure growth responses to antibiotics. "The prototype will hopefully have a test turn-around-time of hours, instead of the current waiting time of days." The team has applied for a patent on the technology.